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Warm Mission - Spitzer Science Center

1. End of campaign reprocessing Data are 2 downlinked every Data products Data Quality Pipeline 24 hrs enter Archive Analysis fj processes amp determines if doe i lt 1 wk e 0 c data are good calibrates data Figure 2 2 The life cycle of a Spitzer observation The detailed definition of observations will occur at the time of each annual proposal solicitation In the warm mission there is no longer a Data Quality Analysis check prior to archiving the data Figure 2 2 shows the basic stages through which a Spitzer observation passes These processes are described in more detail in Chapter 5 An SSC supplied software tool called Spot section 5 2 1 is used to enter all the parameters e g integration time choice of modules etc needed to fully specify the observation This process is called filling out the AOT front end Spot also provides wall clock time estimates for the total duration of observations Spot does not allow observers to create impossible observations however observer supplied constraints can render an observation unschedulable see section 5 4 3 An observation that has been fully defined by supplying parameter values for an AOT is known as an Astronomical Observation Request AOR and is the basic scheduling unit for Spitzer After the Time Allocation process has been completed the AORs submitted for all of the approved Spitzer observations are loaded into a
2. Figure 6 31 Dither pattern dialog box in Spot The dialog box in Spot for the dither patterns is shown in Figure 6 31 which illustrates how the pattern and scale are selected 108 Spitzer Observer s Manual 6 2 3 5 3 Sub pixel dithering Sub pixel dithering combined with the drizzle technique Fruchter amp Hook 2002 PASP 114 144 to reconstruct the images can improve the sampling of the mosaics that are obtained from IRAC or any other observations Such strategies have been used for the WFPC2 and NICMOS instruments on the HST for some time for details see the HST Drizzle Handbook Dithering is also needed to calibrate intra pixel sensitivity variations and needed for programs requiring accurate photometry and astrometry Anderson amp King 2000 PASP 112 1360 To be effective however accurate pointing and low image distortion are required The offsetting accuracy of Spitzer is in the range 0 1 0 4 This combined with the image distortion in the IRAC arrays places a limit of about 1 4 pixel on the sub sampling that is likely to prove useful in practice For example the distortion of the IRAC camera is lt 1 see Figure 6 5 Thus for the largest dither patterns which typically offset up to 64 pixels from the starting point the offsets will be up to 0 6 pixels from the nominal values Thus only in the small scale patterns where the offsets are less than 16 pixels will the sub pixel sampling work
3. 40 Spitzer Observer s Manual Nominal Aperture or Pick Off Mirror Locations Viewed Down the Telescope IRS Slit Short High gt 5 PCRS Sun Shield RS IRAC 3 6 5 8 um Peak up Blue Red IRS Slit _ MIPS 24 um Ist Short Low MIPS 70um IRAC 4 5 8 0 um IRS Slit Long High E E PCRS IRS Slit Long Low lst order 2nd SSS Nominal Field Radius 16 arcminutes Note This is an approximate mechanical layout of the focal plane as viewed from above It is not a projection onto the sky Figure 4 5 Schematic view of Spitzer focal plane from above looking down the boresight The solar panel is on the IRS side of the spacecraft This figure shows the region of the focal surface where the pick off mirrors for each instrument are located This is in contrast to where the apertures project onto the sky See Figure 2 1 for comparison Chapter 4 Telescope and Focal Plane 41 Table 4 3 Spitzer focal plane layout approximate offsets from boresight to aperture centers Aperture Z YC IRAC 3 6 um 2 86 3 67 IRAC 3 6 um subarray 4 97 5 88 IRAC 4 5 um 2 72 3 05 IRAC 4 5 um subarray 4 80 0 92 Table 4 3 gives the measured offsets from the center of the field of view to the centers of the main Spitzer science apertures The values were determined post launch based on the results from the focal plane mapping survey
4. 5 2 _ SCIENCE USER TOOLS iiscsencivcsinessussecesutesscauscentbupaseedvenansseessvovercoepesesDopsaeevusgusetengsnbisreneeveetbepaners 45 DDL SSPOb Leste EE ee ake ater tities A Ra ae A we he Bae eet Ma ee 45 5 2 2 Performance Estimation Tool PET 46 5323 Ble ON Converter annu ine onen a N A RL BER es 46 5 2 4 Leopard Spitzer Archive Interface c ccccccccceccccsccssesesscssseeseeesseesessescessseesesessereneeseereas 47 5 3 MODIFICATION OF OBSERVING PROGRAMS cccsscssesesesessesescsessseseeesecsesesscsesseessesesesesseeseegs 47 5 4 SCHEDULING CONSIDERATIONS sarrioaren ro a niatan ea a 47 5 4 1 Scheduling Methodology u c cccccccccscccsscsscssessssesesecenssseescueeseeseessenseesseeseeesneeseessaserenees 48 5 4 2 Notification That an AOR Has Been Scheduled cccccccsceceseteeseteteeseeetetetneeseneesees 48 5 4 3 Requesting Scheduling Constraints for Science REASONS scce 49 3 37 SOLAR SYSTEM OBJECTS seresa ie e es eaa apa a EET Aa E e aa er E aE Nasa 52 5 5 1 Tracking Performance ccccccccccceccssescesseevesesecseeecsessessscesceeeseeseesseeesnesseesseeseneeseereateaes 52 5 5 2 Ephemeris Management ccccccccccccssesccseesseeseteeseessseeseseeseeseesseesseesenssaceesnesseeesasereetes 52 5 5 3 Target Specification and Scheduling Considerations c e 53 5 5 4 Science Instrument SSUCS ccccccccccccccccseesscesecsceseeessseeeseeesecseeseeesecssesessseeseeeseeesecesessseess 54 SNE 0129111 VIIE A A EE E E A A 54 5
5. Spitzer Space Telescope Observer s Manual Version 10 0 Warm Mission Release Date 27 January 2010 Issued by the Spitzer Science Center California Institute of Technology Mail Code 314 6 1200 E California Blvd Pasadena California 91125 USA http ssc spitzer caltech edu help spitzer caltech edu Spitzer Jed IL Center il Issue Dates Version 10 0 Release 27 January 2010 Version 9 0 Release 7 July 2008 Version 8 0 Release 15 Aug 2007 Version 7 1 Release 8 Dec 2006 Version 7 0 Release 1 Nov 2006 Version 6 0 Release 1 Nov 2005 Version 5 0 Release 1 Nov 2004 Version 4 6 Release 6 Aug 2004 Version 4 5 Release 28 Jun 2004 Version 4 0 Release 19 Dec 2003 Version 3 0 Release 8 Nov 2002 Version 2 2 Release 15 Apr 2002 Version 2 1 Release 1 Aug 2000 Version 2 0 Release 30 Jun 2000 Version 1 0 Release 28 Apr 2000 Acknowledgements The Spitzer Science Center would like to acknowledge the contributions of the following groups to this Observer s Manual The IRAC Chapter 6 was written by members of the IRAC Instrument Team at SAO and Instrument Support Team at SSC ill Version 10 0 Summary of Changes This is a summary of the major changes in version 10 0 based upon version 9 1 The main purpose of this release is to update information for the Warm Spitzer Mission that commences with Cycle 7 science observations Chapter 1 Document list version numbers were updated Chapter 2 Up
6. 6 1 3 2 2 Effects of overexposures 6 1 3 2 3 Short term residual images vest 6 1 3 2 4 Pixel to pixel sensitivity variations cceeseesesesesessseeeeeeeeeseneneneneeceeeeeeseeseeeeeeeens 76 6 1 3 2 5 Intra pixel sensitivity variations cece eseeeseseeeeteeeeeeseseseneseneeeeceeeeesnceeeeenetenenens 77 6 1 3 2 6 Pixel mask ee 613 ZK Dans saevesvectceseea ses tsa nenen cae castes duayd a e S E RE UARRA 01328 PAS i iea oE AENEA R EN AAEN E AN 6 1 3 2 9 Effects of cosmic rays ae 6 1 3 2 10 Thresholds for triggering image artifacts eeeerceceeseseeeseeeeteteeeeeeeeeneeaeans 80 6216322411 a Firstar ame erect ss s npin enean EOE AE nes aed Deane eens tetra 81 6 1 3 2 12 Stability 83 6 11 33 Hard warese ace recieves erect E eE OERE eed ahd ee A E A AT 84 6 13 31 Fowler Sampling i it 3e detsi secestectiedlicanAiniiecarcuel ieee au ania cniiien 85 ix 6 1 3 3 2 Subatray mode a neie weaning E N e E A A Geese Ta 6 1 3 3 3 Exposure time and frame time EILIA Calibration Lamps sene aenn corners tas aah a Nee a it ta 6 1 3 4 Print Walt esea eee E E A EA n da Sash Sass eased gos Bev eda ak ee 62 HOW TO USE IRAC rttr iee cub saxerens savaginn NE EEE EERS sponser iE 6 2 1 Performance of the INStrument ccccccccecccccceseeseeseseeseeseesesecseesseeseeeesecsesseesseesseeeeaeesees 6 2 24 Observing Time Estimi sis sos tessesis evestevsccsstegesestiuss sons sts isahdetpeestdve
7. AJ 126 1090 see also 2MASS All Sky data release web document http www ipac caltech edu 2mass releases allsky doc explsup html Chapter 7 Appendices 131 Table 7 2 Johnson system zero points Passband Effective wavelength Zero point um Jy U 0 36 1823 B 0 44 4130 V 0 55 3781 R 0 71 2941 I 0 97 2635 J 1 25 1603 H 1 60 1075 K 2 22 667 L 3 54 288 M 4 80 170 N 10 6 36 O 21 0 9 4 References Allen s Astrophysical Quantities Fourth edition 2001 Arthur N Cox ed Springer Verlag Campins Rieke amp Lebofsky 1985 AJ 90 896 Rieke Lebofsky amp Low 1985 AJ 90 900 Table 7 3 UKIRT system zero points Passband Effective wavelength Zeropoint um Jy V 0 5556 3540 I 0 9 2250 J 1 25 1600 H 1 65 1020 K 2 20 657 L 3 45 2 90 L 3 80 2 32 M 4 8 163 N 10 1 39 8 Q 20 0 10 4 References UKIRT web page http www jach hawaii edu UKIRT astronomy calib phot_cal conver html 7 2 Infrared Backgrounds Various astronomical sources emit radiation in the infrared part of the spectrum Cool stars M class have their peak emission just short of the near infrared However stars with dusty envelopes or shells and circumstellar disks can be quite 132 Spitzer Observer s Manual bright in the infrared Regions of star formation HII regions and planetary nebulae are strong infrared sources The relati
8. different amplitude above and below the source and decreases in a quadratic fashion with distance along the column from the trigger In addition strong cases of pulldown at 4 5 um can persist long enough that they appear as column pullup in subsequent frames The switching of polarity is due to a combination of the pulldown decay and the Fowler sampling of the data collection Based on examination of 12 sec HDR warm data it appears that the column Chapter 6 InfraRed Array Camera IRAC 81 pulldown and pullup see Figure 6 17 are triggered by pixels above a certain fluence These levels are likely to be appropriate for all frame times and should be sufficient for planning purposes Table 6 5 provides estimates of how bright a pixel has to be to trigger various levels of column pulldown Thresholds are in units of fluence MJy sr sec To find the threshold for a particular artifact for a given frametime divide the appropriate value by the frametime If the surface brightness of a pixel is above a threshold that pixel will produce a specified artifact To see if a point source may induce an artifact convert the estimated flux density to surface brightness assuming that the source is centered on an IRAC pixel by scaling the flux density in uJy by 0 12 Keep in mind that if a source is not centered on a pixel its flux will be distributed more uniformly and be less likely to induce an artifact The upper limit of the flux falling into the
9. in the CTA and the Warm Electronics Assembly WEA mounted in the spacecraft Harnesses connect the detectors and calibration subsystem in the CA to the WEA The WEA communicates with the spacecraft over three RS 422 serial lines that allow receiving commands from and sending acknowledgements and image data to the spacecraft Command amp Data Handling C amp DH computer The IRAC Cryogenic Assembly depicted in Figure 6 1 consists of the following major subassemblies the Pickoff Mirrors the Shutter the Optics Housings which hold the doublet lenses beamsplitters filters and cold stops the Focal Plane Assemblies FPAs that include the detector arrays and associated components the Transmission Calibrator with its Source and Integrating Spheres and the Housing Structure consisting of the Main Housing Assembly and the wedge shaped MIC Adapter Plate SHUTTER FOCAL PLANE ASSEMBLY DOUBLET LENS BEAMSPLITTER PICKGFF MIRROR OPTICAL HOUSING FILTERS CONNECTORS STRUCTUR MIC ADAPTER TRANSMISSION PLATE CALIBRATOR MIC MOUNTING FEET SOURCE SPHERES Figure 6 1 IRAC Cryogenic Assembly model with the top cover removed to show the inner components Chapter 6 InfraRed Array Camera IRAC 63 6 1 2 Optics 6 1 2 1 Optical Layout The IRAC optical layout is shown in Figure 6 2 and Figure 6 3 Light from the telescope is reflected into the IRAC structure by the pickoff mirrors for the two fields of view FOVs Each
10. slews less than 30 settle to within 0 2 rms within 10 sec Settling may take longer in some cases The AOTs make use of on board slew completion and stabilization indicators to proceed with the observation as soon after a slew as is possible Note that the time required for small slews dithers offsets settling etc within an AOR is considered part of the observation Time estimates provided through Spot account for these transparently and accurately The best available pointing system models are incorporated into Spot However note that Spot estimates may change if e g Spitzer s slew rate changes 3 3 2 Pointing Accuracy and Stability The blind pointing accuracy is the same as the on board attitude knowledge lt 0 5 lo radial rms with a stability of lt 0 1 1o radial rms over 200 sec In the incremental pointing mode the PCS performs controlled repositioning of the boresight with an offset accuracy usually no worse than 0 55 1o radial rms across angular distances of up to 30 and usually better than 0 4 3 3 3 Tracking Capabilities Spitzer does not have a true tracking capability for Solar System Objects SSOs However Spitzer simulates tracking by scanning in linear track segments at rates up to 1 s The linear track segments are linear in equatorial coordinate space they are commanded as a vector rate in J2000 coordinates passing through a specified RA and Dec at a specified time The SSO epheme
11. the realized signal to noise level will be higher using N 1 dithered observations than N observations with repeats In place repeats are specified in the AOT window under For Each Pointing Number of Frames dithers are specified lower in the AOT window under Dither Pattern Depending on the number of bright sources in your planned field and the placement of your observation in the schedule relative to other observations that may have bright sources there may be persistent images in the arrays during your observations The calibration data flats and darks are carefully planned to avoid 110 Spitzer Observer s Manual such artifacts If your observation is contaminated by persistent images the effect on the final data quality is reduced in a dithered observation by at least a factor of 1 N where N is the number of dithers Greater reduction occurs when robust averaging outlier rejection is used Taking into account possible residuals of the first frame effect and image artifacts due to bright sources we recommend that sensitive background limited maps be made using either as many dithers as possible or a fine map grid combined with the cycling dither pattern Shallow read noise limited observations or raster maps specially designed to identify phenomena with a certain time dependence such as asteroid motion may be best done with combinations of medium or large dither patterns and small map steps The small scale di
12. with suitable datasets have shown that the SSC pipeline results are comparable The SSC will not as a matter of course undertake such special processing self calibration as part of the automated pipeline The SSC generated data products will always use the dedicated calibration data 6 3 1 3 Skydarks Dark current and bias offsets are calibrated via the standard ground based technique of dark subtraction As part of routine operations the SSC observes a dark region of the sky skydark near the north ecliptic pole every seven days These data are reduced and combined in such a way as to reject stars and other astronomical objects with size scales smaller than the IRAC array The resulting image of the minimal uniform sky background contains both the bias and dark current When subtracted from the routine science data this eliminates both of these instrumental signatures Naturally this also subtracts a component of the true celestial background The SSC includes a COBE based model estimate of the true celestial background which is the same as that returned by Spot Note that our lack of an isolated measurement of the dark current and bias offset during shutterless operations limits the ability of IRAC to measure the true celestial background Our understanding of the history dependent bias variations section 6 1 3 2 11 is currently incomplete and will improve as our temporal sampling improves 6 3 1 4 Distortion and PSF Map The P
13. 2 Celestial coordinates 03 DATA eenaa A EEEa E ERN TE AOE EEEE RE Era E E RE RE EEE NEE 6 3 1 Anstrument Calibr tion srecni a a a a a a a n raai 6 3 1 1 Astronomical Flux Standards ccccccccccscesscsscsecsscesccsecsscssscscssscssceseecsseessenseesecescessessees 6 33 12 Sky Plats ni code e t ate elie en oR ee hae eee eens 6 3 1 3 Skydarks i 6 3 1 4 Distortion and PSF Mapu ecesseesssseseseeeeseseseseeseseeseseseeeesesecseseseseeeeseseeseseeeeaeseeeeseeeeneeaes 6 3 1 Emearnzatloi enana ehr ake sesh td dose as ie eave ose sa vais basand snc shoe Wk vA a 6 3 1 6 Routine Frequent Calibrations for Normal Observations p POSARE B1110 N EAO 11E RAE SEENE I ES EEN NEN E EA REAA 6 33 21 Raw Data oero a E N S a ee ae 6 3 2 2 Basic Calibrated Data or BCD 6 3 2 3 Calibration Files cccsecseesessssesseseeseeseeseseeseeees 6 3 2 4 Extended Pipeline Products Post BCD Pipeline is O 3 33 Data Processing oncunonn We en eh BH Gah Bea a A EN EE O3 3N OVEN IEW nar i nE EEEE EA EREE ONAE NAER E E ERN 6 3 3 2 Science DCE Processing 6 3 3 3 Data Reduction Software 6 3 3 Data ReductrOtiss Actvcccacasecssscesieegescdecsnchssicesh osvicessadessieseduvendessugesk aoei i 6 3 3 3 2 Data Analysis s aore neie dune acces d EE aE ie E a Eiaa EAE desde 7 APPENDICES aaae O A EESE EE aSa eE ERE SEE aat 129 FA INERARED FLUX UNITS ciiccssasaaiveiviiieialaevcinishevcaust reir tere GERAR E 130 Tea INFRARED BACKGROUND Sterse irinin atiri
14. AOR in the published schedule is not the only notification to observers that the AOR has been officially scheduled An email is sent to the observer notifying him her that his her observation has been scheduled Once an AOR has been scheduled only a significant anomaly e g a missed downlink opportunity or a safing event or a rapid turnaround ToO could cause the schedule to change Observers will generally not be notified if this happens Only under extreme circumstances can AORs be modified after they have been scheduled but this is highly discouraged In the case of AORs that are highly time constrained for science reasons if a specific sequence or dates have been provided to the observatory in advance the Observatory Planning amp Scheduling Team OPST will make every effort to schedule those AORs accordingly This information should be provided at the time of proposal submission and always requires a very strong scientific Chapter 5 Observing with Spitzer 49 justification A Spitzer observer may assume that any specific dates and times requested in an approved proposal will be accommodated unless otherwise notified by the SSC however see note in section 5 4 3 below regarding constraint limitations In the event of a serious conflict the SSC will contact the affected observer s as soon as possible to resolve the problem However even for heavily constrained AORs the only formal announcement of their place in the Spitzer schedul
15. IRAC SENSITIVITY CALCULATIONS ccsssssseseeseeesseeseeseeseeenes 89 TABLE 6 7 BACKGROUND BRIGHTNESS IN IRAC WAVEBANDS ccsscesseessesseesceecesecseeeseeeseeeseenseenes 89 TABLE 6 8 FOWLER NUMBERS FOR IRAC FRAMES cc00 0000 90 TABLE 6 9 IRAC HIGH DYNAMIC RANGE HDR FRAMESETS cescssseseeseeseeseeeeceeeeseceeeeeeeseeaeeaees 91 TABLE 6 10 IRAC POINT SOURCE SENSITIVITY LOW BACKGROUND 10 JY 92 TABLE 6 11 IRAC POINT SOURCE SENSITIVITY MEDIUM BACKGROUND 10 LJY eceeeeeereeteeeees 92 TABLE 6 12 IRAC POINT SOURCE SENSITIVITY HIGH BACKGROUND 10 PJY eeeeceseeteteeteenees 93 TABLE 6 13 MAXIMUM UNSATURATED POINT SOURCE IN MJY AS A FUNCTION OF IRAC FRAME TIME cies eeitis Sev hele ad A eee Bad ee ee ese 96 TABLE 6 14 CHARACTERISTICS OF THE DITHER PATTERNS 104 TABLE 6 15 PRF INTENSITIES uenon eae he cadences hn AWA ail hese edd 113 TABLE 6 16 IRAC BASIC CALIBRATED DATA BCD PIPELINE PRODUCTS csscesseseeseereeteeteestaes 121 TABLE 7 1 2MASS SYSTEM ZERO POINTS csccsssesseesseesseesecesescsseceseeeeeeesseeesecesecsssecesecssesessennsesees 130 TABLE 7 2 JOHNSON SYSTEM ZERO POINTS c ccscssssesseesscsseeeseescesecseeeeeesaecseeesecesecsesecesecsseeesseeeesees 131 TABLE 7 3 UKIRT SYSTEM ZERO POINTS ccccessssssesseesceescesseseceseeeseeeseeaecseeesecesecessseceaeesseceseeeseeaees 131 TABLE 8 1 ACRONYMS USED IN THIS MANUAL ccssscesssceessecessecesscssecesseccsseceseseessece
16. Scales and pattern selection in Spot Each of the IRAC dither patterns is available in three sizes large default medium and small For most of the patterns the scaling of the large medium and small patterns is approximately in the ratio 4 2 1 Exceptions are the small cycling pattern which is about 1 5 of the size of the large cycling pattern and has a lower weighted inner region to reduce the numbers of small separation dithers and the 4 point subarray pattern for which the scaling is 4 3 1 5 For all the patterns the sub pixel dithering is maintained independent of scale Chapter 6 InfraRed Array Camera IRAC Large cycling pattern 100 rrr dither distribution 80 dither distribution 20 50 100 150 200 dither distribution 0 0 50 100 150 0 QO 2 4 6 8 1012 107 separation distribution 3000 2000 1000 0 riiitiiiil Q 100 200 300 separation distribution 5000 5 4000 3000 2000 1000 poparation distribution 6000 E 4000 2000 F 0 0 5 10 15 20 25 Figure 6 30 Characteristics of the cycling dither patterns in pixels i Full Array 5 Positions Gaussian 9 Positions Random 12 Positions Reuleaux 16 Positions Spiral 36 Positions Reuleaux Cycling Number of Positions 3 Starting point in dither table 1 Subarray 4 Positions Gaussian 9 Positions Reuleaux Scale Factor Large Medium Small
17. The frames are commanded by specifying the number of Fowler samples for the pedestal NF and the number of wait ticks in between the pedestal and signal frames Nw then the frame time is T 2N N t where T is the readout time 0 2 sec for full array 0 01 sec for sub array mode The exact number of Fowler samples and wait ticks for each frame time has been optimized during the initial warm checkout The frame times have remained constant and consistent with their labeling in Spot The number of Fowler samples has been decreased for the 0 4 and 2 0 second subarray frametimes to maximize the signal to noise for photon noise limited observations The number of Fowler samples have been increased at 3 6 um for the 30 and 100 second frametimes to improve signal to noise for low background observations For very bright sources that saturate the detector during the pedestal reads the apparent brightness in those pixels will appear to be very small as the pedestal and signal reads difference to near zero In those cases the most saturated pixels will 86 Spitzer Observer s Manual appear as holes in bright sources The BCD pipeline does not reliably flag these hard saturations but the cBCD pipeline does attempt to identify them _ _ ___ n On gt 4 3 Tex 2 1 4 3 2 Voltage 1 Time Frame Time Figure 6 21 Fowler sampling times for one pixel Fowler N 4 The P n 1 2 3 4 show the Pedestal reado
18. This annulus rotates with the Sun over the period of a year the edges of the OPZ move along the ecliptic at about 1 day as Spitzer orbits the Sun For an object near the ecliptic plane the length of the visibility period is 40 days twice a year modulo periods when undesired bright moving objects are also present and bright object avoidance has been selected see section 3 2 6 The visibility periods increase to 60 days twice yr at an ecliptic latitude of 45 100 days twice yr at latitudes 60 then becoming a single long window 250 days long for latitudes near 60 70 finally reaching constant viewing near the ecliptic poles About a third of the sky is visible to Spitzer at any time Figure 3 8 illustrates how the total number of days of visibility varies over the sky in three coordinate systems Chapter 3 Observatory Description and Constraints 23 Constant Viewing Zone Contiguous viewing for ee X 4 telescope boresight 1 telescope 1 boresight A l F R Anti Sun Avoidance Zone 25 Sun Avoidance Zone I 40 Operational Pointing Zone 35 Figure 3 5 The main geometric observing constraints form an area called the Operational Pointing Zone OPZ J2000 Locations of X on 24 Nov 2003 75 JON pr S E KA WLLL i CT ee E va PFPE T Sar II AAAA 90S j OPZ
19. and a discussion of the data reduction pipeline and the characteristics of the data More extensive discussion of the data is presented in the IRAC Data Handbook which is available on the SSC website Chapter 1 Philosophy and Scope 3 1 3 Document Update Plans This manual is maintained by the SSC and is intended to support the seventh Call for Proposals CP Proposers should note that we have updated this document with the latest information known as of January 2010 in a very few cases references are made here to information that is posted on the SSC website In addition to maintaining the Warm SOM the SSC maintains instrument web pages at http ssc spitzer caltech edu No significant updates are anticipated between the release of this document and the CP 7 deadline 1 4 Other Relevant Documents If more recent versions of any of these documents are available on the SSC website please use those versions Spitzer Space Telescope Observer s Manual Warm Mission this document version 10 0 Spitzer Space Telescope Cycle 7 Call for Proposals version 1 0 dated 22 January 2010 Spitzer Space Telescope Warm Mission Observing Rules version 10 0 dated 22 January 2010 Spot User s Guide to go with Spot version 18 updated 18 June 2008 Leopard User s Guide to go with Leopard version 18 updated 18 June 2008 Spitzer Space Telescope Observation Planning Cookbook version 8 0 dated 15 August 2008 Many other helpful doc
20. and flux conversion values are stable in time to within our ability to measure them The gain is constant to within the precision of our superflat 0 25 and 0 12 at 3 6 and 4 5 um respectively The photometry of IRAC is stable on both short hours and long months timescales to our ability to measure fluctuations Figure 6 19 displays the photometry of an IRAC primary calibrator after the final setpoint assertion To within the measurement error there is no discernible variation in IRAC photometry The absolute offset in the IRAC images also appears stable Figure 6 20 displays the variation in median value of the routine 12 second skydark measurements taken since the final setpoint assertion The variation in sky brightness at the North Ecliptic Pole due to the variations in Zodical light has been removed The remaining variations are extremely small a few DN as compared to the photon noise of the lowest backgrounds 84 Spitzer Observer s Manual 3 6 um 4 5 um 0 790 POT TTT TTT TTY mye 0 785 J 0 475 0 780 4 0 470 X lt 0 775 5 x X x lt 5 x X i 0 770 0 465 E 0 765 Eo 0 460 0 760 J 0 755 kroli hiia aaia J rottie nearest sass ned lta pin alsda e aeaea in 0 10 20 30 40 50 60 0 10 20 30 40 50 60 Time days Time days Figure 6 19 Time series of NPM1 67 0536 one of the IRAC primary calibrators at 3 6 um left and 4 5 um right The asterisks are the s
21. are requests that the SSC will work to accommodate In the event of a serious conflict the SSC will contact the affected observer s as soon as possible to resolve the problem However even for heavily constrained AORs the only formal announcement of their place in the Spitzer schedule will be their appearance in the latest online weekly schedule as described above Any scheduling constraint imposed on an AOR will increase the difficulty of scheduling the AOR Highly constrained observations are at significant risk of non execution in Cycle 7 The following types of scheduling constraints are supported for both inertial and moving targets Timing Constraints Timing constraints consist of defining a window or series of windows for the start time of an AOR Absolute time observations that will be executed at a specific time or no more than 3 seconds later can only be supported for moving targets To specify an absolute time observation in a moving target AOR set the open and close times for the timing window to be identical Spitzer s scheduling architecture generally operates on relative time so for inertial targets the inertial target AORs will simply run in order Timing constraints for inertial target AORs should be macroscopic days weeks months not microscopic seconds minutes hours Relational Constraints Relational constraints are ordering or grouping constraints that are applied to a group of AORs There are four basic typ
22. arrays operated at a temperature of 15 K Warm will refer to data and operations after cryogen depletion when the telescope is at 27 5 K and the IRAC arrays are operated at 28 7 K 6 1 Instrument Description 6 1 1 Overview 6 1 1 1 Instrument Description IRAC is a four channel camera that provided simultaneous 5 2 x 5 2 images at 3 6 4 5 5 8 and 8 um After the depletion of cryogen the two shortest wavelength channels at 3 6 and 4 5 um remain operational The two operational channels image two adjacent fields of view Both detector arrays are 256 x 256 pixels in size with a pixel size of 1 2 x 1 2 The two short wavelength channels use InSb detectors The IRAC instrument was originally designed to address the four major scientific objectives defining the Spitzer mission These are 1 to study the early universe 2 to search for and study brown dwarfs and superplanets 3 to study ultraluminous galaxies and active galactic nuclei and 4 to discover and study protoplanetary and planetary debris disks The utility of IRAC is in no way limited to these objectives which we only mention to explain the scientific drivers for the instrument design IRAC remains a powerful survey instrument because of its high sensitivity large field of view and mapping capabilities 62 Spitzer Observer s Manual 6 1 1 2 Technical Overview IRAC consists of the Cryogenic Assembly CA installed in the Multiple Instrument Chamber MIC
23. as a pointing reference the pointing drift derived from the gyros is lt 3 mas sec over 8 hours The drift rate when using the IRU only mode should generally be better than 1 mas sec over 200 sec All telescope pointing is defined and calibrated relative to redundant PCRSs located in the focal plane During the course of the mission the PCRS is periodically about every 12 hours used to calibrate the telescope to star tracker boresight alignment that may drift due to thermo mechanical effects Each PCRS detector is a Si PIN photodiode array divided into two 4x4 subarrays for redundancy Each pixel is 250 um square with a plate scale of 10 per pixel The PCRS calibration measures the star position with an accuracy of 0 1 10 per axis and is sensitive down to 10th visual magnitude at a wavelength of 550 nm Spitzer also has WASSs which measure the Sun s position with respect to the spacecraft These sensors were used during initial attitude acquisition after launch as well as for Sun avoidance fault protection and safe mode during the mission Each WASS provides a field of view of 27 sr with an accuracy of 0 1 at null They are placed at the top and the bottom of the solar panels to maximize the coverage with their boresights aligned to the spacecraft Z axis Four reaction wheels provide the primary control actuation for all modes of operation They are mounted in a pyramid orientation about the X axis each canted at 30 toward
24. be executed in any order Follow on This constraint forces a follow on AOR to be scheduled within a given time after a precursor AOR It can be thought of as a statement that Follow On AOR must be scheduled within Time Window after the end of Precursor AOR The follow on constraint can be used to prevent early execution of an observation when the success or content of the follow on is dependent upon the successful execution of a precursor observation This constraint could also be used for periodic observation of a target where the interval between observations is relatively short hours to a small number of days One AOR may serve as the precursor to more than one follow on but a follow on may have only one precursor e g one follow on constraint can tie together only two AORs Shadow The shadow constraint is a special case of the follow on constraint and is used to obtain background measurements for moving targets The primary AOR is executed as specified The shadow AOR will be executed to repeat the track of the primary observations The selected AOR parameters must be identical in the two AORs The shadow may be executed before or after the primary AOR Note that the shadow does not re observe the target at a later date but rather the background of the primary observation As with all constraints shadow observations must be strongly scientifically justified in the observing proposal Timing and Relational Constraints can be combined
25. by both fields of view is shown in the right hand panel Figure 6 28 Schematic diagram of sky covered by the two IRAC fields of view during a 3x5 map On the left is the sky covered by the 3 6 um field of view in the middle is the sky covered by the 4 5 um field of view and on the right is the sky covered by both fields of view In the left and center panels lighter shades indicate a higher depth of coverage while darker shades indicate less coverage The right hand panel is white when both fields of view cover the sky Selecting both fields of view changes the way the pointing is commanded If you have selected a celestial coordinates map the AOT performs your map grid once with the 4 5 um field of view centered on each map grid position then again with the 3 6 um field of view centered on each map grid position This may lead to significant redundancy and cost in observing time for large maps You may prefer to choose a single field of view as your primary field of view and make your celestial map grid large enough that both fields of view will cover your desired area regardless of the scheduled date of observation Chapter 6 InfraRed Array Camera IRAC 103 If you have selected array coordinates and both fields of view then the map is centered on the point directly between the two IRAC fields of view If your map is row by 1 column the map center will not be imaged by either of the IRAC arrays Therefore if
26. cada a a Adee a a aiae 3 3 4 Pointing Reconstruction sarni ora E aa e OE A E a aa 4 TELESCOPE AND FOCAL PLANE esseesosesoesscesscescocesesoesccesccesccesocesosesoesscesesocesocesceescesseesoe 33 Al OPTICAL DESIGN ssescescsscsstenstacias eocecvencaveosnssen sosunestaedcaabencdsetas saducuas sveuueusacasadecoabeatensstesbayeessanseade 34 4 2 OPTICAL AND THERMAL PERFORMANCE cssssssesseesseessessecseseceseceseeeesseesseeeseceseenssseseaeenseees 35 ADA Surface ACCUL ACY ssccsssasscecotecscucgieeshsosdexesesngysasedesvandesndveengunstadeveagaseencedasechda coueusdastastedayeees 35 42 2 Wavefront EFFO S 01 cosh beth cok ts te sash tis chain Shag haeds cast ep ete Hh e eai Heed E ed teetees 36 4233 ERFOUGKPUE escterstinvs 2 gees hbeleeseechial ona nib ddan nlnnadan EAEE 36 4 2 4 StrayTight Rejection s sts tesesevinh sea dasseniarln sda eh e eles a E e 37 4 2 5 Telescope Temperature Thermal Background o r 39 4 2 0 Eelescope FOCUS sites tact ev nieh weetesieseeetent teal dh OG Beebe e Ae AM ES DE E E 39 4 3 FOCAL PEANE LAYOUT wvcsscescesesusews a E sees tes inveeeunannes tuusbupsgecesanetva OEE raae ESKESEN 39 5 OBSERVING WITH SPITZER oisssisosiseesscovsssscesssenstassgcssivedeveavsedossdessoseueasesssd sedessouestassvosesosaes 43 5 1 ASTRONOMICAL OBSERVATION REQUEST AOR ssssssssssssssssssessessssseseresreseesresessessesreseesesee 44 5 2 SCIENCE USER TOOLS onela na E EO ahaha de O ERE E aiT 45 I E S 0 E EE E E S E
