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User`s Manual for the NRAO 12 Meter Millimeter

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1. 705 Mars 6 57 8 49 53 13 04 21 0 011 08 Nov sys VIG ERE PI 289 DEC APEX AZIMUTH ELEVATION 5 TOTAL 10 30 06 5 1115535710 NS 400007 COMMAND 125 18 18 0 50 59 36 2 819500 0 00 00 0 0 00 00 0 0 515 126 18 18 2 58 59 36 6 S APPARENT 10 30 06 5 11 15 35 7 EW 40 000 ERROR 0 00 00 7 0 00 00 0 Z 133 94746 1 06432 __ 0 000 10FFSET 0 00 00 0 0 00 00 0 4 5 OFFSET 0 00 00 0 0 00 00 0 3 POINT 0 00 14 3 59200 20 5 z PSREF 0 00 00 0 00 00 0 5 ATM RCVR RBAY FO TC TAUO FWHM 75795 1 00 0 00 OX 6 36 S E 117 4 43 3 400 0 0 223 0 30 690 1 00 0 00 QY 5 20 6 20 gt 2 54 400 0 0 127 0 15 610 WL OBJ V ANT V CAL EFF SB LO IF S FREQUENCY 0 60 14 7 0 3 Vane 32 USB 3 100 265 886432 7 USB 1500 265 886432 DBE_RUN 1956 293860 1956 293860 TOL FOCUS TORR RH T REFRT MODE SCANS SAMPLES SEC TIME INPUT 0 05 45 7 613 3 13 4 46 2 1065 4 40 7 0 4 40 9 0 00 45 6 WIND gt 114 21 46 2 0 0 0 0 00 Figure 6 5 Continuum status monitor display The numbered boxes indicate sections of information on the display which are explained in 86 7 Appendix A Pointing Equations for the 12m Telescope Primary Pointing Equations The basic pointing model in use at the 12m is much the same as
2. pases erases 28 Terss simae _ Hsc issus rset 9 ooo so 52 2145254 Fors zooma 106 30 21 O 42 CHAPTER 4 TRACKING POINTING AND FOCUS Table 4 1 Galactic and Extragalactic Continuum Pointing Sources continued Emeaw maema mear 12 soo cass munaiepeeaas OO of the source To make a five point map proceed as follows Continuum Five Point 1 Select an appropriate source to point on such as a planet or other strong con tinuum source 2 Using the pointing charts make an initial guess for the azimuth and elevation pointing offsets 3 Ask the operator to setup for a continuum five point measurement on the chosen source The operator will set the integration time per sample and the number of OFF ON ON OFF sequence pairs to measure on the source For the stronger sources 5 seconds and 1 pair is sufficient For the weaker sources 10 seconds and 2 or 3 pairs may be needed If the source is somewhat extended ask the operator to set the grid spacing to be the semi diameter of the source All other parameters will default to reasonable values Once everything is set ask the operator to start
3. 5 9 1 C Tre js Tsk Tues 14 amp To 5 10 MN f ss where and are the image and signal sideband gains is the receiver noise tem perature is the antenna temperature of the sky Equation 6 9 7 is the rear spillover efficiency and mrss is the forward spillover efficiency With this definition the resulting antenna temperature is on the same scale T5 as with the chopper wheel calibration 5 7 SIGNAL PROCESSING 79 This calibration scheme has a certain appeal because it is direct and easy to under stand However the method has several difficulties which prevent it in general from being as accurate as the chopper wheel method First in the absence of an automatic HOT COLD SKY load system which the 12m doesn t have you must make several time consuming observations These include tipping scans to measure the atmospheric optical depth and manual HOT COLD loads to measure the receiver noise temperature and pos sibly the sky temperature If these measurements are not made frequently receiver and atmospheric drifts may introduce calibration errors In addition measurement of an av erage atmospheric optical depth may not adequately correct for local cloudlets and other atmospheric anomalies Finally the chopper wheel method depends only weakly 7 whereas the direct calibration method depends strongly on it Although we do not recommend the no cal method for most cases you should