27. current detector characteristics These include a map of the pixel to pixel response variation flat field and an estimate of the dark bias consisting of a sky dark and an estimate of the first frame effect These calibration files are supplied to the observer Observers are also able to request from the Archive the files that were used to generate the sky darks sky flats and absolute calibration The photometric calibrators are not included with each science observation but they are available via a separate request to the Archive 6 3 2 4 Extended Pipeline Products Post BCD Pipeline Pipeline processing of IRAC data by the SSC also includes more advanced processing of many individual IRAC frames together to form more reduced data products Known by the generic title of post BCD processing this extended pipeline refines the telescope pointing attempts to correct for residual offset variations and produces mosaicked images We do not attempt to improve relative to the BCD the point source or extended emission flux calibration by automatically comparing to a reference source catalog The mosaic only includes data from a single AOR so that observers who break their map into multiple AORs for example those whose maps cannot be completed within the 24 hour time limit of an IRAC AOR or those who are making multiple epoch observations need to recombine them to obtain their ultimate images 122 Spitzer Observer s Manual All I
28. database known as the Science Operations Database SODB and are stored there in the form of a set of specific parameters and values These parameters are used by the software AOR IER Resource Estimator AIRE to expand the set of parameters into a set of instrument and spacecraft commands which execute the observation on board Spitzer as well as to provide extremely high fidelity estimates of execution time for the scheduling process Observers also access AIRE through Spot for their time estimates The Spitzer planning and scheduling process produces observing schedules based on these resource estimates After the content of the schedules has been approved and finalized AIRE takes the AOR parameters and produces command product files which are 12 Spitzer Observer s Manual then processed by the JPL Mission Sequence Team and finally uplinked and executed by Spitzer It is important to realize that the process of creating commands to carry out an observation based on the AOR parameters is done by software not by support astronomers at the SSC After an observation has been scheduled and carried out the resulting data are pipeline processed and are placed in the Spitzer science archive and made available to the observer In the warm mission the SSC will no longer perform a quality assessment of the data prior to it being made available to the observer 2 5 SSC Science User Support Services The Spitzer Science Center acts as the interface be
29. date there is a forbidden zone in the anti Sun direction as well as a second more extended looking forbidden region on the side of the spacecraft towards the Sun the Sun ward forbidden region is in the center of the plot on 1 Sep and on the edges for the 1 Mar plot Note that even targets at high declinations may fall in the forbidden zones part of the time although objects at extreme declinations are generally visible for most of the year Although these plots are accurate observers should always use Spot to determine visibility of a target on a particular day see Chapter 5 unobservable unobservable region unobservable unobservable region Dec in degrees OPZ 01 Jun 04 unobservable region unobservable unobservable Dec in degrees unobservable 5 10 15 20 0 5 10 15 20 RA in hours RA in hours Figure 3 9 Example of time evolution of visibility zones over a year see text Chapter 3 Observatory Description and Constraints 27 3 2 5 Orientation of Focal Plane and Slits against the Sky Spitzer has very limited ability to rotate the focal plane of the telescope At any given time the center of the sunshade X Z plane see Figure 3 3 is always kept within 2 of the Sun This limited freedom in the roll angle is used to maintain a consistent orientation for long observations AORs and is not selectable by the observer For each target the orientation of the focal plane on the sky is a function of
30. drifts away from Earth at the rate of 0 12 AU year This substantially helped prolong the coolant lifetime by supporting an operating regime in which most of the cryogen was used to take up the power dissipated by the detector arrays rather than lost to parasitic heat loads In addition this orbit has less constrained visibility compared to what would have been the case in a near Earth orbit allowing all parts of the sky to be visible for at least two extended periods each year and some zones to be visible continuously 3 2 2 Pointing Constraints Spitzer s view of the sky is limited by two hard pointing constraints illustrated in Figure 3 5 and in a different fashion in Figure 3 6 The angle between the boresight and the direction of the Sun may never be less than 82 5 NB this was updated in early 2004 The angle between the boresight and the direction of the Sun may never exceed 120 The area defined by these hard constraints is called the Operational Pointing Zone OPZ In addition some bright objects such as the Earth are normally avoided because they would degrade the quality of the observation due to direct exposure or stray light but this is not a strict constraint see section 3 2 6 and Appendix 7 3 Note that the definition of the OPZ precludes observing Mercury or Venus with Spitzer A second order effect on the OPZ is provided by the limited roll angle of Spitzer around the Y axis see Figure 3 3 which is ju
31. faint source near a much brighter one The choice of observing strategy and assessment of the technical feasibility for such a search requires great care First it is important to determine the brightness of the point response function PRF of the main source at the distance where the putative companion is being sought Table 6 15 gives the intensity of the PRF normalized to the peak for a source centered in a pixel at several distances from the center Figure 6 32 shows the relative surface brightness of a point source in each channel both along and off a diffraction spike A well designed experiment will be able to cancel the PRF down to a certain level For example if the PRF is down by a factor of one thousand from the peak at the distance being searched it is plausible to search for features at a level of 1x10 if some type of PRF removal is done And furthermore having knowledge of the potential shape of the companion e g a point source or disk will obviously discriminate it from the remaining PRF of the bright source Remember that the Poisson noise of the bright source at the location of the faint target will always limit the accuracy of the PRF removal Here is a recommended strategy for this type of observation Center the bright source on the array and observe it with a dither pattern with medium scale factor Use a pattern with good subpixel sampling to enable future super resolution techniques for example consider
32. http ssc spitzer caltech edu website SSO Solar System Objects Observations ST Star Tracker ST IRU Star Tracker Inertial Reference Unit TAC Time Allocation Committee ToO Target of Opportunity TPG Time Pattern Generator TP TPM Total Power Mode QE Quantum Efficiency WASS Wide Angle Sun Sensor WEA Warm Electronics Assembly 9 Index 2MASS 31 108 120 125 130 anneal 44 aperture 8 27 34 35 36 39 41 56 88 101 archive 7 8 13 47 116 119 121 array coordinates 100 103 110 112 114 115 Astronomical Observation Request AOR 11 20 30 44 48 49 50 52 56 57 82 97 101 108 119 123 Astronomical Observation Template AOT 9 44 90 97 108 Basic Calibrated Data BCD 13 69 119 120 121 122 best observing practices IRAC 108 Solar System objects 54 boresight 21 28 29 30 39 bright object 21 22 27 28 56 108 111 132 calibration 9 13 17 28 29 44 49 53 62 72 86 101 104 108 116 121 122 celestial coordinates 100 113 115 cirrus 132 confusion 7 37 90 93 113 constraints 11 16 21 27 28 30 44 45 47 49 50 54 112 114 chain 50 follow on 51 group within 51 sequence 50 shadow 51 53 timing 27 50 54 56 coordinate system 17 22 28 39 cosmic rays 79 114 121 124 cryogens 6 8 16 21 34 35 cryostat 16 37 dark 37 77 78 88 90 118 121 123 124 Data Collection Event DCE 121 123 Data Handbook 2 3 13 64 7
33. in Horizons 3 Request that SSC ask the JPL Solar System Dynamics team to provide observational support to improve the object s ephemeris Note that this capability is limited Spot now queries the Horizons database at JPL for ephemerides of solar system moving targets if they are not already at the SSC If this is the case Spot will usually take a few seconds longer to return visibility information because it has to query Horizons for the information deposit the ephemeris file at the SSC and only then calculate the visibility However once this is done for a target the ephemeris file is in place at the SSC and it can be immediately accessed for any subsequent visibility calculation request 5 5 6 2 Late Ephemeris Updates Late ephemeris updates are most useful if your object has a high probability of behaving unpredictably during the 5 weeks prior to the observation as might be the case for an active comet near perihelion Another good justification for a late ephemeris update is a target that just prior to observation by Spitzer has relatively few observations and a correspondingly large positional error However if your target is known to have poor positional accuracy at the time you submit your proposal to a Spitzer GO call a late ephemeris update may not be necessary In this case there is likely to be several months of time in which to improve the ephemeris but you should be proactive about acquiring and transmitting the additi
34. nE RE Saor AVE E SaaS ie 131 7 3 SOLAR SYSTEM OBJECTS INCLUDED IN BRIGHT OBJECT AVOIDANCE s ssssesisserereererese 132 ACRONYMS GLOSSARY stinore sisses eaten osons rebier es ioe Toress assii esiosas 133 INDEX NE A EE AE EEE A E E EAA 137 vi List of Tables TABLE 2 1 SUMMARY OF SPITZER CHARACTERISTICS ccccssscesssessssecssscesseessccessecesseesseecessecesseceeceseees 8 TABLE 4 1 TELESCOPE CONFIGURATION seeen rena aae E TEPEE ba veisl ain Gane eee nia lnaen oe 34 TABLE 4 2 TELESCOPE THROUGHPUT snesena eana E EEA Ea E eei sagen einen eons 37 TABLE 4 3 SPITZER FOCAL PLANE LAYOUT APPROXIMATE OFFSETS FROM BORESIGHT TO APERTURE CENTERS AE E E E E sa eae Wes E E EAT 41 TABLE 44 FIELD OF VIEW INDICE Sua reae aa aa tS E EE a ATO AEE E eN eS 41 TABL 5 1 CLASSIFICATION OF TOOS neira re a E EAEE EE e EE S EA ET Steen 57 TABLE 6 1 IRAC IMAGE QUALITY PROPERTIES ccssccssssesssccessecessecsscessecesecesseccsssecessccssseseeessseessseeess 65 TABLE 6 2 SOURCE LOCATION ON THE ARRAYS ccccsssesseesseesseesecscecsessecaceeseseseeeseenseenseseaeesceseseseeseenaes 68 TABLE 6 3 IRAC CHANNEL CHARACTERISTICS ccsccssssesssscessecessecescesecesecesseccsssecssseceaecseecesseeeseeeees 71 TABLE 6 4 IRAC DETECTOR CHARACTERISTICS cscccsssccsssscessccessecsseesecessecessecessecessecesseccsesesseeeseeeses 73 TABLE 6 5 FLUENCE PER SECOND IN MJY SEC SR OF A PIXEL NEEDED TO TRIGGER ARTIFACTS 81 TABLE 6 6 USEFUL QUANTITIES FOR
35. obscuration including spiders mirror reflectivity as a function of wavelength and losses due to contamination Chapter 4 Telescope and Focal Plane 37 Table 4 2 Telescope Throughput Wavelength End of Life um Throughput 0 55 gt 0 18 3 5 6 5 gt 0 70 4 2 4 Stray Light Rejection The cryostat telescope multiple instrument chamber and science instruments are designed and baffled such that at all wavelengths celestial stray radiation and internal stray radiation Do not except for lines of sight near bright sources increase by more than 10 the photon noise of the natural background in the direction of the line of sight of the telescope This requirement implies that the combination of celestial stray radiation and internal stray radiation must be lt 21 of the natural background at the instrument detector arrays Display no gradients or glints in the celestial stray light that will increase confusion noise over natural levels or produce false sources Do not significantly decrease the contrast of the first dark ring of the diffraction limited point spread function The conformance of the Spitzer design to its stray light requirements was verified by analysis using the APART stray light analysis program and an analytical test source designed to approximate the brightest celestial source expected in each of Spitzer s wavelength bands The actual scattered light performance of the Spitzer telesco
36. of the campaign The SSC continues to use defined instrument campaigns in the warm mission even though the campaigns are always IRAC Defined campaigns allow us to process archive and release to the commuity blocks of calibrations and science observations in a timely manner 14 Spitzer Observer s Manual The SSC validates the pipeline processing for each observing mode prior to the release of data In the warm mission basic quality assessment is no longer performed on Spitzer data before they are delivered to the observer Observers should contact the Helpdesk help spitzer caltech edu if they notice unusual features in Spitzer data The SSC will investigate and if needed update the quality assessment information in the database for future archival use In order to obtain data investigators should use the Leopard interface which is not only very similar to Spot see above but also downloaded and installed as part of the Spitzer Pride the same package as Spot The Leopard User s Guide is also available on the SSC website Certain Popular Products i e Legacy Program data are available through the Leopard interface and through the SSC website After obtaining data investigators may be interested in the IRAC Data Handbook available on the SSC website off the IRAC instrument webpage 3 Observatory Description and Constraints 16 Spitzer Observer s Manual This section addresses the Spitzer Space Telescope design and t
37. on planning editing and submitting Spitzer observations Spitzer investigators using data from the cryogenic mission science instruments i e archival investigators should use Version 8 0 of the SOM as a reference to the cryogenic science instruments performance and data acquisition calibration and processing Many documents including the SOM are available on the public Spitzer Science Center SSC website at http ssc spitzer caltech edu We have updated this document with the latest information known as of January 2010 For readers interested in data analysis considerable additional documentation is available on the SSC website including data handbooks for each of the instruments data analysis tools documentation and data analysis cookbooks 1 2 Document Organization The Warm SOM is divided into four major parts the Introduction Chapters 1 and 2 the observatory chapters Chapters 3 to 5 and the instrument specific chapter Chapter 6 The observatory section includes a technical description of Spitzer its operational capabilities and constraints and information about how Spitzer observations are planned and specified The IRAC science instrument has its own chapter prepared by the instrument team and SSC IRAC instrument support team It includes a technical description of the instrument an overview of the instrument s performance and capabilities a description of its observing modes advice on how to best use them
38. scale structures by making a raster map with spacing between rows and columns that is 4 array or less together with a superposed small scale cycling dither pattern Scattered light from bright sources near the edge of the field of view sometimes cannot be avoided but it can be prevented from contaminating multiple frames by using a dither pattern larger than the characteristic size of the regions that produce stray light Scattered light is well rejected by the post BCD pipeline if dithering and mapping offsets are large enough for example with the medium and large dither patterns but not small to ensure that stars in subsequent images are moved out of the scattering zones and the redundancy is high enough to allow for effective outlier rejection five dithers is sufficient We discourage using in place repeats successive frames taken at the same position especially in observations that have less than ten different dither positions both for the reasons mentioned above and also because the residual first frame effect will lead to a bias pattern difference between the repeats and the first frames of a repeat set Taking in place repeats is recommended only in the case of time series measurements with stringent requirements for stability such as observations of planet transits In general the better handling of pixel to pixel variations using dithers will more than compensate for the reduced amount of integration time that is
39. submission deadline Prospective investigators are encouraged to download Spot and to start planning their Spitzer observing programs well before the proposal submission deadline Spot is currently supported on multiple Windows Linux Sun Solaris and Macintosh platforms For details see the Spot User s Guide and Release Notes available at the SSC website with the downloadable software While running Spot a network connection to the Spitzer Science Center is required to obtain observing time estimates visibility orientation or background estimate information and to submit and update proposals a network connection is not necessary for selecting observation parameters 5 2 2 Performance Estimation Tool PET The most important web based SSC tool is the Performance Estimation Tool PET The PET is actually a set of Javascript tools to aid in designing Spitzer observations There are three PET components The Sensitivity PET the Extragalactic PET and the Stellar PET Users of the PET should pay close attention to the various notes and warnings on both the main PET pages and the help pages The Sensitivity PET SENS PET is an imaging sensitivity estimator It takes as input the IRAC imaging instrument configurations and a background level It produces as output the instrument sensitivities both for point source and extended objects and the total exposure depth per pixel The Extragalactic PET EX PET makes predictions for imagi
40. that does not saturate the source should be used to maximize the signal compared to the read noise The field should be inspected to make sure that stray light will not contaminate the observation and that artifacts from bright sources will not overlap the target If the total duration of the observation is greater than 24 hours multiple staring AORs should be supplied with close follow on constraints A request to transform the multiple observations 114 Spitzer Observer s Manual into a single instrument engineering request IER should be added to the AOR comments and the technical section of your proposal These observations necessarily produce high data volumes In these cases the SSC will recommend that data collection in only the field of view imaging the target is selected 6 2 4 2 Shallow Survey Example The goal of a shallow survey is to cover sky rapidly while maintaining some redundancy in order to reject cosmic rays and reduce effects of pixel to pixel gain variations For this survey we request coverage of our target region with both IRAC channels The survey will be conducted with a rectangular grid with the grid steps aligned with the focal plane array in order to make coverage uniform We will step by about 95 of the array width for each grid step specifically we will use a step size of 292 8 244 pixels At each map grid point we will observe 4 dither positions using the cycling dither table Because we used only a sm
41. the IRAC flight InSb arrays However there is a significant offset or bias which can be positive or negative in a dark frame and which therefore must be subtracted from the observations For reference Figure 6 15 shows dark frames obtained during the warm calibration using 100 seconds frames In general the dark offsets have a larger magnitude and more structure than the cryogenic offsets but are as reliably subtracted by the IRAC pipeline Especially for shorter frames the dark images are mostly due to electronic bias differences rather than true dark current therefore the number of electrons in a dark image does not scale linearly with exposure time Note that without the shutter isolated dark bias data as shown in Figure 6 15 cannot be taken in flight In addition no lab dark data exist for 30 K operations Instead skydarks are subtracted The skydark subtraction is done using calibration observations taken lt 1 week from a given science observation As no lab darks exist at 30 K a dedicated calibration program is being used to develop an in orbit calibration of the bias and first frame effect see 6 1 3 2 11 Observers are reminded that absolute sky brightness measurements are not feasible with IRAC due to the shutterless calibration and lack of lab bias measurements 6 1 3 2 8 Flats Flats are obtained from observations of the sky during normal operations Observations through the entire telescope and instrument are necessa
42. the ground no science data can be collected _ Dust Cover ejectable ae Secondary Mirror amp Focus Mechanism Barrel Baffle Solar Panel Shield Outer Vapor Cooled Shield Outer Shell a Primary Mirror _ Multiple Instrument Chambe Pa Aperture Door amp Photon Shutte Inner amp Middle vapor Cooled Shields A Helium Tank Telescope Focal Surface _ Yacuum Shell Gamma lumina ad Support Truss i a a Spacecraft Shield 5 7 Figure 3 2 Cryogenic Telescope Assembly Chapter 3 Observatory Description and Constraints 19 sfC to CTA interface plane at station lt 200 cm Figure 3 3 Observatory Coordinate System If a downlink opportunity is missed e g due to a failure at the ground station Spitzer preserves the collected science data continues collecting new science data as scheduled and attempts to downlink it at the next scheduled opportunity Once a missed downlink opportunity has occurred the spacecraft sequence is modified to provide additional downlink time within the next few opportunities if necessary No data are lost or overwritten unless the risk of overfilling the entire mass memory 16 Gbits exists See also section 3 1 1 below 3 1 1 Data Storage The Spitzer spacecraft includes the command and data handling subsystem C amp DH which shares the flight computer a RAD 6000 with the pointing control subsystem PCS The C amp DH
43. the motion is sufficient between the observations to image the background The Spot visualization tools and instrument sensitivity information can be used to determine whether a particular Solar System observation would benefit from a shadow observation As with all observing constraints the need for a shadow constraint must be scientifically justified Execution of the shadow observation after observation of the science target is the default although there may be situations where this is not the optimal execution order However shadows specified to execute before the primary observation are much more difficult to schedule than shadows executed 54 Spitzer Observer s Manual after the primary and although allowed require even more compelling scientific justification in the proposal than shadows executed after the primary 5 5 3 2 Timing Constraints SSO observations can be scheduled with timing constraints as long as these constraints are scientifically justified Observers are encouraged to specify observing windows that are as generous as possible within the scientific constraints to both enhance the efficiency of our scheduling system and to increase the probability that the observation can be scheduled However when precise timing is scientifically required an absolute time can be specified for the SSO observation and the flight computer will start the track and initiate the data taking at that time or no more than 3 seconds late
44. the two epochs can be subtracted While any detector artifacts and most of the light from the bright source should subtract out two images of any real faint object s should remain The extended PSF for IRAC can also be used to remove bright point sources and improve contrast for the detection of faint companions Chapter 6 InfraRed Array Camera IRAC 113 Table 6 15 PRF intensities Radius 2 44 4 88 7 32 9 76 12 20 Ch 1 TIo 1 272e 02 6 799e 04 1 390e 04 3 899e 05 1 231e 05 Enc Energy 0 855 0 942 0 974 0 991 1 000 Ch 2 TIo 1 628e 02 7 040e 04 9 919e 05 6 316e 05 1 928e 05 Enc Energy 0 836 0 930 0 968 0 987 0 998 Values are derived from 5x 0 24 pixel oversampled PRFs created from observations of calibration stars I Ip is calculated as the flux density pixel in a 1 pixel annulus at the respective radii divided by the flux density measured in the central pixel Enc Energy is the ratio of the flux density enclosed within the representative apertures to the flux density in a 12 2 radius aperture 6 2 4 1 6 Observing single faint sources Despite residual image mitigation procedures observations of a faint source may be affected by long term residual images at 3 6 um from prior observations of bright sources For observations of single faint sources we suggest offsetting the target position by 5 10 to reduce the chance of placing a residual image from a prece
45. validates and executes either previously stored or real time commands receives and compresses data and writes the compressed data into the mass memory The C amp DH also provides a stable clock to correlate data and events During normal Spitzer science operations the observatory collects compresses and stores 24 hours worth of science data prior to downlinking it Spitzer has 8 Gbits of solid state memory available for each of two redundant flight computers for a total of 16 Gbits of storage for both science and engineering data During each 24 hour PAO up to 6 Gbits of the memory are filled Enough additional storage capacity is left unused during that 24 hour PAO to permit missing a downlink opportunity e g due to a problem at the ground station and continuing to observe without overwriting previously collected science data Since the full mass memory 16 Gbits is available to both redundant computers in general multiple passes would have to be missed before the risk of losing science data that had not been transmitted to the ground becomes significant 20 Spitzer Observer s Manual 3 1 2 Data Transmission Spitzer has two antenna systems for data uplink and downlink The high gain antenna HGA supported a maximum downlink rate of 2 2 Mbit s for the first 2 6 years of the Spitzer mission Since Spitzer drifts away from the Earth at about 0 12 AU per year the data transmission rates on the HGA have had to decrease as a func
46. was found that the telescope focus point was about 1 85 mm above toward the back of the primary mirror the optimum focus position for the science instruments Thirty eight days after launch when the telescope had become thermally stable the secondary mirror was moved toward the primary mirror to bring the telescope into focus A re focus was not needed for the warm mission 4 3 Focal Plane Layout Figure 4 5 shows the actual location of the pick off mirrors that feed the science instruments as viewed from above looking down the boresight The Y and Z axis directions are the same as the telescope coordinate system described in Chapter 3 the X direction comes out of the page Figure 2 1 shows the Spitzer entrance apertures as projected onto the sky and appears inverted compared to Figure 4 5 due to the combined effects of looking out from behind the focal plane and the projection of the sky onto the focal plane through the telescope optics To understand this inversion recall the 3 D geometry and the fact that a Cassegrain telescope inverts its image The natural background at 8 um near an ecliptic pole is 5 3 MJy sr which is imaged to a flux density of 5 3 MJy sr x Jt 4x12 2 9x10 Jy in the focal plane of the f 12 Spitzer telescope The scattered light from Vega at 1 off axis is far below the natural background The first stray light trouble from Vega at 8 um should come from the outer parts of its diffraction limited PSF
47. way tiles mesh together 6 2 3 3 Selecting the IRAC Field of View As described above see also Figure 2 1 and Figure 4 5 the two IRAC arrays are fed by two entrance apertures which are located one above the other in the focal plane with a gap of about 1 5 For subarray mode there are two separate fields of 102 Spitzer Observer s Manual view corresponding to the subarray section of each of the two arrays In subarray mode you select one of the two subarray fields of view and the dither pattern will be performed with each of the selected fields of view pointed at the target When making an IRAC map not allowed in subarray readout mode note however that you can make small maps of bright sources in subarray mode using a cluster target you can use the field of view checkboxes to control the way the IRAC AOT performs the map Regardless of your selection data will be collected with both arrays If you select the 3 6 um field of view as primary then your specified map will be performed with the 3 6 um field of view A nearby portion of sky significantly overlapping for large maps will be covered with the other 4 5 um field of view This is illustrated in Figure 6 28 for which the sky coverage of a 3 column x 5 row map is shown If you select the 3 6 field of view the target position will be centered in the left hand panel The serendipitous sky coverage by the 4 5 field of view is shown in the center panel and the sky covered
48. well though even on the larger scales some improvement of the images will probably be noticeable 6 2 4 IRAC AOT Recommendations and Examples In this section we present some advice and some completely worked examples of IRAC observations Further detailed examples are found in the Spitzer Observation Planning Cookbook 6 2 4 1 Best Observing Practices with IRAC We recommend the following guidelines when planning IRAC observations More information on these items can be found below and elsewhere in this Chapter 1 Always check your planned observation by overlaying it on an appropriate astronomical survey image such as one from the 2MASS DSS MSX surveys Check for the range of allowed observing dates visibility windows Verify that bright objects e g from the 2MASS or IRAS catalogs will not ruin the planned observations in terms of scattered light section 6 1 2 2 3 persistent images section 6 1 3 2 3 image artifacts section 6 1 3 2 10 or saturation section 6 2 1 2 Use dithering instead of in place repeats see section 6 2 4 1 1 below 3 Use medium or large scale dither patterns unless performing a map with very tight requirements for uniform coverage see section 6 2 4 1 1 below 4 If the field you are planning to observe includes very bright sources consider using the HDR mode to allow for flux measurements of bright sources and identification of saturating sources section 6 2 4 1 2 below If you plan to obse
49. working channel has a doublet lens which re images the Spitzer focal plane onto the detectors A beamsplitter reflects the short wavelength light to the InSb detectors Channels and 2 The edges of the two IRAC fields of view are separated by approximately 1 5 with no overlap on the sky The IRAC pixel scale is nearly the same in both channels 1 2 per pixel providing a 5 2 x5 2 FOV InSb Detector Pickoff Mirror Si As i Detector I LLL 35 00 MM Figure 6 2 IRAC optical layout top view The light enters the doublet lens and the short wavelength light is reflected to the InSb detectors Channels 1 and 2 Note that the long wavelength detectors 5 8 and 8 0 um are not operational during the Warm Mission Telescope Beam Channels 2 and 4 Fiducial Pickoff Mirror a SP 5 35 00 MM Channels 1 and 3 Figure 6 3 IRAC optics side view 64 Spitzer Observer s Manual 6 1 2 2 Image Quality The IRAC optics specifications limit the wavefront errors to lt A 20 in each channel IRAC provides diffraction limited imaging internally and image quality is limited primarily by the Spitzer telescope The majority of the IRAC wavefront error is a lateral chromatic aberration that is most severe at the corners of the IRAC field The aberration is due to the difficulty of producing an achromatic design with a doublet lens over the large bandp
50. you want the same desired area to be covered by both arrays you will have to increase the number of rows in the map For example suppose that you want to cover an area with a diameter of D and you want the spacing between map grid positions to be 6 The number of columns in your map is just 1 D A One IRAC field of view size is A 5 2 and the gap between the two fields of view is G 1 5 The number of rows required to cover this area with both fields of view is therefore 1 D G You should round the number of rows and columns up for any fraction or else some area at the top and bottom of the desired region will not be covered by both arrays For example if your region has diameter 14 and you are using a 280 map grid spacing then your map should have 3 columns and 5 rows There are special cases for which observers should be careful with their choice of field of view For example if you want to make a sparse map with spacing greater than the array size 6 gt A then each of the 3 choices of field of view 3 6 um 4 5 um or both will yield a significantly different map To make a sparse map with both fields of view ensuring that both arrays cover the exact same sky you should split the observation into two AORs one per field of view Also fora very small map 6xM A where M is the number of rows be sure to make the map large enough that both fields of view will cover the target If the number of extra rows required usin
51. 0 99 01 UR 2 48975 66 23 7 9 1 94 75 79 86 140 000 99 97 GSFC Read noise predicted to be similar to cryogenic operations table values are 5 higher than cryogenic values for 3 6 um Values from pre flight laboratory measurements The QE and actual well depth are not uniquely measured in flight 6 1 3 2 2 Effects of overexposures After observing a bright source a residual image will remain on the array in subsequent frames In the cryogenic mission very bright sources produced a very long term tens of hours residual image at 3 6 mm that had to be mitigated by thermal annealing and judicious scheduling of bright soruces For warm IRAC there are no very long term residual images The arrays are not annealed as part of nominal operations and there are no strong constraints on observing very bright sources The predicted saturation levels for point sources are given in Table 6 13 Very bright sources will produce noticeable short term residuals as well as slew latents noticeable as streaks in images and produced in the motions from one observation to the next While it is not always possible steps should be taken to avoid generating persistent images including keeping bright sources off the arrays and using multiple short exposures instead of longer ones Also observations should be planned so as not to be susceptible to persistent images generated by preceding observations Well dithered observat
52. 0 118 119 123 Director s Discretionary Time DDT 6 12 56 distortion 64 67 108 118 122 125 dither 30 45 69 73 89 97 98 102 103 106 108 115 ephemeris 13 30 52 55 extended source 46 94 119 130 fault protection 28 29 86 87 filter 62 64 70 124 130 filter transmission 71 first frame effect 81 99 110 First Look Survey FLS 6 FITS format 13 119 126 flat 78 89 90 103 104 105 117 121 124 focal plane 10 27 40 Fowler sampling 81 85 90 99 123 General Observation General Observer GO 7 13 47 56 generic target 44 57 ghosts 70 124 Guaranteed Time Observers GTOs 6 Helpdesk 12 47 52 55 57 High Dynamic Range HDR 82 90 96 98 108 110 In Orbit Checkout IOC 6 7 39 41 67 instrument campaign 13 IRAC Mapping 97 99 100 105 108 110 Legacy 7 Leopard 14 47 linearity 115 120 mapping 27 45 noise pixels 65 88 94 Observer Support 6 8 12 Operational Pointing Zone OPZ 21 23 Performance Estimation Tool PET 46 pipeline 8 12 13 116 119 120 121 122 123 124 125 126 point response function PRF 64 110 111 point source 36 90 96 114 125 130 point spread function PSF 34 37 64 118 125 pointing 16 30 31 44 90 100 108 Pointing Control System PCS 16 post BCD 13 31 69 109 119 121 Proposal Kit 13 45 52 residual image 73 roll angle 17 100 114 roll control 27 saturation 5
53. 0 21 0 676 0 563 0 748 2 4 493 1 015 23 0 731 0 540 0 859 6 1 2 3 Chromatic Elements The IRAC system throughput and optical performance is governed by a combination of the system components including the lenses beamsplitters filters mirrors and detectors The system parameters are summarized in Table 6 3 and Figure 6 9 The system response is based on measurements of the final in flight system including the beamsplitter filter ZnS amp ZnSe coating transmissions mirror reflectance BaF and MgF gt coating transmissions and detector quantum efficiency The system out of band transmission more than 2 5 times the central wavelength of the band is less than 10 relative to the transmission at the center of the band Tests during IOC SV showed that the out of band leaks are less than the astronomical background at all locations for sources of any temperature detectable in the IRAC bands 6 1 3 Detectors 6 1 3 1 Physical Characteristics The IRAC detector arrays were developed by the Raytheon Santa Barbara Research Center SBRC in Goleta CA under contract to SAO Hoffman et al 72 Spitzer Observer s Manual 1998 Proc SPIE 3354 24 Estrada et al 1998 Proc SPIE 3354 99 Channels 1 and 2 use InSb arrays operating at 28 7 K The arrays use the CRC744 CMOS readout circuit and the same physical pixel size of 30 um The arrays are anti reflection coated with SiO The power dissipation for each array i
54. 10000 F Pec TT TTT TT TTTiit T ETIU T garnu UEILL TITIN TITT TTT T TTINY Koos Keos 3 6um 4 5um 1000F T 1 0183 i a 0 Ih 0 4s aoe Sass ee 0s Z 10 Udy we 2s 10 F 1 C 100s S i 23 CONMSION 2 2 4 l Luul ji raul ji Lol it Lil LLLE i raul i uul i Laul L Luul a a 0 1 1 0 10 0 100 0 0 1 1 0 10 0 100 0 Tas x Nevame s Jan 27 2010 Figure 6 24 IRAC point source sensitivity as a function of frame time for high background Full array frametimes are in black subarray frametimes are in blue To convert to MJy sr see equation 6 8 _ Wf BT f ST F cen where W is the well depth Table 6 4 fw 0 9 is the fraction of the well depth to which we can linearize the intensities and f en is the fraction of the source flux falling onto the central pixel Table 6 1 For warm data Wfw 30 000 DN as discussed in Section 6 1 3 2 1 Table 6 13 shows the point source saturation limits of IRAC at each frame time If you are observing an extremely bright piece of sky such as an H II region then the saturation limit is lower you should include 6 9 96 Spitzer Observer s Manual your sky brightness in the background B for the saturation table we used the medium diffuse sky background from Table 6 7 Note that the saturation value is conservatively computed from the worst case in which the PSF is directly centered on a pixel While users may find that in their actual data the satu
55. 104 Spitzer Observer s Manual For the full array mode there are two types of dither patterns available Five such patterns are fixed patterns which are performed identically at each mapping position The cycling pattern is a set of dither positions also referred to as points a different subset of which is performed at each map grid position Different patterns are available in subarray mode as the angular scales covered by the arrays are quite different Two fixed patterns are available for this mode Table 6 14 Characteristics of the dither patterns Dither Pattern Scale Max dither Median dither Sub pixel pixels from separation dither 0 0 pixels pattern Cycling Small 11 10 5 pixel Medium 119 53 2 pixel Large 161 97 2 pixel 5 point random Small 26 23 2 pixel Medium 52 46 2 pixel Large 105 92 2 pixel 9 point random Small 16 14 1 3 pixel Medium 34 28 1 3 pixel Large 69 59 1 3 pixel 12 point Small 13 15 4 pixel Reuleaux Medium 27 30 pixel Large 55 59 2 pixel 16 point spiral Small 16 12 Y4 pixel Medium 32 23 4 pixel Large 64 45 4 pixel 36 point Small 17 19 Ys pixel Reuleaux Medium 34 39 V4 pixel Large 67 78 Y4 pixel 6 2 3 5 1 Dithering strategies The dithering strategy adopted for a particular observation will depend on the type of object and the required depth and resolution of the final image The characteri
56. 4 94 scattered light 37 54 68 108 109 124 scheduling 20 27 30 44 45 47 49 50 52 53 115 Science Verification SV 6 7 sensitivity 53 61 76 87 90 92 98 108 114 115 125 shadow observations See constraint shadow shutter 82 84 86 118 123 Solar System Object 30 44 52 53 132 Spot 3 44 45 50 97 108 SSC website 2 12 13 stray light 21 37 39 68 88 subarray 85 91 96 99 101 104 Target of Opportunity ToO 12 21 44 48 56 temperature 6 8 35 36 71 tracking 8 28 30 52 55 visibility 21 22 24 25 26 27 45 51 52 100
57. 4 5 um for a source as a function of its position on the pixel Calibration data have been taken to correct this effect to better than 0 1 6 1 3 2 6 Pixel mask In each array there are pixels that are totally inoperable or whose characteristics QE noise dark current fall outside of the allowed range This is represented by the Operability column in Table 6 4 These pixels are evaluated and masked out during reduction in the pipeline using pixel masks Interested observers can download the quasi static bad pixel masks from http ssc spitzer caltech edu irac products pmask html These masks have been updated for post cryogenic operations The number of bad pixels has increased by a factor of 5 at 3 6 um and a factor of 4 at 4 5 um but is still a small fraction of the array When the applied biases were tuned for each array one constraint was 78 Spitzer Observer s Manual to keep the number of bad pixels comparable to the number of pixels affected by radiation hits in the longest frametime used 100 seconds 500 pixels ET Figure 6 14 IRAC super skyflats for channels 1 and 2 taken during the warm final setpoint calibration The extended dark spots seen in the channel 2 flat are most likely due to particulate impurities on the pick off mirror 6 1 3 2 7 Darks The detector dark currents at 28 7 K are insignificant compared to the sky background Ground tests at 30 K measured dark currents of lt 0 1 e s for sister arrays of
58. 5 6 Best Observing Practices for Solar System Objects 54 3 6 TARGETS OF OPPORTUNITY oore ae e E A A EA E evndnsoeseae 56 36 1 Classification Of 10 OS iieii e eE E E NARRE A NENTA 56 3 06 22 Activation Of AORS o rn ar E i a A E R A eames 57 ST GENERIC TARGETS sran nenni CEN a ER events ERE AR ER ep EAEE a TES 57 INFRARED ARRAY CAMERA TRAC ssessssssssesesososososososessosesesesesesosesesesesesesesesseseosoessosossse 59 6 1 INSTRUMENT DESCRIPTION seisne nanosa e aao eren ESTEE EEE Eaa aa pa ora OLD SOVEIVICW 321i ek eee Bie eT DRS Res Bee BO Ae eee BO EE OLDS OPUS sisi Aten eral nati h tie deaths dibe ohn bee aenea a he rete 6 1 3 DDCLCCIONS oxi 3 15 as el eh tee Sather Ee ee Bah We Be BE 0 2 HOW TOUSE TRAC oetan r ea Erare EEA ETNE TE EERTE TEERAA VARA AT TE EEEE 6 2 1 Performance of the Instrument 6 2 2 Observing Time Estimation 6 2 3 Astronomical Observation Template AOT Description n se 97 6 2 4 IRAC AOT Recommendations and Examples o e 108 Ode IDATA ore AA Se R a A T E E A AS 116 6 3 1 Instrument Calibration cciana a hese E a aa BARES Bes wa each 116 6 3 2 Data Produc aE ATO E AOE E earnest salen tebe A E E a Me 119 6 3 3 Data Processing mircea ai gia TE E EE a i i i E EE i EER 122 APPENDICES R A E E AEA E E EOE TTT 129 Tel INFRARED FLUX UNITS visciscuseds stds chesezas cova enctoescstonedassads sberres EEE A EEA A E i eE 130 7 2 INFRARED BACKGROUNDS fesecscs df tsisscsessevs cecteacedecsostat Ep AEAEE ar E
59. 6 Spitzer Observer s Manual and three dithers per map the first grid position will use points 310 311 and 1 from the table the second 2 3 and 4 and so on This pattern has a 4 sub pixel sampling pattern superposed on it starting with point 1 and repeating continuously every four points at point 311 the final cycle is simply truncated early thus patterns which wrap around the table will miss a sub pixel dither point The five point Gaussian pattern is a general use pattern suitable for shallow observations where the exact sub pixel sampling is unimportant It has a 4 sub pixel pattern with the 5 point at sub pixel 14 4 Figure 6 29 shows the dither patterns at the default large scale Figure 6 30 shows the cycling dither patterns and the distribution of both the dithers and of the separation between dithers for each scale Cosmic ray and residual image rejection is greatly facilitated by taking highly dithered observations The SSC mosaicking software provides good outlier rejection in cases in which each position on the sky is observed with several dithered frames The SSC mosaicker works well with five or more dithered observations per position on the sky though will manage some rejection with fewer It has not been found necessary to take repeated non dithered observations at each position in a map or dither pattern to obtain good cosmic ray rejection if dithering is being carried out with sufficient redundancy 6 2 3 5 2
60. AC 97 6 2 2 Observing Time Estimation The Spot tool provides fast and accurate estimates of observing time for IRAC To optimize AORs however it may be useful to bear the following information in mind Each AOR duration includes a flat 215 sec slew overhead to the initial position an additional 300 sec overhead is charged for moving targets Command overheads between exposure repeats are very short a few tenths of a second except for the 2 sec full array and 0 02 sec subarray frame times in which a delay is put in to ensure that the data can be transferred from the IRAC buffer to the spacecraft for which the maximum rate is 4 sec per 2 full array frames Slew durations and allowed settle times are a function of slew length Slews between points in dither patterns maps or cluster targets apart from those between cluster targets specified by celestial coordinates are carried out under gyro control in low rate mode Each slew and settle has a command overhead of approximately 2 sec Slews of 100 or less have a settle time of 4 sec slews between 100 and 300 5 sec and slews of more than 300 6 sec The slew times themselves are a function of the slew length Slews from 0 to 100 take 0 4 2 seconds excluding overhead and those from 100 to 300 take 4 2 8 4 seconds Slews of a degree take 54 sec For cluster targets specified by celestial coordinates a different slew mode is used with larger command overheads
61. C pipeline provides data with a full astrometric solution including the array distortion using the SIP convention Shupe et al 2005 ASPC 347 491 6 1 2 2 2 Location of sources on the arrays For an observation specifying a target with no offsets or dithers the source will be positioned at the pointing center of each field of view modulo the blind pointing uncertainty of Spitzer 0 5 rms In BCD coordinates the source will appear as listed in Table 6 2 The x and y are the row and column positions respectively in the BCD counting from the lower left hand corner 1 1 with integer values specifying the center of a pixel Offsets in the positive column direction will move the source in the negative y direction on the BCD Offsets in the positive row direction will move the source in the positive x direction on the BCD 68 Spitzer Observer s Manual Table 6 2 Source location on the arrays Array y pixel pixel 3 6 um 131 0 128 0 4 5 um 128 0 129 0 The orientation of the BCDs on the sky is shown in Figure 6 6 The figure also displays the positioning of the subarray fields of view with respect to the full array The 3 6 um subarray subtends the location 9 40 to 9 40 of the 3 6 um full array BCD The 4 5 um subarray orientation with respect to the full array is the same as in the 3 6 um array The pointing centers of both arrays for both the full and subarray are free of hot or noisy pixel
62. E E E 45 viii 5 2 2 Performance Estimation Tool PET 46 SS Ruk Unit CONVENES sennen nmd nan anaE e aA EEan 46 5 2 4 Leopard Spitzer Archive Interface c ccccccccccccccccsccsesessessseeeeeeseesenseseessseesenesserenesseeteas 47 5 3 MODIFICATION OF OBSERVING PROGRAMS cccscsssesesssesesesesesesnesesesesesesenensssessessssssssnenenenees 47 5 4 SCHEDULING CONSIDERATIONS oesie as SEn a EEEE R TENETE AEE E S 47 5 4 1 Scheduling Methodology c ccccccccccccccscsscessversssessscesesseesssececesesecsseseeeseesseeeseeaeeseaeeesiees 48 5 4 2 Notification That an AOR Has Been Scheduled cnenneeee 48 5 4 3 Requesting Scheduling Constraints for Science Reasons ccce 49 5 4 3 1 Recommendations for the Use of Scheduling Constraints 00 0 ccseeeeeeeeeeeseeeeeeeeeeteneeers 49 5 4 3 2 Specification of Scheduling Constraints eee 5 5 SOLAR SYSTEM OBJECTS 0004 5 5 1 Tracking Performance 5 5 2 Ephemeris Management 5 5 3 Target Specification and Scheduling Considerations 5 5 3 1 t Shadow Observations asni enor etn ar a aE ARA EE EEA ER EEGEN 55 3 2 Timing Constraints 5 25 2 csiveiescdsseeceiatecensnecscecdsosvecsunees AAEE EE E ARA EEE ER 5 5 4 Science Instrument SSUCS ccccccecceccescescesceseesevseseeseeeeseeeeseeseeceeseeseeeeseeseeeeeeesneeneeeeeees DED Die OVP COGS sacke ait vas eaten cu cbuk gs aE Bessa aa bas Seon ka pb en asta don Snes Aan tae cd eR a 5 5 6 Best Observing Practices fo
63. For example a series of AORs used to obtain images of a comet over a long track which needed to be broken up into segments due to curvature might be constrained as a chain with an associated timing constraint related to the acceptable range of solar elongations Spot software checks for viability of relational constraints Spot will warn the user if a timing constraint is set such that the target is not visible for some portion of time within the timing window but Spot will still allow the constraint to be set Observers should make sure that the timing windows they introduce overlap sufficiently with the visibility of the target s in order for the complete AOR or set of relationally constrained AORs to be completed Timing windows that barely overlap a visibility window for a given AOR will make scheduling of that 52 Spitzer Observer s Manual AOR very difficult or perhaps impossible e g through small visibility window changes caused by a more accurate Spitzer orbit calculation 5 5 Solar System Objects Spitzer supports observations of SSOs tracking in linear segments at rates up to 1 s as described in Chapter 3 Updated information on Solar System specific observing issues can be found on the web in the Solar System Observing with Spitzer document in the Solar System subsection of the Proposal Kit section on the SSC website 5 5 1 Tracking Performance Spitzer s SSO tracking capability is similar to its scanning cap
64. OPERATIONS OVERVIEW csssssssessessseeseeeseceseneeeseesseeeseceseceeesseesseessesessseeeaes 8 2 4 1 Science Instrument Operations ccccccccccccccceccsccsessseesesecsceesseeseussscesensssersnsesenssneesenseaeets 8 2 4 2 The Life Cycle of a Spitzer Observation oeeie 11 2 5 SSC SCIENCE USER SUPPORT SERVICES cccsccsssssseseeseesecseeeeseeseeeececeeseeseeaecaecaeeeenesseseeeeeeees 12 23d Documentations neinet EN E O A EAEE NR 12 DDD HOIDAESK rrn ENE E ETAR T sad E ORE E EE 12 293 PROPOSQIK AL aire EE NE E TE A R EEA ERE E E 13 DSA Data Products rasei A EE E a A E Ea 13 3 OBSERVATORY DESCRIPTION AND CONSTRAINTS eeseessoesseesscesocesocesoccesocesccesccesoe 15 3 1 OBSERVATORY DESIGN AND OPERATIONS CONCEPT ccssssssssseessesssesseesesseeeseeseeeseeeeeeseenes 16 SAL Data Storage sce treet cine E pene usd NASR SG EAA ENON E 19 Sele 2 Data LHANSIMISSION aA crt carta i TE a IAEE NE sees eet OR Gait oe ata ET red cated 20 BD OKY VISIBILE 230 5 e a a O a a dosh eteens E R aes 21 Be VOl Orbitan A a le vate tes Comte let aa i A antes E ole ae a ios 21 3 2 2 Pointing COMStraint nie series o ieia Ee EEA E E RERE a i 21 3 2 3 Viewing Periods r e a ae RER ERE E E E N E A a EATA 22 3 2 4 How Visibility Evolves with Time eeeeeeeeesiererirerrresrerrserersreresrsrerersrererers 26 3 2 5 Orientation of Focal Plane and Slits against the Sky soose 27 3 2 6 Bright Object Avoidance eeeeeeeeesersrerersrsrsrsrsrsrersrsrsrisrsrsrerer
65. RAC BCD images contain a pointing estimate based on the output of the Spitzer pointing control system startracker and gyros i e the boresight pointing history file This initial pointing estimate is accurate to about 0 5 The post BCD pipeline performs additional pointing refinement for all IRAC frames This is achieved by running the SSC point source detector on the channel 1 and 2 frames and comparing the resultant list of point sources to the 2MASS catalog The results are then averaged and the known focal plane offsets between the two channels are applied to produce a superboresight pointing history file which is then applied to the data during end of campaign reprocessing This improves the pointing accuracy of the frame to better than about 0 3 This refined RA Dec appears in the header as the CRVAL1 CRVAL2 keyword values The SSC pipeline mosaicker produces a single image 1 per band from many input images First the BCDs are corrected for overlap consistency The parts of the images that overlap are forced to have the same background value via addition of an offset Then a fiducial frame is derived This is the definition for the output frame in terms of its physical size projection and orientation Because IRAC has such a large field of view projection effects are non negligible and the mosaicking and coadding process must reproject the data The fiducial frame finder seeks to minimize the amount of blank ar
66. SF over the field of view of each of the two channels has been characterized and the optical distortion has been measured This has been done by observing an open star cluster for which we have good ground based astrometry The distortion is included in the WCS header keywords The positions of both IRAC fields of view have also been determined relative to the Chapter 6 InfraRed Array Camera IRAC 119 PCRS and other science instruments To our ability to measure the distortion map and PSF have not varied from the cryogenic measurement 6 3 1 5 Linearization During cryogenic observations non linearities in the IRAC detectors were extensively calibrated during ground testing using special ground support equipment and also in flight using extended astronomical objects These calibration data allow the detectors to be linearized to better than 1 for 90 of their full well capacity A similar set of calibrations was performed as part of the final warm setpoint calibration however the warm linearity solution was best derived from science observations of extragalactic fields that had also been observed in the cryogenic mission see Section 6 1 3 2 1 The linearity solution will be monitored throughout the warm mission using our network of calibrators and appropriate science observations In the unlikely event that we find a significant deviation from the previous linearity characterization new solutions will be derived 6 3 1 6 Routine Frequ
67. SNS SOEECOEEEEF EERE DENA TET VN ENNEN ALLA EAL ERM AVM NIE SAMA NMRA ere er ay eee A PUN ANNAN SLSLLEEEELEAN EAA IVAN RAN BMRA ee re es a ee ee ee E AE TN NN RACNAY PERRE EE RERS E ER ENN Varennes Ov ma Aa s ree ye vpyyvvyyy O Yee sess ey ps 0 100 200 0 100 200 X POSITION pixel X POSITION pixel 67 Figure 6 5 Optical image distortion in IRAC channels The panels show the image distortions as calculated from a cubic polynomial model that has been fit to in flight data The magnitude of the distortion and the direction to which objects have moved from their ideal tangential plane projected positions are shown with arrows The length of the arrows has been increased by a factor of ten for clarity The maximum positional deviations across the arrays for this cubic distortion model are less than 1 3 and 1 6 pixels for channels 1 and 2 The derivation of the pixel scales that are listed in Table 6 1 fully accounts for the cubic distortion effects shown here The sixth and seventh columns in Table 6 1 were determined from astrometric measurements of star fields 6 1 2 2 1 Distortion Due to the off axis placement of IRAC in the Spitzer focal plane there is a small amount of distortion over the IRAC FOV The maximum distortion in each IRAC channel is lt 2 2 pixels compared to a perfectly regular grid over the full FOV Figure 6 5 shows the distortion across the two short IRAC channels as determined from data taken during IOC SV The IRA
68. ability and the performance is described in section 3 3 3 One significant consequence of this for the SSO observer involves sources whose tracks on an equatorial map have significant curvature during an AOR Such AORs may need to be broken up into a series of short linear AORs The spacecraft does not carry any target ephemerides on board so the track is defined at the time of scheduling and formulated as a vector rate in an equatorial frame A start time and equatorial J2000 start point and time are also provided for use by the PCS Once the track command has been issued the on board system maintains knowledge of where the telescope should be at what time and catches up with the specified track and maintains it Any further updates to SSO tracking performance will be posted in the Solar System subsection of the Proposal Kit area on the SSC website 5 5 2 Ephemeris Management Spitzer uses a database of ephemerides for known SSOs derived from the Horizons database maintained by the Solar System Dynamics group at the Jet Propulsion Laboratory For proposal planning purposes Spot can retrieve the ephemerides for a specified target by resolving the NAIF ID which in most cases corresponds to the Horizons SPK ID These ephemerides are used to calculate visibility windows and resource estimate calculations for SSOs through Spot If the object is newly discovered is not released through the Minor Planet Center MPC and does not y
69. ach of the four outputs of an array pinstriping and a small spatial gradient across the image True dark current and multiplexer glow carry with them the usual shot noise in the number of charges collected at the integrating node but the rest of the Fowler bias presumably is due to relaxation in 82 Spitzer Observer s Manual the multiplexer and temperature changes The noise in pixels with low dark current and low glow is the same as the read noise from the multiplexer so the dark offset variations do not add to the pixel noise In general the first frame of any sequence of images tends to have a different offset from the others because it tends to have an interval frame time or Fowler number different from its preceding image The largest dark offsets occur when a frame is taken with a very short interval from the preceding image which occurs when multiple frames are commanded at once using repeats In lab testing this effect came to be known as the first frame effect for this reason We eliminated commanded repeats from IRAC science observations so there is always a finite interval between one frame and its predecessor Most observers should be dithering between frames meaning that the interval between frames will be in the range 8 to 60 sec The first frame of every AOR with frame time gt 12 sec is taken with the HDR mode see Table 6 9 to mitigate the first frame effect The first frame effect was calibrated
70. ainst the failure of component AORs As a general rule of thumb groups of constrained AORs that occupy more than about half of the time period during which they can be scheduled will have a strong negative impact on efficiency and may not be feasible to schedule In other words any constraint that involves a time range should be twice as long as the AORs you want scheduled within that time range For example 40 hours of observations that must be done within a 40 hour period cannot be scheduled whereas the same 40 hours of AORs may be quite feasible if they can be scheduled anytime within the same one week period In addition shorter AORs of 2 3 hours duration are easier to schedule than many 12 hour AORs When designing scheduling constraints the observer must take into account the natural 24 hour time scale for PAOs and that PCS calibration and instrument and spacecraft maintenance activities can take 15 or more of the available time 50 Spitzer Observer s Manual during a given PAO but usually takes just a few percent of the available time within a given campaign 5 4 3 2 Specification of Scheduling Constraints Scheduling constraints should be submitted at the time the AORs are submitted using the constraint editing capabilities of Spot Please see the Spot User s Guide for details on how to specify constraints and the Spitzer Observation Planning Cookbook for an example of creating constrained observations Note that constraints
71. al 100 Q 100 Figure 6 29 IRAC dither patterns for the large scale factor The cycling patterns are designed for AORs having many mapping dithering observations but may also be of more general use The large and medium patterns are Gaussian distributions with dithers gt 128 pixels removed The small pattern is specifically designed for mapping where only a few dithers are taken at each map position It is also based on a Gaussian distribution but the center is downweighted to decrease the fraction of small dithers in the pattern and it is truncated at a maximum dither of 11 pixels to ensure that maps with up to 280 spacing have no holes even if there is only one dither per map point All the patterns are constrained to have no pair of dithers closer than three pixels in any run of four consecutive points To use these patterns you specify the number of dithers to perform at each map grid position and the starting point in the cycling dither table You may specify a single dither at each map grid position This will result in a single observation at each map grid position but the location of that observation will be slightly shifted from the rectangular grid Such a fuzzy grid should allow a better flat field determination and reduce jail bar or other pattern noise in the resulting mosaic The cycling dither table will wrap around once the final 311 element is reached For example selecting a starting point of 310 10
72. al passes before the ground receives all the science data for a given observation Whenever the HGA is used for downlink it is also possible to uplink commands and files to Spitzer Normally up to 5 communication periods per week are used to send up all the sequences and information needed to support the next upcoming week of observations No communications with Spitzer are planned outside the scheduled downlink sessions 3 1 2 1 Impact on Observers The downlink strategy has little impact on observation planning with Spitzer It is one component in determining the maximum length of an AOR A 24 hour interval between downlinks does imply that it can be a day or longer before science data are available on the ground after an observation is complete i Currently the maximum planned length of an AOR is 24 hours for IRAC Please note that this is the maximum length for an isolated AOR In general there would not be several contiguous periods of this length available Chapter 3 Observatory Description and Constraints 21 The uplink strategy also does not directly affect observation planning but it drives the time scale on which changes to the stored science schedule and its contents can be made e g for a ToO 3 2 Sky Visibility 3 2 1 Solar Orbit An important innovation enabling Spitzer to accomplish ambitious scientific goals at a modest cost is its Earth trailing heliocentric orbit shown in Figure 3 4 in which the observatory
73. all overlap between map grid positions we want to constrain the dithers to keep the coverage as uniform as possible but we also want to separate large scale photometric variations from sky variations so we choose the medium dither pattern scale factor as a compromise The frame time at each position is 30 sec We can make an 11x11 map grid covering 0 7 deg with both arrays in 5 7 hours as determined using Spot Except for the small overlap regions between map grid positions and a crust at the edge of the map the observing time per sky pixel is 4x30 seconds Using the sensitivity tables above assuming medium background and a perfect flat field the S o point source sensitivity is 4 9 uJy at 3 6 um The observation described here could be used as a tile for a survey of a larger area Suppose you want approximately 9 times the area so that your survey region could be broken into a 3x3 set of these tiles The entire observation could not be done in a single AOR because it would exceed the number of allowed instructions in a single observing request To implement this large survey you would generate 9 identical AORs and constrain them to occur within a reasonably short period of time in order to keep the relative roll angle between the AORs small Further you would specify offsets for the center of each AOR You will need to specify in addition to the AOR parameters already described the array coordinate offsets for each AOR th
74. asses being used The effect is small with the total lateral chromatic dispersion less than a pixel in the worst case The sky coordinates of each pixel have been accurately measured in flight using astrometric observations of an open cluster resulting in distortion coefficients for each image The main effect is that the PSF and distortion may be slightly color dependent which may be detectable for sources with extreme color variations across the IRAC bands At the current temperature of the IRAC optics we observe no measurable difference from the cryogenic image quality This is expected as the thermal expansion and variation in refractive index for the optical components is imperceptible from 15 K to 30 K A much larger variation in the flux of sources measured in different parts of the array is due in part to the tilt of the filters which leads to a different spectral response in different parts of the field of view Hora et al 2008 PASP 120 1233 Quijada et al 2004 Proc SPIE 5487 244 The flat field calibration is done with the zodiacal light which is relatively red blue sources have a flux variation of up to 10 from one side of an array to the other see the IRAC Data Handbook for more details Figure 6 4 shows the IRAC point response functions PRF reconstructed from images of a bright star obtained during IOC SV Here we use the language common in the optics field the point spread function PSF is before sampling by t
75. at the higher operating temperature The temperatures quoted in this document are based on the IRAC Cernox temperature sensors which are estimated to be uncertain by 0 5 K at these operating temperatures This section presents our current knowledge of the performance of IRAC during the warm mission and should be sufficient for planning science observations Our understanding is derived whenever possible from data at the final operating setpoints A brief calibration campaign was conducted immediately following the final setpoint assertion From this campaign the linearity flux conversion and flat field were derived In addition a much longer calibration campaign was conducted at a slightly higher operating temperature 31 K and lower applied bias 400 mV at the beginning of the warm mission The optical properties and array photometric variations were measured at this time As a result of this initial characterization it was determined that the instrument could be operated at a lower temperature with a significant improvement in noise properties resulting in the final setpoint assertion For quantities that cannot be measured on orbit or have not yet been done on orbit due to scheduling pressures we rely on the limited 30 K testing of similar detectors conducted by the University of Rochester group and or cryogenic measurements Our current understanding is that the arrays are slightly less sensitive overall and the optical properties have chang
76. at would place them onto the desired grid Thus if the two array coordinate axes are called Y Z and the desired spacing between map grids is G then the map center offsets would be G G G 0 G G 0 G 0 0 0 G G G G 0 G G for the nine AORs respectively Using the constraints editor you would constrain that the observations all occur within a reasonable time typically lt 1 week of each other but you should not specify the exact date 6 2 4 3 Deep Image Example Suppose we want to make a sensitive image at 3 6 um of an object that is less than 4 in size In this case we will not map but instead will perform many dither Chapter 6 InfraRed Array Camera IRAC 115 steps in order to minimize the effect of pixel to pixel gain variations For this example we will use the 36 position Reuleaux triangle dither pattern medium size and 100 sec frames We repeat the pattern four times using a cluster target with small array offsets 1 8 0 0 1 8 and 0 6 0 6 to reach the desired detph Using Spot we determine that the observation takes 8 8 hours The 50 point source sensitivity low background is 0 3 uJy at 3 6 um which is below the confusion limit 3 4 uJy and the per pixel surface brightness sensitivity is 0 003 MJy sr at 3 6 um A neighboring field not overlapping with the target field will be observed at 4 5 um 6 2 4 4 Elongated Object Example We wish to make an image of an elongated
77. ate the total noise by adding the shot noise in quadrature so that o o J F F 6 5 where O is the noise from equation 6 1 and F f S10 6 6 In the bright source limit F gt gt Fp the signal to noise ratio becomes SIN f ST F 6 7 If the exposure time is in seconds and the source flux density is in uJy then for IRAC channels 1 and 2 respectively S N is 0 86 and 0 85 times 4T F Thus for example to design an experiment that requires a S N 1000 for a star with a brightness of 1 mJy at 3 6 um we require an exposure time of 1352 sec This could be achieved for example using 14 frames of 100 sec because the star is below the saturation limit for 100 sec frames In the sensitivity figures 0 6 uJy is the confusion limit predicted by Franceschini et al 1991 A amp AS 89 285 This does not represent a hard sensitivity limit but rather indicates where source confusion affects reliability of source extractions for low background regions Data from IOC SV show noise decreasing as 4N to 0 25 uJy channels 1 and 2 A corresponding deep observation has not been done in the warm mission characterization but ample warm deep data exists from the Exploration Science programs Moderately deep source counts indicate that a 94 Spitzer Observer s Manual source density equivalent to 36 beams source is reached at 20 5 mag or 1 8 and 1 1 microJy at 3 6 and 4 5 um respectively Fazio et al 2004 ApJS 154 10 The conf
78. ay coordinate map The savings are entirely due to the celestial coordinate map covering a smaller area custom tailored to the desired object size ON e amp wo Figure 6 33 Sky coverage for an oversampled celestial coordinate map of an edge on galaxy The map was made with a position angle 60 E of N The scale bar gives the number of times each sky position is observed 6 3 Data 6 3 1 Instrument Calibration The SSC performs routine calibration of IRAC using observations of standard stars and other astronomical objects Additional diagnostic data may also be taken at various times to perform certain calibration operations The data obtained in these observations are used to construct the necessary calibration inputs to the pipeline for the IRAC data processing of science observations The calibration data files as well as the pipeline inputs are available to the general user in the Spitzer archive maintained by the SSC For more information consult the IRAC Data Handbook the Pipeline Description Document and the absoute calibration paper by Reach et al 2005 PASP 117 978 In the remainder of this section we refer to instrument campaigns which are two week segments of the schedule Unlike the cryogenic mission where the instruments were cycled through and IRAC was power cycled at the beginning and end of each campaign IRAC is powered on for the entire warm mission and the campaign nomenclature is a convenience for schedul
79. boundaries drawn 24 Nov 2003 00 00 UTC Figure 3 6 OPZ boundaries for 24 Nov 03 0h UTC with dots representing the actual locations of the telescope boresight for the subsequent 24 hours 24 Spitzer Observer s Manual Ave of Days In OPZ per Year 290200 70s BO a0 AO 90 90 AO OS Oe 20 90 40 SO BO 70S O90 Ecliptic EMO2000 Latitude Figure 3 7 Variation of length of visibility period as a function of ecliptic latitude for all of the targets in the April 2003 ROC This figure is provided as indicative of the general concepts despite the fact that the ROC has changed substantially since April 2003 Chapter 3 Observatory Description and Constraints 25 Spitzer Viewing Statistics for Inertial Targets Ecliptc Coordinates Equatorial Coordinates Galac fic Coordinates Days per year that SIRTF can view objects vrith these coordinates 0 4 ci 12 10 200 20 280 360 Figure 3 8 Total days of visibility per year in equatorial ecliptic and galactic projections 26 Spitzer Observer s Manual 3 2 4 How Visibility Evolves with Time Figure 3 9 illustrates the zone of visibility in equatorial coordinates for four specific dates during the first year of operations Note how the sky passes into and out of visibility as the calendar date advances Each set of contours shows the available and forbidden zones on a particular date in this plot 1 Sep 2003 1 Dec 2003 1 Mar 2004 and 1 Jun 2004 On each
80. bration is derived taking several systematic effects into account The steps are described in detail by Reach et al 2005 PASP 117 978 If this methodology is not applied then point source photometry from the BCDs can be in error by up to 10 The absolute calibration factor for the Warm Mission has been determined in an analogous fashion from the final setpoint calibration campaign and confirmed with the per campaign observations The initial absolute calibration accuracy will probably be of order 5 and will improve as we acquire better statistics and the revised linearity solution is folded into the calibration 6 3 1 2 Sky Flats To get the most accurate measure of the full system gain including the effects of the telescope and the IRAC pickoff mirrors one must use observations of the sky This is done using many dithered observations of a network of 22 high zodiacal background regions of the sky in the ecliptic plane which ensures a relatively uniform illumination with a reasonable amount of flux One such region is observed every 14 days The data are combined with object identification and outlier rejection producing a product analogous to a median sky flat as is commonly constructed during ground based observing and which will be an image of the presumably smooth celestial background further smoothed by the dither pattern The resulting flat field will be divided into the science data From the cryogenic mission data we ant
81. brightness sources To mitigate this potential noise source observations of low surface brightness objects should not count on using the first integration and would benefit from using a large cycling dither pattern as that is the same pattern used by the skydarks If data are taken with the same cadence as the skydarks then the first frame effect is removed entirely by the skydark subtraction In a later version of the pipeline a correction for the warm first frame effect will be implemented Chapter 6 InfraRed Array Camera IRAC 83 180 46 gt aa g 170 See eee gt z a e amp ae 7 7 5 42 ae S g 8 2 goy 3 o dim o gsto 8 CH1 CH2 a 38 150 F 36 io i f PE ETT ne q Lo A 2 i 5 8 g 8 A ge i 3 H 0 o amp i ae ie ieee 3 4 8 L al o 20 4 amp w o 2 4 8 a lo time between frames s time between frames s Figure 6 18 Cryogenic first frame effect electronic offset as a function of time interval between frames 6 1 3 2 12 Stability In flight data indicate that IRAC is very stable at 28 7 K The arrays will continue to be thermally controlled to a level of a few milli Kelvin As in the cryogenic mission we monitor the gain bias and photometric stability using observations of the Ecliptic plane the North Ecliptic pole and the seven primary calibrators for IRAC every fourteen seven for the bias days As in the cryogenic mission the gain bias
82. bservations of single very bright targets Advantages of the subarray mode in addition to the ability to observe sources that would saturate full array readouts are that sets of 64 frames are taken back to back enabling better temporal resolution and decreasing the data volume which means that rapid sampling can continue for longer periods Also because of the more rapid 0 01 sec sampling more Fowler samples are taken resulting in lower readout noise per frame compared to full array mode for the same frame time In subarray readout mode there are 4 selectable frame times 0 02 0 1 0 4 and 2 sec For one commanded image in subarray mode a set of 64 Fowler sampled frames are taken in succession so that each time an image is commanded in subarray mode a cube of 64x32x32 pixel images is generated This means that the durations of a single repeat at each of the four subarray frame times are 1 28 6 4 25 6 or 128 sec respectively The IRAC AOT moves the telescope to point to the subarray region of each requested channel at the target in turn For the 0 02 sec frame time data rate limitations allow only data in the channel actually pointing at the target to be taken For the 0 1 sec 0 4 sec and 2 sec frame times data are taken in both channels at each pointing position although only one channel at a time will point at the target 6 2 3 2 Map Grid Definition If you select No mapping then your map grid will consist of a singl
83. but with the ability to switch to high rate mode for longer slews Overall slews of gt 0 5 deg are faster using this mode AORs with widely spaced cluster targets will therefore be more efficient if RA Dec positions are given for the cluster points Observing strategy is a series of compromises between sensitivity redundant coverage and wall clock time We recommend a given sky position be visited a minimum of three times and preferably five in order to mitigate the effects of radiation hits and bad pixels We also recommend that dithering be performed to mitigate the effects of pixel to pixel gain variations Repeat observations at a given sky position should sample different parts of the array either by using a medium or large dither scale and or by making approximately half array offsets between map positions or between AORs covering the same part of the sky This technique also ensures good straylight removal For more details on recommended observing strategies see section 6 2 4 6 2 3 Astronomical Observation Template AOT Description The IRAC AOT consists of a optional dither pattern superposed on a optional rectangular grid raster In this section we describe the manner in which an IRAC AOR is specified by an observer and the activities that the spacecraft will perform when the specified AOR is executed Figure 6 25 shows an example of an IRAC Post Cryo Mapping AOT dialog box in Spot For more information please see the S
84. central pixel can be roughly estimated by multiplying the flux by the percent of flux in the central pixel from Table 6 1 and the projected area of an IRAC pixel 3 5x10 sr The threshold for strong column pulldown is that for the artifact to be noticeable in high background regions such as the Galactic plane Table 6 5 Fluence per second in MJy sec sr of a pixel needed to trigger artifacts Ch Weak Pullup Strong pulldown pulldown 1 1200 4000 2 1300 2340 4350 The definitions of the columns in Table 6 5 are Weak Threshold for column pulldown that is noticeable in low background pulldown fields Pullup Threshold for column pulldown that will produce a pullup in one or two subsequent frames Strong Threshold above which the column pulldown will be significant in Pulldown high background fields For more information on IRAC instrumental artifacts please see the IRAC Data Handbook 6 1 3 2 11 First frame effect Dark frames in all IRAC channels are not constant A dark image taken with a particular frame time and Fowler number depends on the amount of time elapsed since the previous image and the frame time and Fowler number of the previous image In a sequence of images with the same frame time and Fowler number the variation from one image to the next consists principally of a uniform change over the whole image To a much smaller extent there is also a change in the relative offsets on e
85. characteristics are summarized in Table 2 1 The observatory as a whole is presented in more detail in Chapter 3 the telescope optics in Chapter 4 and the science instrument IRAC in Chapter 6 Spitzer s functional science payload consists of the InfraRed Array Camera IRAC which offers observational capabilities in the near infrared The InfraRed Array Camera IRAC Giovanni G Fazio Smithsonian Astrophysical Observatory Harvard Smithsonian Center for Astrophysics PI IRAC provides images at 3 6 4 5 5 8 and 8 0 um with two adjacent 5 2 x 5 2 fields of view One field of view images simultaneously at 3 6 and 5 8 um and the other at 4 5 and 8 0 um via dichroic beamsplitters All four detector arrays are 256 x 256 with 1 2 square pixels IRAC is described in detail in Chapter 6 Only the two short wavelength arrays 3 6 and 4 5 um operate during the warm mission 8 Spitzer Observer s Manual Table 2 1 Summary of Spitzer Characteristics Aperture diameter 85 cm Orbit Solar Earth trailing Cryogenic Lifetime 5 5 years approx Wavelength Coverage cryogenic mission 3 6 160 um imaging passband centers 5 3 40 um spectroscopy 55 95 um spectral energy distribution Diffraction Limit 5 5 um Image Size 1 5 at 6 5 um Pointing Stability 1o 200s when using star lt 0 1 tracker As commanded pointing accuracy lo lt 0 5 radial Pointing reconstr
86. conducted by the SSC on behalf of the Spitzer observer community The FLS is a 110 hour survey using Director s Discretionary Time The goals of the FLS were to provide a characteristic first look at the mid infrared sky and to rapidly process the data and place it in the public domain in time to impact early Spitzer investigations specifically Cycle 1 planning A detailed description of the motivation and observation planning for the FLS is available on the SSC web pages The first science also included GTO Chapter 2 Introduction to Observing with Spitzer 7 and Legacy Science observing About 6 months after IOC SV was completed Cycle 1 GO observing commenced June 2004 The majority of the original Legacy Science observations which are made public immediately were completed within the first 18 months of the science mission The science Archive opened in May 2004 and initially contained FLS data early release observations and any Legacy data taken and reprocessed with the most current pipelines at that time Cycle 2 GO observations commenced in June 2005 Cycle 3 GO observations commenced in June 2006 Cycle 4 GO observations commenced in July 2007 Cycle 5 observations commenced in July 2008 After the depletion of cryogen on 15 May 2009 Cycle 6 observations commenced in July 2009 2 3 High Level Observatory Description What Can Spitzer Do Spitzer is a 3 axis stabilized pointing and scanning observatory The top level observatory
87. coverage for a map in array coordinates left and celestial coordinates right For large maps which take more than the 24 hour maximum AOR duration you will have to break the observation into multiple AORs For most large maps multiple AORs will have to be used even if a single AOR would be below the maximum duration as there is also a limit to the number of commands in a single observation request This is a hard limit imposed by the spacecraft C DH system Spot will display an error message if you create a request with more instructions than the limit and your observation will have to be done with multiple AORs If your map grid is oriented in array coordinates then you will not know the boundaries of each AOR precisely unless you specify the observing date Therefore care must be taken in planning and managing large observations If the large observation can be broken into separate rectangular map grids they can be specified in individual AORs using the same central target position by specifying a Map center offset see Figure 6 26 An example of how to use the map center offsets is given in the Examples section below In general to calculate observing times for a large map generate an AOR that covers a fundamental tile for your project and multiply the observing time of that tile by the number of tiles needed to cover your desired area Allow for some overlap between tiles both for calibration and for inefficiencies in the
88. d contains one image corresponding to one of the two IRAC arrays the exception being post BCD products described below The FITS headers are populated with keywords including but not limited to physical sky coordinates and dimensions a photometric solution details of the instrument and spacecraft including telemetry when the data were taken and the steps taken during pipeline 120 Spitzer Observer s Manual processing There are three primary image data types that are supplied for each AOR raw BCD mosaic 6 3 2 1 Raw Data Raw data are wholly unprocessed except for those steps necessary to render them into a readable FITS format i e depacketization and decompression This is the form in which data enters the IRAC pipeline Most observers are unlikely to use these data beyond sanity checking of the pipeline data products They are however supplied in the event that observers wish to reprocess their IRAC data in a different manner from the SSC pipeline By comparison to ground based astronomy these are the raw data one gets from a camera and writes to disk while observing at the telescope The raw data consist of 16 bit integer FITS files The data are unsigned integers and the files do not strictly follow the FITS convention however this is the format of the data as delivered The headers are populated with all of the ancillary science telemetry keywords 6 3 2 2 Basic Calibrated Data or BCD BCDs are exposure level data a
89. d the focus mechanism is mounted and thermally connected to the cryostat vacuum shell The barrel baffle of the telescope assembly is separately attached to the vacuum shell at its flange The MIC an aluminum enclosure containing the instrument cold assemblies is mounted on top of the depleted helium tank The CTA was provided by Ball Aerospace and Technologies Corporation The spacecraft and CTA and science instruments together comprise the observatory which is 4 m tall 2 m in diameter and has a mass of 900 kg Chapter 3 Observatory Description and Constraints 17 Telescope ee Outer Shell Spacecraft Shield Spacecraft Bus Star Trackers and Inertial Reference Units High Gain Antenna Low Gain Antennae Figure 3 1 Basic external view of Spitzer The observatory coordinate system XYZ shown in Figure 3 3 is an orthogonal right hand body fixed frame of reference The X axis passes through the geometric center of the top surface of the spacecraft is parallel to the CTA optical axis which passes through the primary and secondary mirror vertices and is positive looking out of the telescope The Z axis intersects the line forming the apex of the two surfaces of the solar panel The Y axis completes the right hand orthogonal frame The X axis origin is defined such that the on axis point between the CTA support truss and the spacecraft bus mounting surface is located at X 200 cm in order to maintain positive X values
90. dated information for CP 7 Chapter 3 Updated information for the Warm Spitzer Mission Chapter 4 No changes Chapter 5 Updated information relevant to CP 7 and the Warm Mission Chapter 6 Updates to provide post cryo IRAC performance information Chapter 7 No changes Chapter 8 No changes iv Table of Contents Major Sections 1 PHILOSOPHY AND SCOPE OF THE SPITZER SPACE TELESCOPE OBSERVER S MA NU AW id sassssrssscsceaicsitccvaciechssesesaccstvesseeuesescsesacdccvssasv esesinbescevencsusscessucstsesvaessctsscocevascsesundscvceseesetesssoetiet 1 141 INTENDED AUDIENCES ssce3 cissavadesesteetacecucdeccakes cast gea E ROE E A E EER OEE 2 1 2 DOCUMENT ORGANIZATION 6 scccsssessesectesceeseccscossesnesatencessenacoansdeseacccesencenssteussanens R irise 2 13 SDOCUMENT UPDATE PLANS ccc sccdcssisceesdssccseeeecacsescobegssctensaves Sada sais decdeuuvevuccaesuavaseds ss saevugevenssueees 3 1 4 OTHER RELEVANT DOCUMENTS cccssssssssesssessseeseesessecsceesceeseesscensscesesesesscesesesessesseesseeestenaes 3 2 INTRODUCTION TO OBSERVING WITH SPITZER eeesseesoessecsscoesocesocesoroesocesocesoeesocesoese 5 2 1 CATEGORIES OF OBSERVING PROGRAMS csccssscesseessesseeecesceseeseeeseesseeesecesecesecseeeeseceseeeeeaes 6 2 2 MISSION OVERVIEW Vestas avn ee cee ha aunt anenaeneuunn va E cease 6 2 3 HIGH LEVEL OBSERVATORY DESCRIPTION WHAT CAN SPITZER DO cccccceesseeseeseeeseeeees 7 2 4 SPITZER SCIENCE
91. ddition to the discussions of best practices in the IRAC chapter There is also a moving target example in the Observation Planning Cookbook found in the Proposal Kit on the SSC website 5 5 6 1 Choosing your targets Whereas fixed targets can have absolute time errors on the order of an hour usually executing earlier than expected moving targets can be executed with an Chapter 5 Observing with Spitzer 55 absolute time error of only 0 3 seconds Please be aware of this if you are planning to observe a Solar System target as an absolute time fixed target observation Generally it is better to request moving target observations i e with tracking for Solar System observations Remember to check for availability of positional error information for the ephemerides for your targets instructions in the document entitled Horizons tips for Solar System Observers found on the SSC website Some objects have extremely poor positional information e g several 100s of arcseconds and would have a very low probability of being acquired by Spitzer There are 3 known ways of dealing with targets in this situation 1 Select another target of comparable scientific interest but with better ephemeris accuracy 2 Use ground based support facilities to acquire astrometric observations of your proposed target Please send these data in MPC ingest format to the Helpdesk help spitzer caltech edu and we will ensure that they are included
92. decsbescteaeenate ceive 6 2 3 Astronomical Observation Template AOT Description n se 97 6 2 3 1 6 2 3 2 6 2 3 3 6 2 3 4 6 2 3 5 Readout Modes and Frame Times s seseeeeeeseerseresresreerreeeeee Map Grid Definition Selecting the IRAC Field of View Selecting Arrays to Take Data With Dither Patterns ow 6 2 3 5 1 Dithering strategies eee is 6 2 3 5 2 Scales and pattern selection in Spot escseseseseseeeeeseeeeeeseeeeeeceseseeseaceearesseeesees 6 253 535 OUD pIKel Cheri Geese Selec e E N Aa Noe e ra a eNe 6 2 4 IRAC AOT Recommendations and Examples 6 2 4 1 Best Observing Practices with IRAC ss sesseesesssesssrsststsseestrsresrerrerttsstesresrrterrenresreseseenres 6 24 11 Dithering vs in place repeats sessesesecesesesesesesescseeeseseeeessesesneeeeereesesenenenees 6 2 4 1 2 HDR mode eee Sie GC 2AM Sy SUGATAN RE eE eee neat eset RON testes Setar ety OD AIA sMappin eerste tathrtectt E el ibe ott csi ncaa E ait cated naa 6 2 4 1 5 Observing near bright targets 6 2 4 1 6 Observing single faint sources 6 2 4 1 7 Confusion and other background issues 6 2 4 1 8 High precision relative photometric monitoring 6 2 4 2 Shallow Survey Example 114 6 2 4 3 Deep Image Example 114 6 2 4 4 Elongated Object Examples idise siveri eoet rea ENE akaraia 115 OAA lt Array coordinata Sa niren ena oe ea octet EE AEE T be eticceetees 6 2 4 4
93. ding observation of a bright star that may have used the same dither pattern as your science observation This offset is most easily accomplished by using a cluster target with array coordinate offsets entering the offset as the first and only cluster position and then selecting the option to observe offsets only 6 2 4 1 7 Confusion and other background issues If you know the accurate better than 0 6 positions of your sources and you are only interested in measuring their fluxes then the classical confusion limit is not really relevant It is generally straightforward to align an IRAC image in celestial coordinates to lt 0 6 and often 0 2 accuracy can be achieved when there are multiple 2MASS sources in the field To separate faint asteroids from more distant targets it is a good idea to observe your field of view at least twice separated by a few hours at least if your source lies at a low ecliptic latitude within 15 of the Ecliptic or so 6 2 4 1 8 High precision relative photometric monitoring For observations of exoplanet transits and other sources requiring high precision relative photometry it is important to minimize systematic effects in the data In this special case we recommend observing with repeats and not dithering By staring for the entire observation variations in photometry due to moving the source to other pixels on the array or other positions on a pixel are minimized The longest possible frametime
94. during IOC SV Each of the fields of view that are used by Spitzer has a code that specifies the position in the focal plane The uplink software uses the field of view index for commanding the target position and the pipelines similarly use it to reconstruct where the telescope was pointing These indices and field of view names appear in the headers of Spitzer data and are listed in Table 4 4 Table 4 4 Field of View Indices FOV name FOV index Telescope Boresight 1 PCRS 1A 4 PCRS 1B 5 PCRS 2A 8 PCRS 2B 9 IRAC center of 3 6 array 67 IRAC center of 3 6 array 68 IRAC center of 3 6 subarray 70 IRAC center of 4 5 array 74 IRAC center of 4 5 array 75 IRAC center of 4 5 subarray 77 IRAC between arrays 81 5 Observing with Spitzer 44 Spitzer Observer s Manual The Spitzer observer s interface to the observatory is the AOT AOT is a central design concept in Spitzer s science operations The use of relatively simple parameterized observing modes enhances the reliability of Spitzer observations and calibration improves the archival value of Spitzer data and reduces cost This chapter describes the IRAC Post Cryo Mapping AOT introduces the concept of an AOR introduces the SSC provided tools for planning and creating observations and describes some of the rules and procedures related to observing with Spitzer Special types of observations such as Solar System Ob
95. e IRAC AOT the 0 6 and 1 2 sec frame times come as part of the high dynamic range HDR sequences The frame sets that are taken for each pointing in HDR mode are shown in Table 6 9 The last column 7 gives the extra time spent taking the HDR frames 200 and 400 second frametimes were tested during WIC but determined to provide no improvement in sensitivity over the 100 second frames and were dropped as they significantly increase the standard calibration time Table 6 10 to Table 6 12 and Figure 6 22 to Figure 6 24 present the predicted sensitivities for the two IRAC channels for each of the three background models The sensitivities in the tables are for point sources extracted from single images but perfectly flat fielded In the figures the sensitivities are for point sources extracted from coadded images perfectly registered We do not include confusion noise due to overlapping images of distant galaxies or other sources of background structure in the sensitivity estimates The detectors are assumed to perform according to the IRAC detector measurements of read noise dark Chapter 6 InfraRed Array Camera IRAC 91 current and quantum efficiency in cryogenic observations with a slight 5 increase in read noise at 3 6 um and the in flight measured throughput for warm operations The first 9 rows in each table show the sensitivity for full array readouts and the last four rows show the sensitivity for subarray readout
96. e position at the coordinates specified in the Target section of the AOT Dithering can still be applied even if the No mapping option is selected With No mapping selected if you select both fields of view first the 4 5 um field of view will be pointed at your target then the telescope will be repositioned so that the 3 6 um field of view is pointed at your target In both cases data from both arrays are collected whether they are pointed at your target or not unless you choose to take data in only one channel see section 6 2 3 4 100 Spitzer Observer s Manual Column Row How Many ig l C Step Size 280 0 277 0 N Orientation Array v Number of times to execute this map 1 Cance Figure 6 26 Map grid dialog box in Spot If you select Yes under mapping mode then you will be presented with a dialog box shown in Figure 6 26 wherein you specify the rectangular map grid The map grid may be specified in either array or celestial coordinates If you choose array coordinates then the map grid will be aligned with the edges of the array such that the map rows and columns will correspond to rows and columns of the array Specifically a column will be along a line of constant solar elongation and a row will be along an ecliptic parallel line of constant ecliptic latitude In the reference frame of a BCD columns are in the negative y di
97. e will be their appearance in the latest online weekly schedule and in the above mentioned e mail message We will do our best to maximize the scientific return via scheduling but specific requests for ordering of AORs in the schedule cannot generally be accommodated 5 4 3 Requesting Scheduling Constraints for Science Reasons The scheduling software is able to handle common types of constraints and logical linkages such as those needed for periodic monitoring of a target However because scheduling constraints restrict the ability of the scheduling system to produce efficient schedules and may render scheduling some observations infeasible the usage of any type of timing or relational constraint requires a very strong scientific justification The justification must be provided at the time of proposal submission or modification if the constraint has been subsequently added 5 4 3 1 Recommendations for the Use of Scheduling Constraints If a scheduling constraint is required it is strongly recommended that the minimum constraint necessary to preserve the scientific goal of the observation s be applied In general larger timing windows and or loose groupings are preferred to non interruptible sequences of observations For very long observations strategies which permit independent scheduling of the component AORs are much preferred this not only enhances scheduling efficiency but also makes the observation as a whole much more robust ag
98. ea in the output mosaic by rotating the output projection such that it is aligned with the map axes This is useful for long thin maps where potentially the output mosaic could be very large but with a great deal of empty space The mosaicker then reprojects all of the input data onto the output projection It reads the SSC WCS which contains the field pointing center rotation scale and instrument distortion and reprojects this onto a standard TAN FITS projection In the process the data are undistorted The reprojected images are interpolated onto the fiducial image frame with outlier rejection rejecting radiation hits that happen in overlapping observations The outlier rejection scheme is specifically designed to work well in the case of intermediate coverage and may not be adequate for all observations and science programs In addition to a sky map in units of surface brightness a noise image and coverage map are also produced The post BCD pipeline modules have been made available for general public use by the SSC They consist of a number of C modules connected via PERL wrapper scripts Namelists are used for input In most cases their operation simply consists of supplying the software with a list of input image files by default they read and understand the IRAC image headers 6 3 3 Data Processing 6 3 3 1 Overview The main processing of the IRAC science data takes place in the BCD pipeline which is a set of modules tha
99. ed imperceptibly The satutation limits are slightly lower while the necessary photometric corrections for high accuracy Chapter 6 InfraRed Array Camera IRAC 61 photometry have increased Long term residual images are no longer an issue but in general residual images latents survive longer Our understanding will continue to improve as ongoing analysis is completed This document will be updated periodically with these revisions At the time of this update January 2010 the IRAC team has just completed an analysis of the linearity for the final warm setpoints We are currently updating the flux conversion values and photometric corrections using this latest linearization These values are likely to change by 2 3 but the values in this document should be sufficient for planning purposes A brief high level summary of IRAC for astronomers appeared in the ApJS Spitzer Special Issue specifically the paper by Fazio et al 2004 ApJS 154 10 entitled The Infrared Array Camera IRAC for the Spitzer Space Telescope and in the paper by Hora et al 2004 SPIE 5487 244 entitled In flight performance and calibration of the Infrared Array Camera IRAC for the Spitzer Space Telescope Copies of these papers are available on the SSC website In the remainder of this chapter we will use cryogenic to refer to data and operations of Spitzer IRAC before the exhaustion of cryogen During this phase the IRAC short wavelength
100. ed to facilitate proposals that address emerging scientific topics 2 2 Mission Overview Spitzer was launched from Cape Canaveral Florida into an Earth trailing heliocentric orbit on 25 August 2003 The observatory was launched with the telescope at ambient temperature only the focal plane instruments were cooled to cryogenic temperatures The telescope gradually cooled to 6 K over a period of 45 days Following launch Spitzer entered a 63 day In Orbit Checkout IOC phase followed by a 35 day Science Verification SV phase during which time the planned capabilities of the telescope were verified the detailed performance characterized and the three science instruments and their operational modes commissioned Following completion of IOC SV Spitzer was commissioned for routine science operations on 1 Dec 2003 The prime science mission continued until the exhaustion of the onboard cryogen used to cool the telescope and science instruments The Spitzer cryogenic lifetime requirement was 2 5 years of normal operations which was passed on 26 April 2006 The cryogen was finally depleted on 15 May 2009 After the end of cryogen the Warm Spitzer Mission began with an in orbit checkout science verification of the spacecraft and single operating science instrument IRAC and then Cycle 7 science observations began in late July 2009 The first science observations during the cryogenic mission included the First Look Survey FLS which was
101. effect The latest version of Spot includes the option to overlay all known moving objects on an image for a particular date Observers concerned about bright moving objects not included in the list above may wish to use this option It should be noted that it is the observer s responsibility to check for any bright inertial targets in the field of view that might compromise the observation There are some that will heavily saturate the instruments and therefore compromise not only that observation but also possibly subsequent science observations as well A list of some of the inertial objects known to saturate IRAC is available on the SSC website You can also visualize bright objects near your AOR in Spot Spot does not exclude observations of these objects The SSC reserves the right to exclude some targets and AORs as a result of impacts that bright objects would have on subsequent observations 3 3 Pointing Capabilities 3 3 1 Pointing Control System The PCS includes the hardware and flight software necessary for precision telescope pointing stabilization slewing tracking and safe mode functions The PCS performed the initial attitude acquisition of the spacecraft following launch vehicle separation It provides periodic boresight calibration for the telescope The PCS provides the capability for both rapid large angle slews and small maneuvers to place and reposition science targets within the science instrument apertures it maintain
102. ent Calibrations for Normal Observations Some calibrations occur extremely infrequently Examples of these are linearization distortion mapping etc Other calibration measurements are repeated more frequently These calibration observations will be performed at specific intervals as required rather than having specific calibration observations for each individual data set Every 24 hours observations of secondary calibration stars will be obtained for example Darks and skyflats are derived every seven and fourteen days respectively 6 3 2 Data Products This section describes the basic data products the observer will receive from the SSC More details are contained in a the JRAC Pipeline Description Document which describes the algorithms implemented in the IRAC pipeline and is available on the SSC website and b the IRAC Data Handbook which provides a detailed guide to the data products and how to use them and is available on the SSC website The data delivery consists of a directory hierarchy with a name unique to that AOR In this hierarchy are the BCD data as well as a number of subdirectories containing the raw data the calibration files log files and the post BCD data The exact contents of the data delivery vary according to what the observer has requested from the SSC data archive via Leopard IRAC data are supplied as standard FITS files Each file consists of a single data collection event i e a single exposure an
103. ernal scattering the only source of an extended to point source throughput change is present for the 3 6 and 4 5 um arrays in either the warm or cryogenic missions The optical properties of IRAC and the telescope should be almost identical to those in the cryogenic mission The 30 K lab tests suggest that the read noise will remain close to the cryogenic values however the actual read noise and its variation with temperature is a property of each and every individual array We have increased the read noise slightly by 5 at 3 6 um to account for the slightly poorer performance measured for deep imaging at 3 6 um At 4 5 um the deep observation sensitivity is unchanged and we use the cryogenic read noise values Chapter 6 InfraRed Array Camera IRAC 89 Table 6 6 lists some useful combinations of IRAC instrument parameters Table 6 6 Useful quantities for IRAC sensitivity calculations Wavelength 3 6 um 4 5 um Conversion factor electrons sec MJy sr 25 29 S electrons sec uJy 0 77 0 89 Gain electrons DN 3 7 Ber J throughput correction for point sources 9 96 0 81 f throughput correction for background 1 l Zero magnitude flux densities Jy 280 9 179 7 Table 6 7 Background brightness in IRAC wavebands 3 6 um 4 5 um low background model 1 fs MJy sr 0 093 0 32 FPC Jy 3 2 11 B elec sec 2 5 9 9 medium background
104. ervations without losing dynamic range we provide a high dynamic range HDR option When this option is selected the IRAC AOT will take extra frames with frame times shorter than your selected frame time The HDR frame times are given in Table 6 9 You can use the shorter frames to measure the brightness of the brighter sources and the longer frames to reach your sensitivity goal No spacecraft repositioning is done between frames and the frames always go from shortest to longest If you elect to perform repeats using the Number of Frames entry box on the AOT form then only the longest frame time is repeated However if you Chapter 6 InfraRed Array Camera IRAC 99 elect to perform dithers then the entire frame set is repeated at each dither position Independent of the HDR option taking frame repeats is recommended only in the case of time series measurements This is because the first frame effect is worse with repeats and image artifact rejection is best accomplished with dithers The repeated frames will need additional processing to remove residual bias patterns For very bright sources which would saturate the array in full array readout mode you can select subarray mode In this mode only a small 32x32 pixel portion of the array is read out so the field of view is only 38 x38 Mapping is not allowed in subarray mode However small maps can be made using a cluster target Therefore use subarray mode only for o
105. es of relational constraint supported by Spitzer Chain Ordered Non interruptible Group A chain can be thought of as a list of AORs that must be executed consecutively in the order specified and without any other kind of activity intervening Note that the total time for the entire ordered non interruptible group cannot exceed the maximum time for an individual AOR for longer observing sequences an interruptible group must be used This type of constraint might be used for an on off source pair of observations Sequence Ordered Interruptible Group A sequence constraint is similar to but less stringent than a chain constraint The AORs will be executed in the order specified and a duration in which they should be completed is specified The sequence Chapter 5 Observing with Spitzer 51 constraint should only be used when the science requires sequential ordering of the AORs For AORs in which the order of observation is not important a group within constraint see below should be used instead of a sequence constraint Group within A group within constraint specifies that a group of AORs will be executed within a specific length of time but with no particular starting date time constraint Once the first AOR has been executed the rest of the AORs in the group will begin within the specified time interval They may be executed in any order within the time interval This is similar to a sequence constraint but the observations may
106. et have a NAIF ID the observer will still be able to send the orbital elements to the Spitzer Helpdesk help spitzer caltech edu and request generation of an ephemeris for planning purposes Requests received at the SSC for non standard ephemerides during the last week of the Proposal Call will be supported on a best effort basis Note that observations which require ephemeris updates lt 5 weeks prior to execution significantly perturb the scheduling process and must be identified in the proposed AORs at the time of submission via the special button in the Spot Chapter 5 Observing with Spitzer 53 AOT window Late ephemeris updates are considered equivalent to medium or high impact ToOs depending upon the amount of lead time given see Spitzer Observing Rule 5 and will incur a special observing overhead for more details see http ssc spitzer caltech edu documents rules overheads html Late ephemeris updates i e specification of the final ephemeris less than 5 weeks prior to scheduling are no longer supported in GO proposals Any observations requiring a late ephemeris update must be submitted via a DDT proposal 5 5 3 Target Specification and Scheduling Considerations SSO observations will normally be performed as tracked observations with both the position and the track rate direction computed from the orbital elements derived as discussed in the previous section 5 5 3 1 Shadow Observations The infrared flux fr
107. even if this makes the observations somewhat less efficient The use of a timing constraint to obtain a specific orientation requires a very strong scientific justification when the observations are proposed See the Call for Proposals for more information 3 2 6 Bright Object Avoidance No object outside the solar constraint zone poses a threat to instrument safety However an observer may wish to avoid observing the Earth and other bright moving objects to avoid compromising observations of faint targets Therefore the visibility windows calculated by Spot avoid certain bright moving targets by default although the observer can choose to override the default With bright object avoidance turned on the default the visibility windows calculated for both inertial and moving targets will exclude regions of time when the positions of a the Earth and Moon and b a fixed list of bright moving objects e g Jupiter Saturn bright asteroids coincide with the target position 28 Spitzer Observer s Manual The visibility windows will be trimmed to delete any time periods when the Spitzer target is within 7 of the Earth or Moon and when the target is within 30 of the other bright objects The complete list of the bright moving objects is in Appendix 7 3 The observer may choose to override the default a Earth Moon or b other bright object avoidance For example to observe Jovian satellites one would turn off b and leave a in
108. folding times for warm residuals are still Chapter 6 InfraRed Array Camera IRAC 75 being characterized Typically warm residuals are lt 0 01 of the fluence of the source after 60 seconds The residual image never exceeds 1 of the illuminating source for integrations beginning immediately after the illumination ends Usually the residual will be orders of magnitude smaller but the residual image of a star can persist as a false point source for up to several hours for the most saturating sources followed by low background deep observations For reference Figure 6 11 shows the residual decay for a strongly saturating source 3 K magnitude star 8 8 Jy observed for 12 seconds at 3 6 um The data residuals are measured using aperture photometry of subsequent 12 second frames and the residuals are averaged over 6 minute intervals In this extreme example the residual persists above the background for more than two hours after the residual is produced 120 EE a a a tv E S S E E E E E E S E D E E E E Averaged residual Averaged background i 100 80 gt lc 5 L L 60 O L 7p ab L an L 40 20 0 fp Pg Sf E E ae ets eae fh ge gt Ih ea liri L i 0 0 0 5 1 0 1 5 2 0 2 5 Time hours Figure 6 11 Residual image brightness decay as a function of time interval since exposure to a bright source at 3 6 um The residual is compared to the sky background in an equivalent ape
109. fter having passed through the IRAC pipeline There is one BCD for each integration taken by IRAC Instrumental signatures have been removed as described in section 6 3 3 2 and the BCDs are absolutely calibrated into physical units i e MJy sr 107 erg s cm Hz sr In addition the pointing refinement see section 6 3 2 4 based upon 2MASS catalogs will be supplied in the BCD header Continuing the analogy with ground based observing the BCDs are data that have been reduced but not yet combined into a final image This is the primary science data product produced by the SSC Ancillary files are supplied with each BCD see Table 6 16 These ancillary files contain several types of information regarding each pixel in each image A version of the BCD with correction of bright source artifacts is produced cBCD In general it should be better than the BCD however the corrections are not always robust An image containing an estimate of the uncertainty for each pixel is supplied A mask image contains status bits indicating the probability that any given pixel has been affected by a radiation hit whether or not the linearity solution could be applied if a pixel is saturated dead hot always on or abnormally noisy Pixels with strong residual images are flagged in the mask file as are those with artifacts induced by bright sources Log files are also supplied and from these and the header keywords the entire pedigree of every data pr
110. g the equations above is excessive you may prefer to create two AORs one per field of view 6 2 3 4 Selecting Arrays to Take Data With A new feature of the warm IRAC AOT is the ability to turn off data taking in one of the channels By default observations take data in both channels This new mode is intended for staring observations of a target in a single field of view such as exoplanet transits such that the off field does not contain useful data and the data volume of the observation is high This option should only be selected when instructed by the SSC to reduce the data volume of a given observation 6 2 3 5 Dither Patterns Dithering performs a number of functions It allows the identification and removal of small scale detector defects such as bad pixels it reduces noise from the effects of pixel to pixel errors in the flat field in fact for large numbers of dithers a flat field can be constructed from the data themselves and sub pixel dithering can be used to recover some level of information which would otherwise be lost through undersampling of the array These considerations drive the optimization of the dither patterns in different ways there is thus no ideal dither pattern For the standard IRAC dither patterns we have chosen designs that we think should please most observers most of the time However the option always remains for observers to define their own dither patterns using array offsets in cluster mode
111. galaxy oriented 60 E of N The galaxy has an optical size of 13 5 by 2 5 and we want to cover about 17 with both fields of view The galaxy is very bright so we need to use a short frame time 12 sec will work to avoid saturating or operating exclusively at the high end of the linearity curve To get the desired sensitivity and source confirmation we take 5 frames using the small scale cycling dither pattern In this example we will perform the observation two different ways to compare the results 6 2 4 4 1 Array coordinates In order to cover the desired area regardless of schedule date we will need to make the map much larger than the galaxy size Using the equations above to cover 17 we need 5 columns and 4 rows We select both fields of view to center the image on the nucleus The duration of the observation is approximately 2390 sec The map grid size is equal to the array size It is possible to make a smaller map tailored to the size of the target if we fix the observing date For a very large observation fixing the date may be the best solution But for a small observation such as this one a better solution is given in the next subsection 6 2 4 4 2 Celestial coordinates Another way to observe the elongated galaxy that is both efficient and independent of scheduling constraints is to observe a map grid in celestial coordinates To get the desired depth of coverage at each position we could use many different opti
112. gt Source Lists Flag artifacts Imask Merg ed Source Lists Post BCD Figure 6 34 IRAC pipeline Chapter 7 Appendices 129 7 Appendices 130 Spitzer Observer s Manual 7 1 Infrared Flux Units The infrared flux density from a point source is most commonly given in units of Jansky Jy where 1 Jy 10 erg s cm Hz 107 Watts m Hz F 10 1 The conversion between Janskys and flux density in W m per unit wavelength is given by F x10 x c F 10 2 The infrared flux density from an area on the sky such as the surface brightness of an extended object or the background emission is commonly given in 10 Jy steradian 1 MJy sr Another common unit is Jy per square arcsecond 1 MJy st 2 350443 x 10 Jy arcsec Table 7 1 Table 7 3 list the zero magnitude fluxes for various common optical and infrared filters Note that the photometric system of filter sets can vary depending on the manufacturer The magnitude of a source can be converted to a flux density using F Fo x 10 10 3 There is an on line Javascript tool available from the SSC website that interactively converts Janskys to magnitudes or any of a variety of flux units and vice versa Table 7 1 2MASS system zero points Passband Effective wavelength Zero point um Qy J 1 235 1594 H 1 662 1024 Ks 2 159 666 7 References Table 2 from Cohen Wheaton amp Megeath 2003
113. hat is well centered in a pixel It was determined from the images of the focus star after the telescope was focused that were the most symmetric and concentrated These values for the flux in the central pixel were used in the saturation predictions in the following sections of this document The flux in the central pixel for a random observation will be lower depending on the centering of the source with respect to a pixel Chapter 6 InfraRed Array Camera IRAC Chil Ch2 T T T LES nk ee eae bee bore eeen vanal LAAVAA AARE L IAE L ee e A a OV KSEETEEETLELLIZ NNNNN ADEE re a a a a ee EEL PA LIS Lb A RSSA EA Cnn ore ae Base a oe ea a a ike do IIASA AANEREN A ATR FOR RR Re Ra ee ae RR A Aa gear ace Sgt ee IES SSS T 200 rsrsrs serirrrrreresiissn gt 200 ae eS D rr wna n ec cee E E ere wc A A E DY a eee Oe hp eK Oe x SR AUR AUS EEEE EEE TE ETERNE ET Se rrr an Wa ain eae ta E S D a A e EA ot Ge EO E O EE as oes wee Raced Se FP aa eek ee wee eel Rene vay erence SN E E cbs ods aa ao as pa gt a gt a EEE E EEEE S O wre rrr re Kerestin e ree ee SE mo O aseran ea p EERE ee e Sree eia Boo Pasaba eer A E oe aa a E E EEE EE E ee des ae BEO Aa hamei aeae e ee A IOO eere i e REA Se ea a gt Ti MOO bak nee eee ek ee ae ey O J ai adaa a BE TETAS Aaa ata S L o Jerressasesrrerisasaasssan A ane R E RATEN EE P aeS OG aap GREE RET BOR E RO A AAAA r gt Joe a RS Se Ro Ee a Wy A N J gt ee EEEE REREE EEEE S
114. he background current is B SI yy f 52 pic Sex 5 7 6 3 and the effective exposure time is T T 0 2N 6 4 6 2 In these equations the spectral resolving power A AA is from Table 6 3 the detector quantum efficiency Q electrons per photon is from Table 6 4 the instrumental throughput n is from Table 6 3 the telescope throughput nr 0 889 0 902 for channels 1 and 2 respectively with Be primary Al coated secondary and 50 nm ice contamination the telescope area including obstruction 4 4636 cm the equivalent number of noise pixels N jx is from Table 6 1 and defined in section 6 1 2 2 h is the Planck constant Ing is the background surface brightness in MJy sr fs 1 2 is the stray light contribution to the background the dark current D is given in section 6 1 3 2 7 the read noise R is from Table 6 4 Q x is the pixel solid angle see Table 6 1 f is the in flight estimated throughput correction for point sources Table 6 6 fa is the in flight estimated throughput correction for the background Table 6 6 The throughput corrections f and f were determined initially by comparing observed to expected brightness of stars and zodiacal light during cryo operations Stars were measured in a 10 pixel radius aperture The point source throughput was updated using the warm measurements of the same calibration stars The extended source throughput was not changed as there is no indication that int
115. he detector array and the point response function PRF is after sampling by the detector array More information is available on the SSC website The PRFs which are undersampled at the IRAC pixel scale were generated by combining 108 individual IRAC images in each band By offsetting each image by a fraction of a pixel width fully sampled PRFs can be extracted from the data The resulting PRFs are the optical point spread function projected onto the focal plane by the IRAC and telescope optics convolved with the response function of a single detector pixel The images were combined using a drizzle algorithm Fruchter amp Hook 2002 PASP 114 144 to minimize smoothing of the PRF during the reconstruction process The resulting PRF sampling was 1 4 the width of an IRAC pixel Images of a bright star at the native IRAC pixel scale are also displayed for comparison FITS images of IRAC PRFs are available on the SSC website The appropriateness of a given PRF is dependent on the observation sampling and the photometric reduction package used see IRAC Data Handbook The best current PRFs are available at the SSC website http ssc spitzer caltech edu irac psf html Chapter 6 InfraRed Array Camera IRAC 65 3 6 microns 4 5 microns Square Root Sealing Leganthmile Sealing 1 2 Pixel Scole Square Root Scaling Figure 6 4 The in flight IRAC point response functions PRFs at 3 6 and 4 5 um The PRFs were constructed using cr
116. he observational constraints that arise from that design The Spitzer Space Telescope consists of five basic elements the cryogenic telescope assembly CTA the spacecraft and the IRAC instrument In this section the spacecraft and CTA design not including the telescope design see Chapter 4 are discussed The design and performance of the pointing and control system PCS are also discussed A brief high level summary of the mission for astronomers appears in the ApJS Spitzer Special Issue specifically the paper by Werner et al 2004 ApJS 154 1 entitled The Spitzer Space Telescope Mission A copy of this paper is available on the SSC website 3 1 Observatory Design and Operations Concept A basic external view of Spitzer is shown in Figure 3 1 The spacecraft provides structural support pointing control and telecommunications and command data handling for the entire observatory The spacecraft including the spacecraft bus the solar panel and PCS including the Pointing Control Reference Sensors or PCRSs which are located in the focal plane were provided by Lockheed Martin Space Systems Company The CTA shown in Figure 3 2 consists of the telescope the superfluid helium cryostat the outer shell group and the Multi Instrument Chamber MIC which hosts portions of the IRAC camera and the PCRS The telescope assembly including the primary and secondary mirrors metering tower mounting bulkhead all made of beryllium an
117. he pipeline will incorporate a better correction However as the corrections are not always robust the BCDs can be used when the corrections are suspect The online post BCD pipeline uses the corrected BCDs cBCDs At this stage the BCD has been produced and consists of a single image corresponding to each exposure made by IRAC The header of the image contains the standard FITS keywords and the image distortion coefficients as well as sufficient information to trace the calibration and pipeline pedigree This image is calibrated in physical units and has a pointing solution attached Further processing to generate the extended pipeline products mentioned above will use the cBCD files as input Post BCD pipeline modules are available on the SSC website to match the background brightnesses of overlapping images take care of column pulldown see the IRAC Data Handbook for more information register images relative to each other using sources in the overlapping region and generate mosaics from all the images in an AOR Another set of pipeline modules is under development to extract point sources from the images and measure their flux densities using dedicated observations of the point spread function place the flux densities into source lists for each wavelength and merge the wavelength dependent source lists The algorithms have been designed to be robust and strive for completeness at the 100 level thus they do not reach the ultimate sens
118. hese constraints represent scheduling directives for an AOR or for a related group of AORs The details of the kind of constraints that are supported are in section 5 4 3 Timing constraints are used to specify a window when an AOR should be executed e g to observe a comet at maximum solar elongation Relational constraints are used to specify how AORs within a group are related to one another e g a series of AORs that define a very deep map and must be executed consecutively 5 2 Science User Tools Science User Tools are software packages and other materials such as tables and graphs that are provided by SSC to help the astronomical community plan prepare submit monitor and interpret the results of their Spitzer observations They are all available from the SSC website specifically in the Proposal Kit area http ssc spitzer caltech edu We highlight a few tools here for illustrative purposes Note there is also a perl based tool available from the Proposal Kit area allows proposers to format the Observations Summary Table portion of the proposal template using the AORs to be submitted 5 2 1 Spot Spot is a multi platform JAVA based client server GUI driven software tool intended to assist potential and approved Spitzer observers in planning and modifying their observations Spot along with the Spot User s Guide can be downloaded from the Proposal Kit section of the SSC web pages http ssc spitzer caltech edu It all
119. icipate a pixel to pixel accuracy of the flat fielding derived from a 14 day span containing two flat field observations of 3 4 and 1 6 lo for channels 1 and 2 respectively Using combined flats 118 Spitzer Observer s Manual super sky flat from the first two years of cryogenic observations the estimated pixel to pixel accuracy is 0 25 and 0 2 in channels 1 and 2 respectively During the final warm setpoint calibration we began to construct the super sky flat for the warm mission The initial flat has errors of 0 25 and 0 12 in channels 1 and 2 Users should note that the flat field data are generated from the zodiacal background and are appropriate for objects with that color There is a significant color term of order 5 10 for objects with a Rayleigh Jeans spectrum in the mid infrared such as stars see the IRAC Data Handbook for more information This color term has changed from the cryogenic version as discussed in Section 6 1 3 2 4 Note that for deep survey observations and other data sets with a large number of frames and a good dithering strategy the system gain could be determined by the actual survey frames themselves rather than using the standard set of dedicated observations of some other part of the sky The dither and mapping pattern need to be specified to optimally relate each pixel in the array to all the others as well as mapping out the region at the required sensitivity Tests of self calibration
120. idance Also see the list of bright inertial objects on the SSC website Earth e Moon e Mars e Jupiter e Saturn e Uranus e Neptune e 4 Vesta e 6 Hebe e 1 Ceres e 7 Iris e 15 Eunomia 8 Acronyms Glossary 134 Spitzer Observer s Manual Note The following lists the acronyms used in this manual A larger list of acronyms is available on the SSC website http ssc spitzer caltech edu Table 8 1 Acronyms used in this manual AOR Astronomical Observation Request s AOT Astronomical Observation Template s BCD Basic Calibrated Data C amp DH Command and Data Handling CA Cryogenic Assembly cBCD Corrected BCD CE Combined Electronics CMOS Complementary Metal Oxide Semiconductor CTA Cryogenic Telescope Assembly DCE Data Collection Event Dec Declination DN Data Number DSN Deep Space Network FAQ Frequently Asked Question FITS Flexible Image Transport System FLS First Look Survey FOV Field of View FPA Focal Plane Array FWHM Full Width at Half Maximum GSFC Goddard Space Flight Center GO General Observer s GTO Guaranteed Time Observer s HGA High Gain Antenna HST Hubble Space Telescope ICRS International Celestial Reference System IER Instrument Engineering Request like an AOR but for specific engineering tasks not able to be accomplished using an AOR IOC In Orbit Check
121. ientation of the BCDs for the cryogenic and warm missions is identical The photometric calibration appropriate for point sources is applied The pixels are then in units of surface brightness MJy sr Chapter 6 InfraRed Array Camera IRAC 125 18 All of the DCEs from the present AOR are used to determine where there are potential residual images The data pipeline does not correct the BCD for residual images since this correction may be quite complex and dependent on a number of different factors and hence cannot be easily executed in an automated fashion However we determine which pixels are likely to produce residual images above the local noise level and we flag those pixels in subsequent DCEs based on the residual image decay curve Flagging for warm data is currently incomplete as the SSC is still determining the appropriate source brightness thresholds 19 The 2MASS data are used to refine the pointing The refined pointing keywords are added to the header of the final BCD product at this stage 20 The BCDs are copied and further processed to remove instrumental signatures caused by bright sources The corrections are empirical and fit directly to the data Artifact corrected BCDs and uncertainty images are created and the mask file is updated In general the corrected BCDs cBCDs should be better and used in place of the BCDs The current version of the warm cBCDs only corrects in part the column pulldown Later versions of t
122. in ground tests with the shutter closed Figure 6 18 shows the dependence of dark offset on the interval between 30 sec frames Each symbol is the mean dark signal in one output of each array averaged over a box near the center of the array The dashed curves are multiple exponential fits to the means in the set of 48 images with the first image excluded The lower portion of each panel shows the residuals in electrons between the data and the fitted curves The residuals are small compared to the expected sky backgrounds in 30 sec frames see Table 6 7 During cryogenic operations channel had very significant variations in offset compared to the low background in that channel Channel 2 had very small variations in offset In the warm mission the first frame effect is less pronounced in total offset in both arrays At 3 6 um the average offset is 40 DN compared to 80 DN in the cryogenic mission for delay intervals of 7 to 40 seconds During the final setpoint calibration data were collected to create a warm first frame correction These data are currently being analyzed and the correction will be incorporated into a future version of the pipeline It is possible that the residual pinstriping and pixel to pixel variations will be more significant than in the cryogenic 3 6 and 4 5 um data For both arrays a gradient in the first frame effect has been noted These residuals will mainly affect measurements of diffuse low surface
123. ing 90 Spitzer Observer s Manual combined flats super sky flat from the initial warm characterization the estimated fr is 0 25 and 0 12 in channels and 2 respectively Using these values for fr in equation 6 1 single frames are dominated by background and read noise When combining multiple frames to generate a mosaic the background and read noises will average down as square root of the number of frames while the flat field noise will only average down for dithered observations For N undithered observations on the medium background flat field noise dominates when the total exposure time N x T exceeds approximately 210 sec using individual campaign flats or 0 8 hrs using the super sky flat For dithered observations the flat field noise will also average down and will only be important for the very deep observations of high background fields Table 6 8 Fowler numbers for IRAC frames Frame Time sec Readout Mode Fowler Number Wait Ticks 100 Full 32 16 436 468 30 Full 32 16 86 118 12 Full 8 44 6 Full 8 14 2 Full 4 2 2 Subarray 8 184 1 2 HDR 1 4 0 6 HDR 1 1 0 4 Full 1 0 0 4 Subarray 4 32 0 1 Subarray 2 6 0 02 Subarray 1 0 For the frame times used in IRAC operations in flight Table 6 8 gives the readout mode and Fowler number For full array readout mode only the 0 4 2 6 12 30 and 100 second frame times can be chosen in th
124. ing processing and calibration Chapter 6 InfraRed Array Camera IRAC 117 6 3 1 1 Astronomical Flux Standards A number of astronomical standard stars are observed in each instrument campaign to obtain a valid absolute flux calibration Stars with a range of fluxes are observed at a number of positions across the array and many times throughout the mission to monitor any changes that may occur Calibration stars with known spectral types and accurate absolutely calibrated fluxes in the IRAC bands have been determined These absolute calibration stars are in the continuous viewing zone CVZ so that they can be observed at any time necessary and will be monitored throughout the mission Four stars are observed in the CVZ every 14 days These standards remain the same throughout the warm mission and provide the absolute flux reference for IRAC Additionally secondary calibrators near the Ecliptic plane are observed once per data downlink typically ever 24 hours The secondaries used are chosen to minimize telescope slews These calibrators are used to monitor any short term variations in the photometric stability Analysis of the flux calibrator data during the cryogenic mission indicates that absolute flux calibration is accurate to 3 Repeatability of measurements of individual stars is good to better than 1 5 and can be as good as 0 01 with very careful observation design e g Charbonneau et al 2005 ApJ 626 523 The absolute cali
125. ions will yield mosaics with much reduced persistent images and are strongly recommended in straight averages the amplitude decreases as the number of dithers is increased and the worst ones are eliminated from robust averages or medians 74 Spitzer Observer s Manual 3 6um Oi a EE E E E E E E E E r E E EE fal 1x10 2x10 3x10 4x10 5x10 1em aA 4 5um 74 4 D a gt Zz 5 4 x lt a Le pe J N w zZ S O gt 4 ro 1 E me PSR Te of ey E E E 1x10 2x10 3x10 4x10 5x10 Raw DN Figure 6 10 Non linearity curves for the IRAC detectors Measured source flux as a function of peak DN is plotted The detector responses are fairly linear until saturation where there is a steep drop off in responsivity The linearity solution is indicated by the green curve 6 1 3 2 3 Short term residual images Both IRAC channels have residual images of a source after it has been moved off a pixel When a pixel is illuminated a small fraction of the photoelectrons is trapped The traps have characteristic decay rates and can release a hole or electron that accumulates on the integrating node long after the illumination has ceased The warm mission short term residual images are different in character than the cryogenic residuals as the behavior of the trap populations is a function of the impurity type and temperature In all arrays the longest cryogenic e folding decay time is about 1000 sec The e
126. itivity of the data 6 3 3 3 Data Reduction Software 6 3 3 3 1 Data Reduction 126 Spitzer Observer s Manual The actual BCD pipeline will not be released due in large part to the degree to which it is tied to SSC specific infrastructure However detailed documentation on the algorithms used by these modules is in the IRAC Pipeline Description Document and the source code for the pipeline is available at the SSC website The software modules that make the post BCD products mosaicker coadder point source extractor and bandmerger have been released to the general community and are available from the SSC website This software consists of modules written in C with PERL wrapper scripts to control them The primary advantage they offer over existing astronomical software packages is the ability to read the IRAC ancillary data mask files etc during processing and a robust handling of image distortion and reprojection The latter is important both due to the non negligible distortion in the IRAC detectors and their large areal coverage The SSC has encoded image distortion using the TAN SIP projection type which is now supported by several software packages It is anticipated that the community will also find this software useful in processing multiple related AORs which the SSC does not do automatically 6 3 3 3 2 Data Analysis Most scientific analysis such as optimal mosaicking and source extraction require additional processing be
127. ity A function quadratic is fit individually to the response function of every pixel based on warm operations calibration This function is then used to linearize the response of the individual pixels Saturated pixels are identified and marked in a data quality image file The processed pixels should be linear to better than 1 Scattered light patterns are removed from data A butterfly pattern in channels 1 and 2 caused by internal scattered light is scaled using the estimated zodiacal background level and subtracted from each image They are subtracted from all calibration data as well The skydark is subtracted from the image the bias is monitored throughout the mission every 7 days using dark sky patches It is chosen from the nearest calibration in time Radiation hits cosmic rays are detected at the single frame level using a median filter Radiation hits are not removed from the BCD Instead a probability that a given pixel is affected by a radiation hit is noted in the data quality file mask The primary method for removing radiation hits is in the post BCD pipeline which uses robust averaging of observations of the same piece of sky The data are flat fielded The flat field image is generated from observations of bright sky patches The channel 1 and 2 images are flipped This transposition is a result of the dichroic beamsplitters As a part of this operation associated mask files are flipped as well The or
128. jects SSOs ToOs and Generic Target observations are also discussed The tool which Spitzer observers must use for planning observations and submitting proposals is Spot which is described in section 5 2 5 1 Astronomical Observation Request AOR When all the relevant parameters for the AOT are specified and linked to a description of the target the resulting fully specified observation is called an AOR which is the fundamental unit of Spitzer observing AORs are created using Spot see section 5 2 An AOR can be thought of as a list of parameters that when properly interpreted completely describe an observation In fact an AOR as represented in the SODB contains a series of keywords and values that are used to create the sequence of commands that are sent to the observatory to carry out the observation This ASCII format is what Spot actually creates and stores in its output file The parameters of the AOT are specified in Spot by filling out a form sometimes called an AOT Front End The AOR is the fundamental scheduling unit for Spitzer An AOR cannot be subdivided will not be interrupted for other activities such as downlinks and is handled as a unit by the observatory Because of this non interruptible nature and the need to perform certain activities periodically e g detector anneals pointing system calibrations and downlinks a maximum duration exists for an AOR That maximum time is 24 hours for IRAC AORs Longer
129. known In more detail the quantity is derived as follows Let the PRF in pixel 7 be P and the intensity of an image in pixel i be J If a point source with flux F is present in the image then J FP If we do a least squares 2 r ET F P y p ye fit to determine F then we minimize i where g is the measurement uncertainty in pixel i We will assume here that o is independent of pixel and set o o this condition is approximately true for background and or read noise limited observations Now we take the derivative of with respect to the source flux and set it to zero to find the optimum value We find 0 U FPP solving for F we find p gt LF f gt Now we derive the uncertainty in the flux Using the well known theorem for propagation of errors Op 5 dF dI o and applying it to the result above P Po a Des re YR 1 ree ioe es to Op ovVN where N T which is the definition of noise pixels The number of noise pixels is used for the sensitivity predictions in the following sections of this chapter o we find that o 5 or equivalently There are two columns for the full width at half maximum FWHM of the PRF The mean FWHM is from observations of a star at 25 different locations on the array The FWHM for centered PRF is for cases where the star was most closely centered in a pixel The fifth column in Table 6 1 is the fraction of the flux in the central pixel for a source t
130. l overhead burdens applied to Targets of Opportunity targets with late ephemeris changes etc The online Proposal Kit also contains information specifically designed for Spitzer Solar System researchers It includes asteroid count estimates a summary of ephemeris management at the SSC and tips for using the Horizon database supported by the Solar System Dynamics Group at the Jet Propulsion Laboratory 2 5 4 Data Products Once data are collected the SSC creates and delivers pipeline processed calibrated data products to the observer and to the Spitzer science archive Three levels of processed data are created Raw data Basic Calibrated Data BCD and Ensemble data also known as post BCD data Raw data are unprocessed data which have been packaged into FITS format BCDs are single frame FITS format data products which have been processed to remove instrumental signatures and which are calibrated in scientifically useful units Ensemble data are higher level products and may include mosaicking or co addition The data products are described in more detail in the IRAC chapter and in the corresponding IRAC Data Handbook available on the SSC website In general the final calibration for an instrument campaign which is typically about two weeks in duration is based upon calibration observations taken at the beginning and periodically throughout the campaign so pipeline data products are typically available two weeks after the end
131. lable to the Spitzer Principal Investigator are scheduled or will be scheduled with high likelihood but not yet executed or analyzed prior to the Spitzer proposal deadline An investigator may propose observations of generic targets describing them in as much detail as possible in a Spitzer observing proposal The investigator must submit AORs with celestial positions accurate to within 2 degrees radius and with integration times specified to within a factor of 1 5 After the complementary observations are obtained and analyzed the Principal Investigator must modify the generic target AOR and include the precise celestial coordinates and integration time before the observations can be scheduled The observations must be completed within the observing time allocation awarded when the proposal was approved If your proposed targets are not known at the time of proposal submission you must provide a credible schedule for determining the targets and delivering the AORs Details will be worked with all successful teams For nominal generic targets the AOR must be fully specified 8 weeks prior to execution 6 Infrared Array Camera IRAC 60 Spitzer Observer s Manual The InfraRed Array Camera IRAC was built by the NASA Goddard Space Flight Center GSFC with management and scientific leadership by the Smithsonian Astrophysical Observatory SAO under principal investigator Giovanni Fazio The information in this manual is based on the design
132. ll be observed For cometary satellite or ring observations in particular check this time sequence for potential unplanned slews across the cometary nucleus major planet or some other bright object both in the instrument s field of view and in the potential zones of avoidance for scattered light see the IRAC specifications in Chapter 6 5 6 Targets of Opportunity ToOs see Observing Rule 5 are transient events whose timing is unpredictable Predictable phenomena whose precise timing is not known a priori e g novae newly discovered comets gamma ray bursts may be requested in a General Observer GO proposal Observations of completely unanticipated phenomena can be requested through DDT procedures see SSC website for details A GO proposal must include an AOR for each predictable ToO observation filled in with as much information as possible An example demonstrating proper construction of ToO AORs is included in the Observation Planning Cookbook 5 6 1 Classification of ToOs ToOs are classified based solely on their impact on the observatory scheduling process which depends on the time elapsed between the activation of a ToO observing request and the desired date of execution of the corresponding observation see Table 5 1 for ToO classification criteria Chapter 5 Observing with Spitzer 57 Table 5 1 Classification of ToOs High impact lt 1 week 48 hour minimum turnaround Medium impact 1 8 weeks L
133. m the data In channels and 2 the pattern is in the form of a butterfly wing at the bottom of the array in raw data coordinates i e at the top of the array in BCD coordinates The scattered light avoidance zones 1A 1B 2A and 2B are larger than the regions the scattered light falls into This results in the diffuse scattered light butterfly wings having amplitudes 5 of the background intensity even though only about 2 of the light from a given point in the stray light zones is scattered into them Additional information and figures on this subject are available on the SSC website and in the IRAC Data Handbook s a a eee ae reds 3 aa ee is h x Ta gt Scattered Light spot ge aay yt AA Ma Di o n Tosg 7 5 r Pi K m AT x wit 4 7 gt F ee wy j 7 b gt 7 ne Pay 5 Bm a s N gt YY r r Ra of Tre wW n aA ji A vinx ear g gt 4 ce i OAS E F aS gt D v 3 4 p m T ar M lt gt 7 d ove Po Figure 6 8 Stray light pattern in BCD data when a bright source falls in the stray light avoidance zone 2A in channel 2 See text and SSC website for more detail 6 1 2 2 4 Ghost images Ghost images are visible near very bright sources in channels 1 and 2 These ghost images are caused by internal reflections within the tilted filters Because of the increase in the optical path length ghost images are not in focus The separati
134. metric linearity calibration InSb detectors have measurable nonlinearity The InSb arrays are nearly linear until they reach saturation Both arrays are linearized to better than 1 up to approximately 90 of their full well capacity defined in electrons in Table 6 4 with the gain listed in Table 6 6 corresponding typically to 30 000 DN Note that the linearizable well depth for both arrays has decreased from 45 000 DN in the cryogenic mission As part of the warm calibration the detector linearity has been determined by observations of a large number of point sources over a large dynamic range compared to comparable cryogenic observations of those sources The warm linearity measurements are shown in Figure 6 10 The arrays were illuminated with a constant flux and successively longer exposures were taken Chapter 6 InfraRed Array Camera IRAC 73 For a perfectly linear system the flux would be directly proportional to the exposure time and the graph would show a straight line In fact the arrays were driven past their saturation levels and the shape of the curve up to 90 of the saturation level was fitted with a polynomial for the linearization module in the pipeline Table 6 4 IRAC detector characteristics FPA Read noise for frame with specified Quantum Well Operability designation frame time electrons Efficiency Depth e 0 4s 2s 12s 30s 100s 1 48534 34 24 7 13 0 10 3 8 6 9 2 87 145 00
135. model 1 fs MJy st 0 15 0 44 FC wy 5 1 15 B elec sec 4 1 14 high background model I fs MJy sr 0 52 1 0 F uy 18 35 B elec sec 14 32 Table 6 7 gives the background brightnesses in useful units for three nominal observing directions The low background model applies near the ecliptic pole the high background case is in the ecliptic plane and the medium background case is intermediate The background model includes contributions from emission and scattering from zodiacal dust and emission from Galactic dust The near infrared cosmic infrared background radiation is not included because it will be partially resolved by Spitzer Note that these values have not changed from the cryogenic mission and the background current B must be scaled by the point source throughput to realize the actual warm background in electrons second The quantity fr is the flat field pixel to pixel variance which depends on the observing strategy In what follows we will set fr 0 which would apply strictly in the case of stable detectors with perfect flat field measurements and should apply practically for highly dithered observations An observation with no dithering will be limited by the correlated noise The accuracy of a flat field derived from a single warm observation is measured to be 3 4 and 1 6 in channels and 2 respectively by comparing flats in several campaigns Us
136. na ee e EEEE EAE AET E EEEE EARE E E E ei 131 7 3 SOLAR SYSTEM OBJECTS INCLUDED IN BRIGHT OBJECT AVOIDANCE c csescssseeseeeseeeees 132 8 ACRONYMS GLOSSARY essessoessessocesocescessoecesocesoessocsseesseesocesocesoscesocesocesocesesssoesseesocesesossse 133 9 INDEX 1 Philosophy and Scope of the Spitzer Space Telescope Observer s Manual 2 Spitzer Observer s Manual The Spitzer Space Telescope is the fourth element in NASA s family of Great Observatories and represents an important scientific and technical component in NASA s Astronomical Search for Origins program The spacecraft carries an 85 centimeter telescope and during the warm mission one operational ambient temperature science instrument capable of performing imaging from 3 to 5 um Spitzer was launched on a Delta 7920H from Cape Canaveral Florida into an Earth trailing heliocentric orbit on 25 August 2003 The warm mission began after the depletion of cryogen on 15 May 2009 After the in orbit checkout phase for the spacecraft and the one functioning science instrument IRAC warm mission science observations began in late July 2009 1 1 Intended Audience This document the Spitzer Space Telescope Observer s Manual Warm Mission Warm SOM hereafter is the essential technical reference manual for Spitzer observers The Warm SOM provides information about the design performance and operational constraints of Spitzer It includes information
137. ng of extragalactic sources As input users may choose an SED model background level and IRAC instrument configurations The output includes flux in the instrument passbands instrument sensitivities S N and total exposure depth per pixel Finally the Stellar PET STAR PET predicts fluxes for imaging of stars For input users select a stellar spectral type MK class K band magnitude or K band flux density The tool calculates the expected flux density in the IRAC 3 6 and 4 5 um passbands 5 2 3 Flux Unit Converter Although Spitzer predictions and products are in Janskys some astronomers may be more familiar with magnitudes Another Javascript tool available at the SSC website converts back and forth between the flux density units of magnitudes and Janskys as well as a variety of other flux density units See also the Appendix of this document for more information Chapter 5 Observing with Spitzer 47 5 2 4 Leopard Spitzer Archive Interface The observer s interface to the archive is called Leopard and is very similar to Spot see above Leopard comes with the Spot software download on the Proposal Kit web pages Detailed information about how to retrieve public and or proprietary data using Leopard can be found in the Leopard User s Manual also available on the Proposal Kit web pages Observers should also use Leopard as their primary source for checking the Reserved Observations Catalog 5 3 Modification of Obser
138. observations can be specified using multiple AORs and relational constraints to identify these AORs as members of a related group see section 5 4 3 When specifying such related groups it should be taken into account that in general several AORs of the maximum length cannot be placed back to back Even taking into account only that the spacecraft generally must downlink every 24 hours ignoring calibrations and maintenance of the science instruments it can be seen that the observatory schedule may be broken up into irregular chunks of time Observing programs that are composed of short independent AORs that can be scheduled any time their targets are visible are easiest to schedule Observing programs that require building related groups of constrained observations are more likely to fit into a schedule if the AORs themselves are short and the constraint is constructed to allow maximum flexibility for details on constraints see section 5 4 An AOR contains three categories of information Chapter 5 Observing with Spitzer 45 Astronomical Target The target of an AOR can be a single pointing or a cluster of pointings within a 1 radius at which the specified observation is repeated identically The single pointing or cluster may be either of an inertial target or a moving target AOT Specific Parameters These parameters include instrument configuration exposure time and dedicated mapping parameters Timing and Relational Constraints T
139. oduct can be derived Chapter 6 InfraRed Array Camera IRAC 121 Table 6 16 IRAC Basic Calibrated Data BCD pipeline products Data Contents BCD Basic Calibrated Data dark subtracted linearized flat fielded and flux calibrated among other things see section 6 3 3 2 cBCD Artifact corrected BCD BCD with empirical corrections for artifacts due to bright sources Uncertainty Image Array of calculated uncertainties for the BCD Imask Residual Image pixels Radiation hit pixels Data quality flag image for the given science DCE Bit that is set to indicate residual image prediction Bit that is set to indicate the probability of radiation hit event for a given pixel All of the data and ancillary files are in FITS format containing a header with keywords and their values followed by a binary image except certain log files that are in simple ASCII format The standard FITS header keywords are all present so that essentially any FITS file reader is able to read the files and all images have 256x256 pixels Subarray data are in a cube 32x32x64 pixels Observers should expect to receive a data volume of approximately 3 5 Mbytes per frame as their BCD including ancillary files as described in Table 6 16 Information about the data that observers receive is included in the IRAC Data Handbook 6 3 2 3 Calibration Files For each BCD the pipeline calibration server generates several estimates of the
140. om background sources and particularly small scale structure in that background frequently limits the sensitivity of Spitzer To assist in background subtraction Spitzer Solar System observers can specify shadow background observations A shadow observation allows the track across the sky taken during observation of a moving target to be replayed or pre played when the target is not there Shadow observations allow the Solar System observer to remove background small scale structure thereby improving moving target sensitivity Shadow observations will help to mitigate the effects of background sources but not completely eliminate them Therefore if possible it is best to avoid viewing faint objects while they traverse crowded fields such as near the Galactic plane and timing constraints should be selected accordingly To reduce potential errors due to time dependent changes in the zodiacal light instrument characteristics and calibration a shadow observation is generally most effective when taken as close in time to the primary observation as is scientifically possible In addition the shorter the interval of time between primary and shadow the larger the visibility window for the primary shadow pair and the higher the likelihood that the observation can be scheduled Often a balance must be struck then between the brevity of the interval separating the primary and shadow observations and the object s motion in that interval so that
141. on between the main image and its ghost is roughly proportional to the distance of the main image from the Spitzer optical axis In terms of pixel offsets in the BCDs the ghost positions are Ax Ay Ax B A y B where x y Chapter 6 InfraRed Array Camera IRAC 71 are the pixel coordinates of the source in a BCD The coefficients for channel 1 are A 0 049686 B 14 6110 4 0 048301 and B 0 54605 The coefficients for channel 2 are 4 0 051957 B 1 20675 A 0 053381 and B 0 45258 The peak intensity of the ghost is roughly 0 05 of the unsaturated peak intensity of the star Examples of ghost images are available in the IRAC Data Handbook The SSC pipeline flags filter ghosts The filter ghosts are treated as part of the point spread function and are included in both the measured PRFs and the nominal flux calibration which is derived from photometry using 10 pixel radius source apertures 6 1 2 2 5 Polarization The tilted elements within IRAC produce a small amount of instrumental polarization that can lead to small photometric errors when viewing polarized sources The filter transmission is shown as a function of wavelength in Figure 6 9 the expected polarizations are 0 794 and 0 282 for channels 1 and 2 respectively Table 6 3 IRAC Channel characteristics Chan Effective Bandwidth Average Minimum Peak um transmission in band trans um M trans mission mission 1 3 550 0 75
142. onal observations required to reduce the ephemeris error so that a late ephemeris update will not be needed Late ephemeris updates i e specification of the final ephemeris less than 5 weeks prior to scheduling are no longer supported in GO proposals Any 56 Spitzer Observer s Manual observations requiring a late ephemeris update must be submitted via a DDT proposal 5 5 6 3 Timing Constraints Make sure that any observer imposed timing constraints are still within the Spitzer visibility period for your target see section 3 2 above Even though a given timing window may be optimal for the science goals the object may not be visible to Spitzer in that timing window It is important also to check that the object flux positional uncertainty and sky plane motion remain within acceptable values during the constraint interval 5 5 6 4 Visualization It is EXTREMELY important to visualize your proposed AORs This is especially true for moving target observations Spot will allow you to overlay the orbital track of your object on the infrared background and will also overlay your AOR in the sequence in which it would be observed on a date that you enter You can use this feature to check for structure in the infrared background around your object If you have a time constrained observation you can use the Animate with Trail feature in the visualization tool to see the time sequence and positions at which individual apertures wi
143. ons but there is one method that will optimize the sky coverage and provide the desired redundancy with minimal overhead We make a finely spaced map grid that avoids holes due to the unknown a priori roll angle on the scheduled date and yields the desired 5 observations of each sky position To do this we use a map grid spacing of A 5 61 4 The number of rows is 13 The position angle is 60 E of N We select the cycling dither pattern small scale with a depth of one Making a 13x1 map in celestial coordinates covers the desired long axis of the galaxy but the perpendicular coverage with the desired depth is only for a relatively narrow strip about the same size as the optical disk Therefore we add 1 extra column to the map and make a 13 by 2 celestial coordinate map Figure 6 33 shows the sky coverage by one field of view for this 116 Spitzer Observer s Manual observation Since both fields of view were selected the map will be performed twice with the galaxy centered first in the 4 5 um field of view and then in the 3 6 um field of view In Figure 6 33 there are two bands of sky coverage the upper band is the sky covered in the first map pass by the 4 5 um field of view the lower band is the serendipitous sky covered by the 4 5 um field of view during the second map pass Using Spot the duration of the observation is approximately 1500 sec even with both fields of view selected This is significantly less than in the arr
144. ons in the light over the FOV transmitted by the telescope and camera optics The gain variation for each array is mapped as described in Section 6 1 3 2 8 Chapter 6 InfraRed Array Camera IRAC 77 However the gain maps do not strictly apply for point sources with stellar like spectral slopes due to the variation in filter effective wavelength across the array A location dependent photometric correction for point sources is provided for each array The peak to peak variation in photometry is 6 at 3 6 um and 8 at 4 5 um Images of the photometric correction are shown for each IRAC InSb detector in Figure 6 13 The pixel to pixel sensitivity variations characterized as part of IWIC are under analysis We expect that the gain will exhibit the same temporal stability as in the cryogenic mission Figure 6 13 Photometric correction images at 3 6 um left and 4 5 wm right The contours start at 0 95 for 3 6 um and 0 94 for 4 5 wm and increment by 0 01 The correction images are normalized to the average response of the array 6 1 3 2 5 Intra pixel sensitivity variations For the InSb detectors the measured flux of a point source depends on the position of the source centroid with respect to the center of a detector pixel This effect is due to the efficiency of the InSb pixels not being uniform throughout the entire pixel The effect is more significant in the warm mission The variation in measured flux is 7 at 3 6 um and 4 at
145. order tst IRS Peak up Blue Red IRAC 3 6 5 8 um MIPS 24 um Instantaneous IRS Slit FOV Short High J Nominal Focal Surface PCRS MIPS 160 um Radius 16 arcminutes Instantaneous FOV MIPS 160 um Full FOV Note This is not an engineering diagram MIPS 24 um Full FOV Figure 2 1 Science instrument apertures projected onto the sky Because of the optical inversion in this projection the section of sky closest to the projected Sun is on the MIPS side of the focal plane e g to the right in this view Because the spacecraft does not rotate about the line of sight this vector is fixed relative to the focal plane on the sky The IRAC sub array fields are shown by the small boxes in the lower corners of both IRAC arrays The 4 5 and 3 6 um sub arrays are on the left While this diagram depicts the apertures of the IRS and MIPS those instruments will not function in the warm mission Chapter 2 Introduction to Observing with Spitzer 11 2 4 2 The Life Cycle of a Spitzer Observation Accepted program Observatory Planning amp Mission Sequence AORSs fill Science m Scheduling Generates 1 m Team Generates Operations Database wk schedule 3 weeks week uplinkable Observer fills S out AOTs and l wee s activities a anaiei are uplinked submits proposal weekly 5 days Observer m retrieves data SPACE TELESCOPE writes papers BN
146. out IPAC Infrared Processing and Analysis Center IR Infrared IRAC InfraRed Array Camera IRAS InfraRed Astronomical Satellite IRS InfraRed Spectrograph IRU Inertial Reference Unit Jy Jansky LGA Low Gain Antennae MIC Multi Instrument Chamber MIPS Multiband Imaging Photometer for Spitzer mJy milliJansky MJy megaJansky Chapter 8 Acronyms Glossary 135 MPC Minor Planet Center OPZ Operational Pointing Zone PAO Period of Autonomous Operations PCRS Pointing Control Reference Sensor PCS Pointing amp Control System PIN Positive Intrinsic Negative Diode PM Primary Mirror PRF Point Response Function like a PSF but having been transmitted through the telescopetinstrument optics PSF Point Spread Function PST Point Source Transmission RA Right Ascension RCS Reaction Control System rms Root Mean Square SAO Smithsonian Astrophysical Observatory SBRC Santa Barbara Research Center S C SpaceCraft SED Spectral Energy Distribution SIRTF Space InfraRed Telescope Facility Spitzer s old name SLO Second Look Observation SM Secondary Mirror SODB Science Operations Database SOM Spitzer Observer s Manual S N Signal to Noise Spot Previously listed here as an acronym meaning Spitzer Planning Observations Tool but now it is simply a proper noun sr Steradians SSC Spitzer Science Center SSC
147. ow for processing of the command sequences by the Mission Operations team at JPL and review of the resulting command products The one week schedule loads are uplinked weekly following final review and project approval In general Spitzer weeks run from Thursday to Wednesday After a schedule has been finalized changes are only made in unusual circumstances e g activation of ToO programs Due to the fact that Spitzer is in contact with the ground for only a brief period every 24 hours it is not practical to make changes to the on board schedule on very short time scales the fastest a ToO can be accommodated is about 48 hours from the time of SSC s receipt of the approved AORs Observations are grouped into campaigns which will be approximately 14 days duration Rules regarding schedule changes on short notice are discussed in more detail in the Spitzer Observing Rules 5 4 2 Notification That an AOR Has Been Scheduled Approximately three weeks before a schedule has been uplinked to the spacecraft the list of scheduled AORs and their nominal execution times will be published on the SSC website Unless an absolute scheduling time has been specified at the time of scheduling the actual execution time of an AOR may differ from the scheduled time by a small amount possibly up to several minutes This is due to the non deterministic nature of the on board indicators for slew completion and settle time The appearance of a particular
148. ow impact gt 8 weeks Special overhead burdens will be assessed against high and medium impact observations See this discussion in the Proposal Kit section of the SSC website Because of the reduction in SSC staffing to support science operations during the warm mission high medium impact ToO requests can only be submitted as DDT proposals Only low impact ToOs can be requested as part of GO proposals in Cycle 7 See the current Call for Proposals 5 6 2 Activation of AORs A request for ToO activation must be sent to the SSC Director via the Helpdesk help spitzer caltech edu If the ToO involves a previously accepted AOR then the request is to activate this AOR Modification e g to position of the AOR is possible and a request for this should be included with the ToO activation request Target of Opportunity activation requests will be supported during normal business hours 5 7 Generic Targets Generic targets denote observations that fail to qualify as Targets of Opportunity i e they generally have more refined and predictive spatial and temporal information than ToOs and can be scientifically described but lack precise celestial coordinates or brightness estimates at the time of Spitzer proposal submission A generic target can be selected from a complementary observing program with Spitzer or with any other telescope but one where the conditional observations assumed to be under the control of or clearly avai
149. ows investigators to construct and edit detailed AORs by entering targets and selecting from a variety of preset instrument specific functions e g exposure times instrument modes dither patterns and observing constraints Spot also includes useful visualization tools to permit an investigator to see how proposed Spitzer observations and the Spitzer focal plane is laid out on the celestial sky These capabilities allow observers to retrieve relevant images from other astronomical surveys in any of a number of wavelengths and archives It calculates estimates of Spitzer observing time including telescope overheads for each AOR in a proposed program along with target visibility information focal plane position angle for a selected observation date and estimates of the zodiacal and cosmic infrared background at the target Spot allows investigators to view AORs from all previously approved programs it also supports check in of active Spitzer programs for approved modifications by investigators see section 5 3 46 Spitzer Observer s Manual Spot allows prospective investigators to plan develop and modify their proposal in an iterative manner That is a proposer can write a portion of their proposal and define their accompanying AORs save the results locally and then re load those results at a later time for subsequent modification The saved cover sheet proposal text and AORs can be modified repeatedly before the proposal
150. pe was characterized on orbit during the early parts of the mission and will continue to be characterized during nominal operation of the telescope In some cases there are modifications or caveats to the requirements above Discussion of stray light as it pertains to each instrument is covered in the instrument chapters later in this document A useful output from the stray light analysis is a set of predicted point source transmission curves The point source transmission function PST is the inverse of the ratio of the flux density W m Hz of an off axis source to the flux density at the telescope focal plane due to light scattered from that source The separate PST curves in Figure 4 3 refer to different azimuthal locations of the celestial point source An azimuth of 0 refers to the anti Sun direction The differences among the azimuths are mainly due to the changing illumination of struts supporting the secondary mirror Figure 4 4 shows the variation as a function of wavelength 38 Spitzer Observer s Manual 0 d 24um 0 Azimuth eee ate aw oA al ee 24um 30 Azimuth 24um_ 60 Azimuth 5 24um 180 Azimuth 3 Sas Site Ha t pa Ae 7 J gt p 3 eS 6 ND 7 8 9 10 a ie i 10 100 Point Source Off Axis Angle Degrees Figure 4 3 Theoretical PST off axis as a function of azim
151. pointings are spaced closely together There are cases for which a celestial grid is preferred specifically for mapping a highly elongated region see the example in section 6 2 4 4 We recommend that all observers considering a celestial grid inspect their survey coverage for a range of possible spacecraft roll angles using Spot visualizations Be aware that away from the ecliptic poles not all position angles are available over all positions on the sky Ifa large observation can be broken into separate rectangular map grids as is necessary when the total map duration is more than 24 hours then individual AORs for each map tile can use the same central target position and a map Chapter 6 InfraRed Array Camera IRAC 111 center offset Allow for some overlap between tiles both for offset correction and for inefficiencies in the way tiles mesh together 6 2 4 1 5 Observing near bright targets If you observe a target next to a very bright target and want to image it with both arrays and it will be impossible to avoid having the bright target in one of the arrays e g due to the small roll angle range it may be better to make the observation in two separate AORs imaging one channel at a time or to take more frames with dithers than would be strictly needed for sensitivity purposes to beat down persistent images Remember to justify this in your proposal A very common and challenging type of observation is to search for a
152. posure times due to the higher rate at which the subarray frames are sampled 0 01 seconds compared to 0 2 second for the full array Other times plotted below are assumed to use multiple exposures of those fixed times Table 6 10 IRAC point source sensitivity low background 10 wJy Frame Time sec 3 6 um 4 5 um 100 0 69 1 25 30 1 7 2 5 12 3 8 5 0 6 8 5 9 8 2 37 39 0 6 210 222 0 4 420 443 2 19 21 0 4 101 102 0 1 553 566 0 02 8393 8447 Table 6 11 IRAC point source sensitivity medium background lo pJy Frame Time sec 3 6 um 4 5 um 100 0 83 1 44 30 1 9 2 9 12 4 1 5 5 6 8 7 10 4 2 37 40 0 6 210 222 0 4 420 443 2 19 21 0 4 101 103 0 1 553 567 0 02 8383 8447 Chapter 6 InfraRed Array Camera IRAC 93 Table 6 12 IRAC point source sensitivity high background lo pJy Frame Time sec 3 6 um 4 5 um 100 1 43 2 16 30 3 0 42 12 5 4 7 4 6 10 3 112 8 2 39 42 0 6 211 224 0 4 421 445 2 21 24 0 4 103 106 0 1 555 569 0 02 8394 8848 available only in high dynamic range mode gt subarray mode set of 64 32x32 images Sensitivity is per frame not per 64 frame coadd For bright sources shot noise due to counting statistics in electrons from the source itself becomes the dominant source of noise We can estim
153. pot User s Guide 98 Spitzer Observer s Manual 8 0 0 IRAC Post Cryo Mapping Unique AOR Label IRACPC 0000 Target M31 Type Fixed Single Position 0h42m44 32s 41d16m08 5s New Target _ Modify Target gt Target List Instrument Settings Readout Mode For each Pointing Field of View Data Collection 0 4 Full Array 2 M 3 6um M 3 6um Number of Frames 2 High Dynamic Range ae ae 5 i y M 4 5um M 4 5um S 2 Subarray Frame Time secs Kan a 30 100 Mapping and Dithering 200 Mapping Mode 400 No O Yes Dither Pattern No O Yes Flux Density Calc Obs Time _ _ Comments _ Sensitivity e Special _ Vis Window _ Exposures Cancel GOK Figure 6 25 IRAC Post Cryo Mapping AOT dialog in Spot 6 2 3 1 Readout Modes and Frame Times In full array readout mode there are eight selectable frame times 0 4 2 6 12 30 100 200 and 400 seconds Note 200 and 400 seconds are not supported in the warm mission You should select the frame time that will allow you to reach your target sensitivity while allowing for rejection of cosmic rays after accumulating all repeat observations of the same piece of sky by dithers frame repeats map overlaps and map cycles The cosmic ray rate is about one hit per array per second It is possible that bright sources will saturate the array at the frame time you choose to meet your sensitivity goal To allow sensitive obs
154. r Note that there are other required activities at the beginning of the moving target AOR which could vary in duration from target to target such as slewing to the target from a previous target or settling and waiting at the acquisition point before starting the track However these variable duration activities are executed before the absolute timing constraint is applied to specify the beginning of the track The scheduling constraints that are supported for both inertial targets and SSOs are described in section 5 4 3 5 5 4 Science Instrument Issues The saturation of the IRAC instrument is discussed Chapter 6 and the scattered light characteristics of the telescope are discussed in Chapter 4 in addition to Chapter 6 IRAC observers interested in milli magnitude photometry of bright sources should be particularly mindful of the stability issues discussed in these chapters 5 5 5 Overheads The time duration of each AOR an observer submits with a program includes a fixed slew overhead For both inertial and moving targets this is 215 seconds in duration An additional 300 seconds overhead is now charged for moving targets This extra overhead accounts for the target acquisition and tracking requirements for moving targets see Solar System Observing with Spitzer in the Proposal Kit for details 5 5 6 Best Observing Practices for Solar System Objects The following best practices for Solar System objects are provided in a
155. r Solar System Objects 5 5 6 1 Choosing your targets 2 sivoi cueiden neta eal eevee age EE OLNE 5 5 6 2 Late Ephemeris Updates ai 5 5 6 3 Timing Constraints nsi eriei SEE AEE SE AE aiaa 5SS 6A Visnalizati Ns en ee ear ai Aa a E Aeaee R AOE EECA NASA A EE EAE O EESE 5 6 TARGETS OF OPPORTUNITY a A A T E E AR E a A AE 36 1 Classification Of TOOS sa cesei sheet wees den an NE E e AS E 5 6 2 Activation of AORs LE GENERIC TARGETS raa E T lapsan see edeetasasvodessaeteddss 6 INFRARED ARRAY CAMERA IRAC 6 1 INSTRUMENT DESCRIPTION ercisccsecsiuattiiseieceivn cities E EN E E ENR ek Oo DL OVERVICW nnna oeiia r hak un EEE AK EEE E 6 1 1 1 Instrument Description 61 12 Technical Overview rereana fe ation tab chien and eatin E E dade alana OD De OPC Sere OA Meese Se Be LIP Noi aa lis tN RO ote sates ORs ell aaa eaten seats tad 6 1 2 1 Optical Layout 6 1 2 2 Image Quality 6 1 2 2 1 Distortion s 6 1 2 2 2 Location of sources on the arrays sssesseesessssssstsstsstesttsrerrestrrrtstesresrertsrtesresresrseesees 67 6 12 23 Scattered and stray lightss 2 fcc hnnnedec cian oneal 68 6 1 2 2 4 Ghost images ws 6122s CRG AMIZ AL OM nsec et ces cesses eal ates ace te cas ata get E ee eet N 6 12 33 Chromatic Elements enaa a EA A REE AEA 71 6l PD CCECIOUS oo REE T PG Se BEE or he a AL e ea nee 71 6 1 3 1 Physical Characteristics 6 1 3 2 Detector Calibration eee 6 1 3 2 1 Photometric linearity calibration
156. r the end of the integration Differencing is performed in the IRAC electronics to generate one integer value per pixel per exposure to store on the spacecraft and transmit to the ground The Fowler N used for an observation depends on integration time and has been selected to maximize the S N based on in flight performance tests during IWIC 6 1 3 3 2 Subarray mode In subarray mode only one corner 32x32 pixels offset by 8 pixels from the edges is read out from one array Pixels 9 40 9 40 of the array are read out The subarray pixel size is the same as the full array pixel size 1 2 Fowler sampling is performed as in full array mode but a set of 64 subarray images are generated and tiled into a single 256x256 image before data are sent from IRAC In subarray mode Fowler sampling is performed at 0 01 sec intervals Subarray mode is useful for observing very bright sources and for obtaining high temporal resolution 6 1 3 3 3 Exposure time and frame time The relationship between the exposure time Tex and frame time is shown in Figure 6 21 The exposure time is defined as the time elapsed between the first pedestal sample and the first signal sample The Fowler samples are taken consecutively at 0 2 second intervals 0 01 seconds for subarray in each group pedestal and signal samples The frame time 7 T is the total time elapsed between resets which could include multiple reads and dead time before and after Fowler sampling
157. rate images IRAC responds to each command with an acknowledgment In the case of a command that requests telemetry the acknowledgment consists of the telemetry packet which is sent on the low speed connection between IRAC and the C amp DH There are two types of engineering data special engineering data which are collected every 4 sec and housekeeping data which are collected every 30 seconds Special engineering data are used for onboard communication between IRAC and the C amp DH while housekeeping data are used on the ground to monitor instrument performance A command that generates images from the arrays is acknowledged on the low speed line and when the frame is complete the data ready signal is sent on the high speed line The frames together with their ancillary data are then transferred one at a time to the C amp DH The rate of transfer is 2 seconds per frame which limits the data collection rate to a maximum of two frames every 4 seconds for IRAC observations with both arrays Autonomous fault protection ensures that none of the monitored voltages or currents enters into a red limit Fault protection is performed by a watchdog demon that is always running when the instrument is on The red limits are stored in IRAC memory Ifa voltage in the focal plane assembly goes into a red limit IRAC will turn off the affected focal plane array The individual pixel values are not monitored so a bright astronomical source will no
158. ration value of point sources is higher the values given in Table 6 13 should be used for planning purposes To apply Table 6 13 for extended sources I 28 6x f Fa for compact diameter lt 30 sources 6 10 ext oe I 28 6 x F u for more extended sources where I is the total surface brightness in MJy sr at which a pixel saturates f Seen and fex are as defined above and F is the saturating point source flux density for in mJy from Table 6 13 appropriate for the channel and integration time Table 6 13 Maximum unsaturated point source in mJy as a function of IRAC frame time Frame Time sec 3 6 um 4 5 um 100 3 2 3 2 30 10 6 10 6 12 26 5 26 6 6 53 53 2 159 160 0 6 530 540 0 4 820 820 2A 160 160 0 4 820 820 0 1 3175 3200 0 02 15890 16000 subarray mode If the bright extended source extends well beyond the 5 x5 FOV then the saturation brightness is lower by the factor fy Use Table 6 13 in conjunction with Table 6 10 through Table 6 12 to determine the dynamic range of an individual IRAC frame If bright point sources e g IRAS sources are in your field of view and you want to measure their fluxes as well as those of fainter sources then you can use the high dynamic range HDR option With this option shorter frames will be taken in addition to your specified frame time Chapter 6 InfraRed Array Camera IR
159. rection and rows are in the positive x direction It is worth noting that the two IRAC fields of view are at approximately constant solar elongation so that a map with 1 column and several rows will make a strip along the direction of the separation between the two fields of view and will yield 2 array coverage along part of the strip if it is long enough If you select celestial coordinates then the rows and columns will correspond to J2000 Right Ascension and Declination You may specify a position angle degrees E of N to orient the raster in Equatorial coordinates Specifically if the position angle is zero a column will be along a line of constant Right Ascension and a row will be along a parallel line of constant Declination The celestial option can be useful for regions whose shape is clearly defined on the sky This option also allows two separate AORs both in celestial coordinates to have a fully specified location of map grids such that they can be made adjacent to one another independent of observing date but see below regarding gaps The most important advantage of mapping in array coordinates is that the sky coverage can be custom tailored and the coverage precisely defined independent of the date when the observation is scheduled In general we do not recommend using celestial map grids A celestial map grid can yield highly non uniform coverage including gaps unless the individual pointings are spaced closel
160. reported in the data products Based on post BCD processing and comparison of extracted sources with 2MASS sources users will receive data with IRAC data pointing reconstruction generally lt 1 with respect to the 2MASS coordinate system Relative pointing relative separation of two objects within an AOR is good to lt 0 5 4 Telescope and Focal Plane 34 Spitzer Observer s Manual 4 1 Optical Design The telescope is a Ritchey Chr tien design known from on orbit measurements to be diffraction limited at 5 5 um over the IRAC field of view and expected to be diffraction limited at 6 5 um over the entire field of view The 85 cm diameter primary mirror and the rest of the telescope structure are fabricated entirely of beryllium utilizing advances in optical design testing and fabrication to produce a lightweight telescope that operates at cryogenic temperatures The optical design parameters for the telescope are summarized in Table 4 1 below The telescope configuration is shown in Figure 4 1 Note that this chapter addresses the telescope itself the point spread function PSF for IRAC is discussed in chapter 6 Table 4 1 Telescope Configuration Optimal Parameter Description Value at 5 5 K System Parameters Focal Length 10 200 mm Focal Ratio f 12 Back focal length PM vertex to focus 437 mm Field of View diameter 32 0 arcmin Field curvature radius 140 5 mm Wavelength coverage 3 um 180 um Aper
161. requirements on the characterization of the flight instrument in pre flight ground tests and on in flight performance at cryogenic temperatures including the In Orbit Checkout IOC Science Verification SV period in August November 2003 and the limited testing of arrays similar to the flight arrays at 30K see McMurtry et al 2006 SPIE 6265 626508 for more information This section describes the performance of IRAC from 18 September 2009 onward On 18 September 2009 the final warm IRAC operating setpoints were asserted and are presumed to remain the same for the duration of the warm mission Both the telescope and IRAC instrument are at stable operating temperatures By passive cooling the telescope is equilibrated at a final primary mirror temperature of 27 5 K At this higher temperature only the 3 6 and 4 5 um arrays will usefully operate The longer wavelength Si As arrays have extremely high dark currents at this higher temperature and saturate in even the shortest frametime and will be dropped from the discussion in the remainder of this section Both IRAC arrays are actively thermally controlled to 28 7K The programmable voltages for each array have been optimized In addition to the operating temperature the primary difference in array operation is a lower applied bias of 500 mV for the 3 6 um array compared to 750 mV of applied bias for the cryogenic mission This change was necessary to reduce the number of noisy pixels present
162. res to be 0 067 um rms over the entire clear aperture meeting the specification of 0 075 um rms Figure 4 2 36 Spitzer Observer s Manual Flight Primary Mirror Surface Error RMS 0 067um i0 075um requirement OD 850mm ID 320mm 0 449 Microns 0 594 Figure 4 2 The deviations in the flight primary mirror surface The RMS error was measured at cryogenic temperatures to be 0 067 um rms over the entire clear aperture meeting the specification of 0 075 1m rms 4 2 2 Wave front Errors The telescope is required to provide a beam to the telescope focal surface that is diffraction limited transmitted wave front error lt A 14 rms at 5 5 um over the entire field at operating temperature At a wavelength of 3 5 um the telescope produces a wave front error of less than 0 134 rms over the IRAC field of view and the image of a point source contains 45 of the encircled energy within a diameter of 2 The actual performance of the telescope is slightly better than the requirements see IRAC chapter specifically Table 6 1 for more information 4 2 3 Throughput The requirements for the telescope assembly are that it shall provide a minimum end of life throughput no less than that given in Table 4 2 Telescope throughput is defined as the ratio of energy from a point source reaching the telescope focal surface to the energy collected by an 85 cm diameter mirror Factors that degrade telescope throughput are the central
163. rides are maintained on the ground rather than on board Spitzer but an observer may specify flexible scheduling constraints and the linear pseudo track specification start point rate and direction in equatorial J2000 coordinates is calculated at the time of scheduling The observation is executed at the scheduled time within a window of 3 0 s and the PCS follows the track as specified assuming the given start point corresponds to the time given in the track command All other PCS movements can be superposed on a specified track including dithers and scans PCS measurements indicate that the blind track acquisition accuracy is lt 0 5 independent of the track rate which is consistent with expected performance on fixed targets The track stability is better than scan mode requirements 0 5 in 1000s The offset accuracy during tracking is better than 0 55 also consistent with the required performance on fixed targets Section 5 5 presents more detailed information on observing Solar System Objects including ephemeris management Chapter 3 Observatory Description and Constraints 31 3 3 4 Pointing Reconstruction Pointing reconstruction refers to the post facto determination of where the telescope was pointed with a greater accuracy than was known by the flight system Spitzer has a requirement to perform pointing reconstruction to 1 4 Based on in orbit results Spitzer meets this requirement The nominal pointing is
164. rrereesrsrrseres DH 3 3 POINTING CAPABILITIES eii rr Eai AEE da uinaanaanciaa TE 28 33 Pointing Control System cocccsceccccseticeccvevseceesagsicasectcnsdiosbivticoussoctesseagcaasedenstuviidestsnendesbenes 28 3 3 2 Pointing Accuracy ANd Stability c cccceccececcecccccee esses seeseeeeneeeeescesseeeseesenseessieesseetaeeees 30 33 3 Tracking Capabilities ioe onii Ea nE EEE REO iE 30 33 4 Pointing Reconstruction airce onina N ERE EE E a E NNE 31 4 TELESCOPE AND FOCAL PLANE esseesceesoesscesscesocesesoesscesocesceesocesosesoesscesesocesoeescesseesscesoe 33 4l OPEICAL DESIGN aeneae E A E ivan Eaa A E EEE aaa S E AN EE 34 4 2 OPTICAL AND THERMAL PERFORMANCE cssssssessessseesseeseccesecsseceseesaeeseeeseenseceseeessseseseenseens 35 ADL SUPACE ACCUFACE N RE Ged E EA E NTS 35 4227 Wavefront Errors miea R CG A E ER T RAM E t 36 42 3 Throughput ae n ene Rn A Ri Bee WE EA R Hal Ae RE R E OAS 36 4 2 4 Stray Light Rejection cccecccccccccssccccesesseesseseeseseeseesseesesecseescaeeesseeseesenesneesseesaesaeenaees 37 4 2 5 Telescope Temperature Thermal Background o oo 39 4 2 6 Telescope FOCUS airo rei e O RE RERE ia i L REN i i 39 43n FOCAT PEANE LAYOUT imona a E E TE a T E 39 5 OBSERVING WITH US PLTZER sesssscssccvecssocassvuaseschessdsusueusescsssctevessensteisvsssdeasbacsesassuesebesssssus stees 43 5 1 ASTRONOMICAL OBSERVATION REQUEST AOR ccccccssccsssccsssccessecescessecesseeesseecssseesseeeses 44 8 9
165. rture Both the sky and residual are averaged over 6 minute intervals The short term nature of the residuals is displayed in Figure 6 12 which displays the residuals produced by a 3 and 4 magnitude star observed in a 12 second integration at 3 6 um pointings In addition to the compact residuals there are 76 Spitzer Observer s Manual streaks that can be produced when bright sources cross the array during slews between AORs Figure 6 12 Illustration of residual images in warm data 3 and 4 K magnitude stars were observed in a single 12 second frame upper left panel The upper right lower left and lower right panels show the decay in the residual in 12 second frames taken 60 120 and 180 seconds after the bright sources were observed The positions of the bright sources on the array are identified by red annuli in subsequent frames The linear features are slew latents produced as a the bright sources move on and off the array The dark patch above the 3 K magnitude residual leftmost residual is a diffuse residual from a previous observation of a 1 magnitude source and the linear features crossing the entire array are slew latents due to 1 and 2 magnitude sources crossing the array as an earlier part of the test 6 1 3 2 4 Pixel to pixel sensitivity variations The gain varies from pixel to pixel on the array due to intrinsic gain differences in the pixels themselves and in the output electronics gain and due to variati
166. rve a single very bright point source consider the subarray mode section 6 2 4 1 3 below 5 Inspect the field being observed including the serendipitous field covered by one pair of detectors while another is observing the target of interest Chapter 6 InfraRed Array Camera IRAC 109 Residual images can persist for some time and ruin your observation section 6 2 4 1 5 below 6 If rejecting asteroids is important plan at least two observations of your target separated by a few hours section 6 2 4 1 7 below 7 For the most sensitive observation plan observations using the 100 sec frame time and dithering as much as possible 8 For highest quality images put your source in the inner 1 3 of the array 6 2 4 1 1 Dithering vs in place repeats Dithering is used to eliminate array dependent or transient artifacts from the true celestial map A well dithered map will mitigate the effect of pixel to pixel gain differences which will average down when a celestial coordinate mosaic is generated Radiation hit and residual image rejection is greatly facilitated by taking highly redundant observations The SSC mosaicking software provides good radiation hit rejection in cases where each position on the sky is observed with several dithered frames The medium and large scale dither patterns with five or more dithers per position are adequate for this It is also possible to achieve sufficient redundancy and accurate maps of large
167. ry to Chapter 6 InfraRed Array Camera IRAC 79 measure the total system response The responsivitity at 30 K is as stable as it is at 15 K in part due to the active temperature control and thermal stability of the arrays The gain maps vary by only a few percent per pixel compared to the cryogenic flats Flat field observations are part of both the initial warm calibration and the routine calibrations for each two week campaign The data are combined to generate a super flat as was done in the cryogenic mission and the current superflat has approximately the same precision as the cryogenic version Figure 6 15 IRAC skydark images for the 100 second frametime The 3 6 um dark is to the left and the 4 5 wm dark is to the right The mean dark levels are 190 and 555 DN Pixel 1 1 in the lower left corner is the first pixel read out by the electronics These images are flipped about the y axis compared to the BCDs 6 1 3 2 9 Effects of cosmic rays Cosmic rays will strike the IRAC detectors fairly regularly about one per array per second each one affecting on average about 5 5 pixels so strategies to deal with them in data reduction will have to be implemented The most common cosmic rays do not affect the pixel performance in subsequent frames and are confined to a few pixels around the peak However other events can cause streaks or other multiple pixel structures in the array and less common energetic events may cause residual image
168. s Table 6 9 IRAC High Dynamic Range HDR framesets Long Frame Time List of frames taken T sec 100 0 6 12 100 15 30 1 2 30 3 12 0 6 12 2 6 0 6 6 2 Low Background 10000 f Ne TITIM T TTT T PTT T garua Seo TIT TITTI TIIM T BoR lt 0 02s F 0 02s 3 6um 4 5um 1000 F 2 0103 oR Ng 04 0 4s No Sas 100 7 7 1 i 0 4s T 0 4s S amp gt 6 2s S y 2s 10 fee ya E L eX 2 30s T 1 a T 100s 2 F Confusion ___ Ke oa confusion ____ 2 _ 100s rul rinl Lou ji Luu eee 1 Lou i Lou 1 rrul nul LLLI 0 1 1 0 10 0 100 0 0 1 1 0 10 0 100 0 io x Nerame s Jan 27 2010 Figure 6 22 IRAC point source sensitivity as a function of frame time for low background Full array frametimes are in black subarray frametimes are in blue To convert to MJy sr see equation 6 8 92 Spitzer Observer s Manual Figure 6 22 Figure 6 23 and Figure 6 24 show the point source sensitivity as a function of integration time for each background model The time axes in the plots represent the frame time for the images which does not include time for moving the telescope The IRAC full array frame times shown are 0 4 0 6 2 6 12 30 and 100 seconds The IRAC subarray frame times shown are 0 02 0 1 0 4 and 2 seconds The subarray frame times are more sensititive than their full array counterparts as they have both higher Fowler sampling to reduce read noise and longer ex
169. s lt 1 mW Table 6 4 gives some of the detector properties The operability is the percentage of the pixels in an array that are within usable specifications as measured from warm calibration data The read noise is our best estimate based on actual flight performance and 30 K ground testing Read noise is not directly measurable from in orbit data and is particularly hard to measure for the longer frametimes The effective warm well depth is smaller than the value indicated in Table 6 4 as discussed in Section 6 1 3 2 1 Channel 2 Transmission 0 4 Transmission Transmission o Fad K 2 a 4 9 5 1 3 1 3 3 4 3 4 5 4 Wavelength microns 3 5 3 Wavelength microns Figure 6 9 IRAC total system response for channel 1 left and channel 2 right including transmission of the optics and quantum efficiency of the detectors Digitized versions of these curves are available on the SSC website 6 1 3 2 Detector Calibration The following subsections outline some of the more important data calibration steps and the nature of residual images The IRAC pipeline provides a science caliber calibration for the detector non linearity bias and flat field The pipeline also flags inoperable pixels residual images and radiation hits Inter and intra pixel response variations and their impact on photometry are discussed in detail in the IRAC data handbook and ancillary documentation at the SSC website 6 1 3 2 1 Photo
170. s in a 10 pixel radius aperture Figure 6 6 The orientation of the BCDs on the sky 6 1 2 2 3 Scattered and stray light Stray light from outside the IRAC fields of view is scattered into the active region of the IRAC detectors in both channels Stray light has two implications for observers First patches of stray light can show up as spurious sources in the images Second background light when scattered into the arrays is manifest as additions to the flat fields when they are derived from observations of the sky Stars which fall into those regions which scatter light into the detectors produce distinctive patterns of scattered light on the array We have identified scattered light avoidance zones in each channel where observers should avoid placing bright stars if their observations are sensitive to scattered light The zones for channels and 2 are shown in the Spot overlays Figure 6 7 Zones 1A 1B 2A and 2B which produce the strongest scattered light typically scatter about 2 of the light from a star into a scattered light splatter pattern which has a peak of Chapter 6 InfraRed Array Camera IRAC 69 about 0 2 of the peak of the star Figure 6 8 They arise due to scattering of light from the edges of the holes in the covers of the focal plane arrays The A and B zones join up to form a continuous region of scattering which is widest at the upper left and right corners of the array and narrows to a waist in the cen
171. s in subsequent frames Single cosmic rays should not permanently affect the responsivity of a pixel but multiple strikes over time and exposure to strong solar proton events may lead to an increase in the dark current of certain pixels and perhaps to a degradation of responsivity Despite significant solar events during cryogenic operations no significant degradation of the IRAC arrays has been observed There is no difference in the effect of cosmic rays on the IRAC arrays between 30 K and 15 K operations Figure 6 16 shows portions of a 100 sec IRAC frame at 3 6 um obtained during IOC SV Stars have been subtracted to show only cosmic rays in the images 80 Spitzer Observer s Manual Figure 6 16 Channel 1 IRAC image 100 sec frame time obtained during IOC SV Stars have been subtracted All the dark pixels are affected by cosmic rays Figure 6 17 Image showing examples of column pulldown and pullup at 4 5 um The panels display two sequential 30 second observations of NGC 1353 from the S4G program 6 1 3 2 10 Thresholds for triggering image artifacts During cryogenic operations a variety of image artifacts were produced by observations of bright sources During warm operations the artifacts are much more benign with only the column pulldown effect persisting Column pulldown is a columnwise offset produced by a bright pixel Pulldown is more prevelant and has a more complex behavior than in the cryogenic mission The effect has a
172. s provided in the IRAC Data Handbook and the IRAC Pipeline Description Document 1 The header of the raw data file is validated to ensure that it is an uncorrupted IRAC image with the shutter open and lamps off 2 Several computed keywords such as exposure time commanded instrument and telescope parameters and the pointing information are added to the image from the telemetry and the SODB Many of the detailed IRAC telemetry channels mostly instrument parameters such as voltages and various temperature measurements are removed from the header they are still present in the raw header 3 The 16 bit integer FITS images are received and then rectified so that increasing flux produces increasing DN which is not the case initially for the IRAC InSb arrays 4 The 16 bit integer FITS images are converted to 32 bit floating point FITS images 5 Detector wraparound is detected and corrected IRAC can generate negative flux numbers as a result of Fowler sampling Although IRAC uses a signed 24 bit internal data representation it only sends unsigned 16 bit integer data and in the process of doing so discards the sign bit The wrap detection module uses the knowledge that very high DN values exceed the full well capacity of the detector and hence they are actually wrapped negatives Fowler number barrel shift and gain have been chosen such that the entire full well capacity of the detectors can be represented by 16 bits 6 The raw data uni
173. s the X axis Over time angular momentum accumulates in the reaction wheels due primarily to the small offset between the center of mass and the center of radiation pressure Unlike an observatory in low Earth orbit which can dump this momentum magnetically Spitzer has a Reaction Control System RCS which uses cold nitrogen gas thrusters to provide the reaction wheel momentum unloading capability opportunities to dump momentum are scheduled during routine downlinks The nitrogen supply is sized to accommodate a mission lifetime in excess of 5 years On orbit measurements show that the PCS is capable of slewing the telescope 180 in 900 sec 1 in 60 sec and 1 in 6 sec while maintaining its inertial pointing knowledge These times include the acceleration and deceleration of the The on board catalog actually uses the ICRS coordinate system so in fact the on board pointing system is really ICRS However the differences between ICRS and J2000 are so small lt 120 mas that no conversions are made and Spitzer is effectively considered to use the J2000 coordinate system 30 Spitzer Observer s Manual telescope but do not include the time it takes for the PCS to stabilize after the slew has completed The pointing system has several operating modes and the AOTs are designed to use the pointing mode most appropriate for each observing mode Settling time varies with operating mode and slew magnitude For IRU only slews
174. s the solar array orientation toward the Sun and it points the high gain antenna toward Earth for downlink The PCS also contains Wide Angle Sun Sensors WASSs that act as a second check on the spacecraft orientation to ensure that the hard pointing constraints see section 3 1 are not violated High level fault protection will place the telescope in a safe mode if a violation is detected The performance numbers presented in this and the following sections are based on our present understanding and measurements of the on orbit PCS performance The PCS is a celestial inertial three axis stabilized control system A high performance star tracker inertial reference unit ST IRU package provides attitude determination and reconstruction capabilities On board pointing commands and variables use the J2000 coordinate system Reference to the J2000 celestial sphere is implemented within the ST through autonomous identification Chapter 3 Observatory Description and Constraints 29 of stars carried in an on board catalog of 87 000 Tycho stars down to 9th visual magnitude The ST is used to point an instrument boresight to a desired location on the sky with an initial accuracy of at least 0 5 1o radial The ST field of view is 5 x5 which ensures that Spitzer can point to any part of the sky and have the ST meet its pointing requirements Typically 40 stars are used simultaneously The gyros provide pointing stability when not using the ST
175. sal submission once Science User Support has reactivated the password The Science User Support Team at the SSC will review the changes and re check for duplications prior to re releasing the AORs for scheduling Once a program has been resubmitted the access for modifications will be disabled and any further modification will require a new request for access accompanied by supporting justification Check the Science User Support section of the SSC web pages for full details on program modification 5 4 Scheduling Considerations The technical issues of telecommunications and telescope performance that drive science instrument campaign planning and the basic scheduling timescales are addressed in Chapter 3 In this section the planned scheduling process is addressed from the observer s point of view The use of scheduling constraints is also discussed in this section Scheduling constraints restrict the ability of the planning and scheduling system to create 48 Spitzer Observer s Manual efficient schedules and to ensure the feasibility of scheduling all AORs Therefore a scientific justification is required for the use of scheduling constraints and must be provided in the science proposal at the time of submission 5 4 1 Scheduling Methodology The Spitzer scheduling process constructs short term observing schedules of one week in length Each week s observing schedule is finalized about five weeks in advance of execution to all
176. soa 0n88 cece colar en a ea Sa tens rob a a otoan m 12 29 2 TLCIDAESK es sextevstecas evextcuapecnsksshth tastaseas ceeh te evbeasessekend bsskaleztaesh eee oiea eye iT 12 2 93 PROPOSAL Kat sxsscstevsi zee eveszceesees TEN tyes eoepnree heuer eta a ae ee geae tae oE bese tees ee 13 294 VIGIGPLOGUCTS reves tie treo ere nann n a a NU co se meted seat a dats eee 13 3 OBSERVATORY DESCRIPTION AND CONSTRAINTS 3 1 OBSERVATORY DESIGN AND OPERATIONS CONCEPT ccssssssssseesseeeesseeessecssseneseseesseeessenes SAD Data Stora g nio oia Een E seg es E O EREE Sas ch E EAE estas geen SLD Data B A01 ISAO AARE S E OER PEL impact OMmODSETVEr S Snie ea aE A e A a E a 3 2 SKY VISIBILITY e asenna a aeaaee SDT Nolar OPD IG ae ines tice Meo ass ana ed aad os ed Ma aa N Sa 3 2 2 Pointing Constraints enssins eiio i sbuidovete cnbsseen Duce s 3 2 3 gt WIGWINE Periods niini n a N E aha e T ited E E RE 3 2 4 How Visibility Evolves with Time eeeeeeeeseeeeerirrreresrrrersersrersreersrsrerererersns 3 2 5 Orientation of Focal Plane and Slits against the Sky 3 2 5 1 Impact on Observers scsi sie ie aaeege nel Nate AA couse eens 3 2 6 Bright Object Avoidance 3 3 POINTING CAPABILITIES onnsa ansent eidet eeaeee rataa Senidea aE a ilasan 3 3 1 Pointing Control System essin aaa iaa na aa NAE A AE ea 3 3 2 Pointing Accuracy and Stability ccccccccccccccceccesceseeseeseseeseeseesecsecseenececeeeeeeneneeeeeeseeaeeaeeas 3 3 3 Lracking Capabilities aiino e n a a E
177. sponse On the other hand easy questions can result in a nearly instantaneous response There is no auto reply on our system Note that as the proposal deadline approaches the turnaround times for an SSC response particularly for definitive answers to complex technical questions will likely increase It is the responsibility of proposers to take this reality into consideration when submitting queries shortly before the proposal submission deadline Chapter 2 Introduction to Observing with Spitzer 13 2 5 3 Proposal Kit The online Proposal Kit provides all of the information necessary for the prospective General Observer GO investigator It can be found on the SSC website at http ssc spitzer caltech edu The Proposal Kit also includes instructions for downloading and installing the Spot software on the user s machine Spot discussed more in section 5 2 1 below is used to plan and prepare Spitzer observations and to submit GO proposals electronically to the SSC Prospective GO investigators are recommended to download Spot and to start planning their Spitzer observing programs well before the proposal submission deadline In addition the Proposal Kit includes many kinds of documentation including this manual the current Call for Proposals Spitzer Warm Mission Observing Rules an Observation Planning Cookbook Proposal Submission Guidelines and the proposal template files guidelines pertinent to the calculation of the specia
178. ssecessessscessecenseees 134 vii Table of Contents All Sections 1 PHILOSOPHY AND SCOPE OF THE SPITZER SPACE TELESCOPE OBSERVER S MANUA e r E E E e a a E E A E 1 1 1 INTENDED AUDIENCE ccccccsssccssscessecessessscessscessecesssecsssecessecsecessecessecessecesaeceaecesecstcenseeenseees 2 1 2 DOCUMENT ORGANIZATION cccssccsssccsssceessecessceccssseceseeesseecessecessecessesaecesecesseessaseessteesseeeneees 2 1 3 DOCUMENT UPDATE PLANS cccccessssscesscesscesecseesscesscesecsecssecsscesscesecsesssceaucessessecssscsseessceaseass 3 1 4 OTHER RELEVANT DOCUMENTS cccccssssessseeessecesscccscessccesseecssecessecessecessecsecessecensecesseeessseeeaees 3 2 INTRODUCTION TO OBSERVING WITH SPITZER csscssscssccssscsccnccsccesccesssssesscees 5 2 1 CATEGORIES OF OBSERVING PROGRAMG cccssscsssscessecessecesseesscessecessecesssecessecesseceaeesecenseeesaes 6 Di2e MISSTIONFOVERVIEW E EE SEA E E si cssscse E E eveaadten dl susosuees E Sieeseesiimehee aes 6 2 3 HIGH LEVEL OBSERVATORY DESCRIPTION WHAT CAN SPITZER DO ccccccssseesseseseeeesees 7 2 4 SPITZER SCIENCE OPERATIONS OVERVIEW cscccssssesssscessecessecessessscsssecessecesssecsseecsssecscessesenaes 8 2 4 1 Science Instrument Operations cccccccccccecccsesccsesssceseseesenssseesenseseessnesseteeeseeesieeseneeaeets 8 2 4 2 The Life Cycle of a Spitzer Observation ooeec 11 2 5 SSC SCIENCE USER SUPPORT SERVICES eessen reia EE e ina a EE i 12 DOT DO GUMETIATION 2
179. st 2 Figure 3 6 shows the actual pointings of the observatory for one day in the life of Spitzer Note the locations of the OPZ and how it constrains where we actually pointed on that day 22 Spitzer Observer s Manual Earth A We Ny 0 6 AU Solar Orbits for an 8 25 03 launch projected onto the Ecliptic plane during the 62 month mission lifetime goal In the rotating coordinate frame shown here the Earth Sun line X axis is held fixed and the Earth is at the origin 0C mas SV Mission Lifetime Req t Mission Lifetime Goal 0 5 AU 0 4 AU circle of 1AU radius 0 3 AU 0 2 AU 0 1 AU j y 4 P 30 day timeticks on trajectory Figure 3 4 Spitzer s solar orbit projected onto the ecliptic plane and viewed from ecliptic North In the rotating frame the Earth is at the origin and the Earth Sun line is defined as the X axis Loops and kinks in the trajectory occur at approximately 1 year intervals when Spitzer is at perihelion Spitzer s orbit is also slightly inclined with respect to the ecliptic 3 2 3 Viewing Periods The amount of time during the year any particular target is visible depends primarily on the absolute value of its ecliptic latitude Figure 3 7 As seen in Figure 3 5 Spitzer s instantaneous window of visibility on the celestial sky forms an annulus the OPZ perpendicular to the ecliptic plane of 40 width and symmetrical with respect to the Sun
180. stics of the available dither patterns are given in Table 6 14 The Reuleaux Triangle patterns were designed with the idea of optimizing the Figure of Merit of Arendt Fixsen amp Moseley 2000 Astrophysical Journal 536 500 They thus sample a wide range of spatial frequencies in a fairly uniform manner and are well suited to the Fixsen least squares flat fielding technique The 9 point and 16 point patterns were designed to be the optimum size for 1 3 and 4 sub pixel dithering respectively The random 9 pattern is based on a uniform random distribution The spiral 16 pattern was designed by R Arendt to provide a pattern which is both compact and has a good figure of merit for self calibration Chapter 6 InfraRed Array Camera IRAC o point Gaussian 100 50 50 100 Pirvuitiiiit 100 Q 100 16 point spiral 100 50 gat ob i 50 nei 100 eee ba Sa EE 100 Q 100 4 point random subarray TTT TTTTy TT rry rt 10 ETT L EL R y L a S L 10 9 point random 100 100 Pirvuitiiiitd 100 Q 100 36 point Reuleaux TTrt T TETA T 100 50 Eai 3 0 E 5 50 KK 100 Lod eas Ss a I 100 Q 100 9 point Reuleaux subarray 10 F a oF 10 E J J 10 0 10 105 1lz point Reuleaux Pirvuitiiiit 100 Q 100 Cycling pattern 100 3 x aie l
181. t trigger a red limit The command sequences will continue to execute therefore it is possible for normal completion of an IRAC observing campaign to occur with only one of the two arrays returning data If the second focal plane assembly has a telemetry datum go into a red limit for 90 seconds then IRAC will send the C amp DH via the special engineering data on the low speed line a request to be turned off If a telemetry point other than one affecting a single focal plane goes into a red limit IRAC will also send the C amp DH a request to be turned off 6 2 How to Use IRAC 6 2 1 Performance of the Instrument To estimate the sensitivity of IRAC for warm operations we have used warm calibration data taken after the final setpoint assertion to measure the change in effective throughput from cryogenic operations For quantities such as QE and readnoise which cannot be directly measured in orbit we use the required performance based on the design specifications and the results of the 30 K ground tests performed at the University of Rochester McMurty et al 2006 The online tool SENS PET at http ssc spitzer caltech edu tools senspet makes this estimation for you and should be used for planning observations The sensitivity to point sources in flux density units is based on the following formula o i JBT FOLIIS R DL Sloly l l where the scale factor is 6 1 88 Spitzer Observer s Manual Ge Onn AAA hi t
182. t work in tandem with calibration and pointing servers The BCD pipeline takes as input the raw data files and generates the Basic Calibrated Data BCD as its final output The pipeline is run automatically Chapter 6 InfraRed Array Camera IRAC 123 upon receipt of individual raw files The BCD pipeline is triggered for all science frames generated as part of an AOR Calibration data are sent to other pipelines for processing and their output is made available for the calibration server When the BCD pipeline is running and requires a calibration file such as a dark current image a query is sent to the calibration server which returns the appropriate file The calibrated images BCDs from an AOR are combined in the post BCD pipeline to create a mosaic The extended pipeline mosaicker source extractor overlap corrector and pointing refinement modules is exported to observers so that they can combine data from multiple AORs The BCD pipeline is not exported because of its inextricable link to the Spitzer Science Operations DataBase SODB which is needed to identify the appropriate calibration files for a given observation A copy of the BCD pipeline code is available at the SSC website for reference purposes but no support is available for running the code outside the SSC 6 3 3 2 Science DCE Processing The following is an explanation of the steps taken by the IRAC BCD pipeline Figure 6 34 Further guidance to IRAC data reduction i
183. ta These two science instruments are therefore not available in the Warm Mission The two shortest wavelength arrays within IRAC 3 6 and 4 5 um are the only operational science instrument and arrays for which new Spitzer observations can be proposed Figure 2 1 shows that the two apertures for IRAC point toward different Chapter 2 Introduction to Observing with Spitzer 9 portions of the sky at the same time so that switching apertures and staying on the same target involves re pointing the telescope A central concept in Spitzer science operations is that of the Astronomical Observation Template AOT The IRAC instrument offers the observer a selected number of choices in configuring and operating the instrument The AOT concept and the IRAC Post Cryo Mapping AOT in particular are described in Chapter 5 and in greater detail in Chapter 6 The SSC plans and executes calibration activities to maintain the instrument calibration In general observers should not need to supplement the instrument calibration but may do so by incorporating observations for that purpose into their proposed observing program The calibration for IRAC is discussed in Chapter 6 10 Spitzer Observer s Manual Nominal Field of View Locations Projected onto the Sky MIPS 70 um IRS Slit Full FOV PCRS Long Low MIPS SED Slit IRS Slit Long High MIPS 70 um Instantaneous IRAC FOV 4 5 8 0 um IRS Slit Short Low
184. tellar photometry averaged over all the data in a single calibration AOR The error bars are the uncertainty in that mean derived directly from the data 3 6 um 4 5 um em m Residual Dark Bias Variation DN ii E i i Ii 1 1 psf ft kl 1 ye agp Nf EE 0 20 40 60 80 100 0 20 40 60 80 100 Days since 25 Sep 2009 Days since 25 Sep 2009 Figure 6 20 Variation with time in the median level of the 12 second warm skydarks at 3 6 um left and 4 5 wm right The uncertainty bars plotted are 1 sigma estimates of the photon noise from the North Ecliptic Pole lowest background region 6 1 3 3 Hardware IRAC has no moving parts other than the shutter which is not operated in flight The instrument takes data by staring at the sky and sampling the arrays between resets IRAC is capable of operating each array independently and or simultaneously The standard observing mode is to collect data in both channels simultaneously The option to collect data in only one channel exists to facilitate Chapter 6 InfraRed Array Camera IRAC 85 high data rate observations of a point source that would be precluded due to data volume restrictions if the off source field of view data were taken 6 1 3 3 1 Fowler sampling Multiple Fowler sampling is used to reduce the effective read noise This mode of sampling consists of taking N non destructive reads immediately after the reset and another N non destructive reads nea
185. ter Zone 1C produces diffuse scattered light across the array but the light is only noticeable if Zone 1C is illuminated by very bright stars See the SSC website for more details Scattered light is well rejected by the post BCD pipeline if dithering and mapping offsets are large enough to ensure stars are moved out of the scattering zones and the redundancy is high enough to allow for effective outlier rejection The medium and large dither pattern scales with 4 or more dithers per position are adequate for this Most cases of scattered light are identified and flagged as part of the pipeline processing No secure model for the distribution of scattered light from point sources exists and for all intents scattered light from point sources cannot be reliably removed from images me J aS f oN y Zone 2B gt a me f ba 7 r y re A N La N wid Zone 2A Zone 1B Figure 6 7 Locations of IRAC stray light avoidance zones as seen in Spot visualizations See text and SSC website for more detail The effects of the scattered light on the flats are illustrated in additional figures found on the SSC website The SSC has implemented a correction in the pipeline to remove diffuse stray light from the BCDs flats and skydarks This correction 70 Spitzer Observer s Manual scales a template of the diffuse scattered light pattern by the expected zodiacal background level and subtracts it fro
186. the 9 point Random 1 3 pixel sampling or the 16 point spiral 1 4 pixel sampling Use the HDR mode so that there are short 112 Spitzer Observer s Manual 3 6 um 45 um 5 0 um iin pA 3 6 um 4 5 wm 5 0 um SESS VED NT 01 1 18X14 YB q U XN 4 0 XN 4 JO OEY 47 6 0 4 n i 4 n 4 L 4 1 O 20 40 60 Radius from center arcsec Figure 6 32 Relative surface brightness of a point source for each channel as a function of radius out to 60 along a diffraction spike bottom panel and off a diffraction spike top panel as measured with the PRF frames at the exact same position as the long frames These may help with centroiding because they will be less saturated in the core Use a long enough frame time for the long frames so that you will gain the required sensitivity on the putative faint source in one or a few frames preferably without saturating the primary source over more than a few pixels Finally and probably most importantly design the experiment as a pair of AORs with timing constraints such that the source will be seen at two very different roll angles The range of possible roll angles depends on the ecliptic latitude of the source and must be verified using Spot visualizations near the beginning and end of the visibility windows for a given source Typically the range of roll angles is about 20 degrees The PRF will remain fixed in array coordinates so the basic calibrated images at
187. the position of the spacecraft along its solar orbit i e the date of observation and of the ecliptic latitude of the target Targets in the ecliptic plane have only two possible focal plane orientation angles which are at 180 with respect to each other Elsewhere in the sky the focal plane has two ranges of orientation angles Science instrument aperture orientation within the focal plane is physically fixed which affects mapping with IRAC 3 2 5 1 Impact on Observers Spitzer observations must be designed with this lack of roll control in mind Observers who require a certain array orientation angle relative to the target must check to determine when if ever the needed orientation is achieved and must request a suitable range of observation date s It is possible that a particular target might not be observable at the same time of the year that an array has the desired orientation The focal plane array orientation on any date can be calculated and projected on the sky using the Spitzer observation planning tools see Chapter 5 Because constraining the orientation tightly also constrains the scheduling time tightly it increases the risk that an observation may be subject to schedule conflicts In addition the presence of many highly constrained observations tends to reduce the observatory s efficiency Therefore observers should design observations that are as robust as possible to the maximum range of focal plane orientations
188. ther patterns should only be used in conjunction with mapping using 1 3 2 3 array offsets The small scale dither patterns do not provide sufficient redundancy under other circumstances 6 2 4 1 2 HDR mode It is possible that bright sources will saturate the array at the frame time you choose to meet your sensitivity goal To allow sensitive observations without losing dynamic range we provide a high dynamic range HDR option When this option is selected IRAC will take extra frames with frame times shorter than your selected frame time HDR mode is recommended if the target or nearby sources will saturate in the desired frametime The short frame data are very useful in recovering the photometry of saturated sources fitting PRFs to saturated sources and in removing artifacts due to bright saturated sources HDR mode is only slightly more expensive in observing time than the full frame counterpart 6 2 4 1 3 Subarray Subarray mode is useful for observing single very bright sources and for obtaining high temporal resolution since sets of 64 frames are taken back to back with no gap between frames The saturation limit is much higher in the subarray mode 6 2 4 1 4 Mapping In most cases we recommend mapping in array coordinates since the sky coverage can be custom tailored independent of the date when the observation is scheduled A celestial map grid can yield highly non uniform coverage including gaps unless the individual
189. throughout the observatory The Sun always lies within 2 of the XY plane i e the roll angle is constrained to 2 Spitzer is operated autonomously for moderately long periods of time 24 48 hours interspersed with short periods 40 80 minutes of ground contact In extreme circumstances Spitzer is designed to survive for up to a week with no ground contact at all During routine science operations Spitzer typically executes a pre planned week long schedule of science observations calibrations and routine engineering activities which has been uploaded in advance and stored on board This master sequence might typically have 7 different 24 hour Periods of Autonomous Operation PAOs containing observations and calibration activities each followed by a 40 80 minute period spent re orienting the spacecraft for downlink and transmitting the data to the ground After the downlink Spitzer returns to the 18 Spitzer Observer s Manual pre planned sequence of observations and calibrations Because efficient communication with the ground requires use of the high gain antenna mounted on the bottom of the spacecraft Figure 3 1 executing a downlink of the collected data requires slewing the spacecraft to orient the high gain antenna toward one of the Deep Space Network DSN stations on Earth Any of the three DSN sites Canberra Madrid and Goldstone can be used when visible During the time that data are being transmitted to
190. tion of time and late in the mission we may have to change our downlink strategy to accommodate this The two low gain antennae LGA give wide angle coverage but only support downlink at 44 kbit s at the beginning of routine observing decreasing to 40 kbit s after 3 years The LGA are used in safe mode but are not normally used to transmit science data during routine science operations Because of the location of the HGA it is necessary to stop observing and point the telescope in the anti Earth direction to downlink the science data Depending upon the details of the observations that are scheduled Spitzer produces 1 to 6 Gbits of compressed science data during 24 hours of observing Initially Spitzer collects data for about 24 hours and then spends 0 5 1 hour downlinking the data The scheduling system predicts the compressed data volume that is generated during each downlink and schedules the downlink contact time accordingly The downlink periods are also used for some spacecraft maintenance activities such as dumping angular momentum and for uplinking commands If low volumes of data are being generated longer periods of time between downlinks may be used At each downlink opportunity Spitzer attempts to transmit all the data in the mass memory Any data that the ground has not confirmed as received will be retransmitted at the next downlink pass It is anticipated that some data could be missed at any pass and it may take sever
191. ts are renormalized to account for IRAC Fowler sampling and bit shifting Observers need not take Fowler sampling into account when reducing their data 124 10 11 12 13 14 15 16 17 Spitzer Observer s Manual Due to the design of the IRAC electronics there is a maximum rate at which they can react to changes in pixel intensity during readout Thus it is not possible to go from full to zero pixel intensity between two adjacent pixels in a single readout channel equivalent to pixel latency This artifact appears as a decaying ghost following a bright pixel along the fast readout channel which is horizontal in the IRAC images This bandwidth limitation is corrected based on measurements of the detector bandwidth in each readout channel taken during IOC SV The correction is assumed to be the same in warm operations The dark current and bias offset are subtracted using a laboratory dark which should be a good first order approximation of the dark current First frame effect timing dependent bias offset is subtracted These offsets are being determined as a function of array timing and will be implemented in a future version of the pipeline These offsets are a constant level per readout column and thus they can be subtracted from the images Multiplexer bleeding correction was applied to cryogenic data but is not in the warm mission as the artifact no longer exists Data are corrected for non linear
192. ture Stop Location Edge of primary mirror Diameter of OD obscuration 850 00 mm Diameter of ID obscuration 320 00 mm Linear obscuration ratio 0 3765 Primary Mirror hyperbola Radius concave 2040 00 mm Conic constant 1 003546 Clear aperture 850 00 mm Focal ratio f 1 2 Secondary Mirror hyperbola Radius convex 294 343 Conic constant 1 531149 Clear aperture OD 120 00 mm PM to SM spacing vertex to vertex 887 528 mm Chapter 4 Telescope and Focal Plane 35 Secondary mirror Primary mirror Secondary mirror katie Spider Hades Figure 4 1 Spitzer Telescope Assembly The telescope employs a single arch primary mirror which reduces mass The primary mirror is supported on three bipod flexures relatively close to its axis The spider blades primary mirror baffle and secondary mirror baffle are integrated into a one piece relatively small diameter metering tower extending through the central hole in the primary mirror 4 2 Optical and Thermal Performance 4 2 1 Surface Accuracy The Ritchey Chr tien design minimizes spherical aberration and coma over large fields of view Field curvature varies quadratically with field angle Similarly the rms wavefront error at best focus varies quadratically with field angle and equals 0 52 waves A 0 6328 um at the edge of the field Essentially all of this error is due to astigmatism The surface figure for the primary mirror was measured at cryogenic temperatu
193. tween the astronomical community and the observatory The SSC offers support at all stages of Spitzer use including proposal preparation detailed observation design and data analysis The SSC provides this support primarily through supplying documentation providing science user tools and operating an online Helpdesk 2 5 1 Documentation The primary form of science user support is through the dissemination of information including this Warm SOM In addition to the Warm SOM the SSC offers a comprehensive set of web pages which includes further technical documentation software downloads Spitzer Frequently Asked Questions FAQ pages observation scheduling status frequently updated news instrument status performance reports plus tools and information to assist with proposal preparation 2 5 2 Helpdesk The SSC Helpdesk service help spitzer caltech edu is the primary interface to the Spitzer Science Center for the community The Helpdesk provides answers to technical or policy questions and is available to address concerns about any facet of Spitzer observing The Helpdesk also provides support for special needs such as requesting modifications to Spitzer observations and the activation of Target of Opportunity ToO observations Once a Helpdesk question has been received it is directed to an appropriate expert An observer can expect a reply within two business days Particularly complex requests may take up to 5 days for a re
194. uction required lt 1 0 Field of View of imaging arrays 5 x5 each band Telescope Minimum Temperature 5 6 K cryo 27 5 K post cryo Maximum Tracking Rate 1 0 sec Time to slew over 90 8 minutes 2 4 Spitzer Science Operations Overview The Spitzer Science Center SSC conducts the science operations for Spitzer and is charged with 1 acting as an interface and advocate for users 2 capturing and conducting the science program efficiently 3 producing and securing the Spitzer science legacy and 4 conducting public and scientific outreach for the Spitzer program The SSC is located on the campus of the California Institute of Technology in Pasadena California USA In carrying out its charter the SSC issues annual Calls for Proposals organizes science and technical reviews selects approved programs based on a Time Allocation Committee review and administers data analysis awards The SSC provides tools for detailed planning of Spitzer observations and proposal submission and offers Science User Support services In addition the SSC schedules observations on the telescope provides basic pipeline science data processing and data quality assessment and creates a data archive that has been publicly accessible since May 2004 2 4 1 Science Instrument Operations After the end of cryogen the Infrared Spectrograph IRS and Multiband Imaging Photometer for Spitzer MIPS will not be able to produce unsaturated da
195. uments can be found on the SSC website at http ssc spitzer caltech edu Previous years also mentioned the Spitzer Space Telescope Reserved Observations Catalog It is easier and recommended at this point in the mission to use Leopard to search the set of objects observed or planned to be observed with Spitzer Leopard is packaged with Spot and also has batch search capability However we are still generating the plain ASCII ROC obtainable from the SSC website the latest version is dated 1 October 2009 Additional relevant documents include IRAC Data Handbook version 3 0 20 January 2006 2 Introduction to Observing with Spitzer 6 Spitzer Observer s Manual This chapter includes an overview of the Spitzer mission a summary of the observatory s technical capabilities and a description of the Science User Support services that are offered by the SSC The relevant details of the mission and telescope design appear in Chapters 3 5 A brief high level summary of the mission for astronomers appears in the ApJS Spitzer Special Issue specifically the paper by Werner et al 2004 ApJS 154 1 entitled The Spitzer Space Telescope Mission A copy of this paper is available on the SSC website 2 1 Categories of Observing Programs All observing time on Spitzer is now General Observer GO Up to five percent of the total observing time is allocated by the SSC Director as Director s Discretionary Time DDT and is intend
196. usion estimates by Franceschini et al and Fazio et al are for low background extragalactic observations only For observations of higher background or more cluttered regions such as the Galactic Plane the confusion noise will be much more significant We recommend that observers examine the archive for available public data from regions similar to their targets to determine appropriate measures of confusion noise Medium Background 10000 f PTT TT TTT T UREE T Pry T US TTT TT TTT T TTT T TTT T apeu Koos Keos 1 3 6um 4 4 5um 1000 F F 7 100 Ne S 3 0 48 i 0 4s 10 uy 4 4 2s 2s 10 J 6s 12s 30s 1 ig pl ig 100s J i __ confusion___ ___ confusion 100s J L Lou it Lol i uul it Luul LEENH ji raul i raul ji Laul ji Laun eBags sit 0 1 1 0 10 0 100 0 0 1 1 0 10 0 100 0 Trrame X Nevaie s Jan 27 2010 Figure 6 23 IRAC point source sensitivity as a function of frame time for medium background Full array frametimes are in black subarray frametimes are in blue To convert to MJy sr see equation 6 8 For diffuse emission the surface brightness sensitivity per pixel in MJy sr is 0 03f SaN pix x the point source sensitivity in pJy 6 8 Chapter 6 InfraRed Array Camera IRAC 95 The noise pixels Npix are defined in section 6 1 2 2 The saturation limit for IRAC is calculated as follows Using the same notation as earlier in this section High Background
197. uth for 24 wm 0 ISS E E E EEE male Ss 2 3um 0 Azimuth on axis 14 S aa a e a a aa 8um 0 Azimuth on axis x O hes 24 um 0 Azimuth on axis 2 4 Se Ps gt N 70um 0 Azimuth off axis H 160um 0 Azimuth off axis 3 4 A a a 34 64 4 8 4 9 t t i 0 1 1 10 100 Point Source Off Axis Angle Degrees Figure 4 4 Theoretical PST as function of wavelength for fixed azimuth on axis Chapter 4 Telescope and Focal Plane 39 These plots can be used to estimate the stray light contribution from a given source For example at 8 um Vega has a flux density of 62 Jy From Figure 4 4 the 8 um PST at 1 off axis is 2x10 so the predicted flux density in the Spitzer focal plane due to Vega at 1 off axis is 120 mJy 4 2 5 Telescope Temperature Thermal Background The telescope and science instrument operate at ambient temperature The telescope temperature is approximately 27 5 K during the warm mission 4 2 6 Telescope Focus Spitzer is equipped with a secondary mirror focus mechanism which was operated both on the ground and on orbit Prior to launch the end to end image quality was measured on the ground and the mechanism was set to the position that was predicted to give optimal focus following in orbit cool down During the cryogenic In Orbit Checkout IOC it
198. uts and the S show the Signal readouts Ty is the effective exposure time and 7 T is the frame time or total time to obtain one IRAC image The reset part of the sketch is not at the same time and voltage scale as the rest of the figure 6 1 3 3 4 Calibration Lamps IRAC contains two types of internal calibration lamps The transmission calibrator lamps are designed to illuminate all four arrays and provide an internal responsivity measurement There are two transmission calibrator spheres each of which contains two lamp elements To illuminate the arrays the shutter is closed a transmission lamp is turned on and the light from that lamp bounces off a mirror on the back of the shutter The flood calibrators individually illuminate each detector The flood calibrators can be controlled individually and they can be used whether the shutter is open or closed Neither set of calibration lamps is used on orbit as all calibrations will be performed using observations of the sky 6 1 3 4 Firmware The IRAC firmware controls the focal plane assemblies calibration electronics and warm electronics boards Apart from autonomous fault protection the IRAC firmware responds only to commands sent by the Spitzer Command amp Data Handling C amp DH computer The C amp DH sends setup commands to configure the Chapter 6 InfraRed Array Camera IRAC 87 electronics requests for each telemetry packet and integration commands to gene
199. vely cool interstellar medium in galaxies has an infrared component There are also ultra luminous infrared bright galaxies that are very strong sources of infrared radiation As in the optical the infrared zody is concentrated toward the ecliptic with weaker emission by approximately a factor of 4 toward the ecliptic poles The infrared zody is strongest from about 5 um to about 30 um with peak emission at about 10 um The infrared zody has structure on most scales and as observed from Earth varies from season to season The intensity of the infrared zody will also vary with solar elongation or how close to the Sun one is pointed The infrared zody is difficult to model As one moves to longer wavelengths 100 um diffuse Galactic emission from dust clouds in the interstellar medium becomes the dominant contribution to the infrared background This infrared cirrus is patchy with higher concentrations found in the Galactic disk and toward the Galactic center However it is important to realize that the cirrus is ubiquitous and it is critical to examine the IRAS maps or radio maps of cirrus tracers when planning longer wavelength observing Far infrared emission from external galaxies in the field of view will add to the overall background flux For more information on infrared backgrounds see the Backgrounds section of the Infrared Compendium available on the SSC web pages 7 3 Solar System Objects Included in Bright Object Avo
200. ving Programs The SSC envisions that it will execute most observations exactly as they are specified in response to the Call for Proposals Therefore the SSC will expect observers to provide sufficient information at the time of proposal submission to implement the observations 1 e completed AORs also see the current Call for Proposals for more information In cases of conflicts or special needs it will be possible for observers to request a modification of their not yet scheduled AORs via the Spitzer Helpdesk The request must be accompanied by a strong justification for the proposed changes Major modifications by observers are only allowed in exceptional circumstances and only outside of blackout periods Minor modifications can be made at any time See the SSC web pages for details on blackout periods and the definition of major vs minor modifications An observer can at any time request again with strong justification that a scheduling hold be placed on observations that need to be delayed Only certain types of modifications are permitted and the observer should check the Spitzer Observing Rules prior to requesting a modification of his or her observing program Upon completion of approved modifications the observer will need to resubmit the revised AORs Viewing modification and resubmission of AORs can all be done within Spot Modification of programs requires the program username and password set at the time of propo
201. y together There are cases in which a celestial grid is preferred specifically for mapping a highly elongated region We recommend that all observers considering a celestial grid inspect their survey coverage for a range of possible spacecraft roll angles Overlaying your AORs on a sky image in Spot can do this The roll angle itself can be obtained in the Target Entry Visibility section of Spot Chapter 6 InfraRed Array Camera IRAC 101 by choosing an observing date when the target is visible The roll angle is essentially fixed if the source has an absolute ecliptic latitude smaller than 10 when the total roll angle variation as a function of observing date is less than 8 Figure 6 27 shows two examples of 3x3 map grids one in array coordinates and the other in celestial coordinates For the celestial grid the spacecraft roll at the time of observation had a position angle of 30 east of north Not only are there large gaps in the celestial grid but the coverage is non uniform in the observed region as well To eliminate the gaps you could tighten the spacing of the map grid positions sacrificing some area covered or some dithering or you could fix the date of the observation such that the roll angle will be equal to your desired celestial position angle But be aware that away from the ecliptic poles not all position angles are available over all positions on the sky oF oF oo oo Figure 6 27 Simulated IRAC
202. yogenic observations Warm versions of the PRFs are in development The PRFs were reconstructed onto a grid of 0 3 pixels 1 4 the size of the IRAC pixel using the drizzle algorithm We display the PRF with both a square root and logarithmic scaling to emphasize the structure in the core and wings of the PRF respectively The bottom panels display an instance of a point source as constructed from the PRF at the IRAC pixel scale of 1 2 The reconstructed images clearly show the first and second Airy rings with the first Airy ring blending with the core in the 3 6 and 4 5 um data Table 6 1 shows some IRAC image quality properties These numbers were derived from cryogenic measurements of bright stars Preliminary analysis of warm measurements has shown that the optical properties of both IRAC and the telescope have not measurably changed The noise pixels column in Table 6 1 gives the equivalent number of pixels whose noise contributes to a linear least squares extraction of the flux of a point source from a Table 6 1 IRAC image quality properties Channel Noise FWHM FWHM of Central Pixel Maximum pixels mean centered pixel size distortion mean PRF flux pixels relative peak to square grid 1 7 0 1 66 1 44 42 1 221 1 3 12 1 72 1 43 43 1 213 1 6 66 Spitzer Observer s Manual 13x13 pixel portion of an unconfused image and assuming the PRF is perfectly
203. yond the Basic Calibrated Data Since data products are delivered in standard FITS format observers may use any of the large variety of data processing packages available to the general community e g IRAF IDL etc The SSC pipeline provides mosaicked images of entire AORs and the part of the pipeline that takes the cBCDs and combines them into the mosaic is provided to observers Users should be able to use the post BCD software themselves for additional multi AOR data reduction or to improve the quality of mosaics as necessary for the desired science goals The software is designed without arbitrary limits to the size of datasets that it can process Chapter 6 InfraRed Array Camera IRAC 127 Add Header Keywords i Detect and Correct _ Flip sense of InSb flux Conver to foating point wraparound condition Normalize by Fowler _ Correct electronic _ Subtract laboratory Lab Dark number and barrel shift bandwidth limitations dark bias frames from calibration server Correct multiplexer bleed lt Linearity Solution from calibration server Sky Dark Subtract sky dark Detect radiation hits eet Flatten Flat Field Raw Science Data Validate Data gt from calibration server Flux Calibrate Correct for dichroic flip Basic Calibrated Data from calibration server Detect Latents Coadded Mosaic Mask straylight Extract Sources

Warm Mission - Spitzer Science Center

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