4. Opserving Basics sis xxx bee UR SSEORA ket s eS 6 3 1 Switching Modes 6 3 2 Checking the Subreflector Throw 6 3 3 Continuum Sensitivity 6 3 4 The Digital Backend 6 3 5 Software Signal Processing of Digital Backend Data 6 4 Observing 6 4 1 Point Source ON OFF Observing Procedures 6 411 ON OFF Sequence 6 4 3 Mapping Extended Sources 6 4 2 1 6 4 2 2 Continuum On The Fly Mapping 6 5 Utility Observing ol Sky Tip Procedures 4c RR oe asd ee AR s 6 5 1 1 SPTIP Analysis Procedure 6 5 1 2 STIP Reduction Procedure 5 0 Calibration e sli eo tu o p d dia deti dag Oe gh GS Vare Calibration room ete Set eas 6 6 2 Hot Cold Load Calibration 6 6 3 Calibration of the Flux Density Scale 6 7 Continuum Status A Pointing Equations for the 12m Telescope A l Primary Pointing Equations A 1 1 Secondary Pointing Corrections 1 74 74 79 TT TT 78 79 79 79 80 80 81 82 87 87
5. 6 5 F noisetute 2 exp 6 6 where is the average noise tube calibration signal from each sample VANE is the signal measured from the vane SKY is the signal measured from the sky A is the airmass and the zenith optical depth Te is the calibration scale factor defined above When using noise tube calibration the verbs switched and totalpwr compute a system temperature and store the result in the header under the label tsys The computation of tsys is by way of the equation t Sys qu REF REF CAL REF p The verbs switched and totalpwr also calculate a signal to noise figure of merit called tpsn which is stored in the adverb vrms 2 tpsn is the switched power signal divided by the total power signal and is given by Dy SIG REF Se 6 8 SIG REF s The basic condar data reduction commands are given below 1 is either 1 or 2 for continuum channels 1 or 2 In the c 1 command below 1 can be either 1 2 or b for both tpsn 0 5 x 96 CHAPTER 6 CONTINUUM OBSERVING Scan number get table produces a table listing of scan number Scan number s 1 displays a switched power ON OFF sequence Scan number miti displays a mapping row Scan number fii displays a five point map scan number sptip 1 displays vane switched tipping scan Scan number foca l displays a focus check scan scan number citi stacks scans with sp
6. OH 0 06 ws oseta osorg o amos soo1nog Surjuroq e1329dg 91991 44 CHAPTER 4 TRACKING POINTING AND FOCUS EL Point 3 T 8 AZ Point m AZ Point Center Point El EL Point AZ Five Point Mapping Scan Layout and Order of Measurements Figure 4 4 Five point mapping scan sequence 4 6 POINTING 45 2 Ask the operator to setup for a spectral line five point measurement on the chosen source Set the integration time per sample the number of ON OFF pairs to measure on the source the number of scans per calibrate and for a total power five point the ON and OFF integration time Depending on the source size you may also have to set the grid spacing appropriately All other parameters will default to reasonable values 3 Choose an azimuth offset angle for the reference to be used for position switch ing 4 Using the pointing charts make an initial guess for the azimuth and elevation pointing offsets 5 Ask the operator to start the measurement To process these measurements there are a number of condar and line procedures available 4 6 1 1 Continuum Five Point Analysis Continuum five point measurements are processed automatically by the on line dataserver The results from these on line fits are sent automatically to the operator for entry into the control system To process a continuum five point measurement manually use the condar resident proce
7. Thess T fss Tlemb Ne Atmospheric optical depth at the zenith Maximum antenna gain Efficiencies Radiative efficiency P QdQ C 2 Rearward scattering and spillover efficiency Don P Q dQ Jus P Q dQ C 3 Tr Mrss C 4 Forward scattering and spillover efficiency llo P Q dQ Toe Pal dQ Main beam efficiency Lan Pag Q dQ San P Q dQ 6 Efficiency at which the source couples to the main diffraction beam of the telescope 2 9 B V d V Van 0 40 C 7 Efficiency at which the source couples to the telescope beam P Y Pal dQ C 8 C 9 Temperatures Source radiation temperature Observed source antenna temperature Tr exp If Q B V dU C 10 2 RELATIONS BETWEEN TEMPERATURE SCALES 121 T4 Observed source antenna temperature corrected for atmospheric attenuation 11 T4 Observed source antenna temperature corrected for atmospheric attenuation radiative loss and rearward scattering and spillover 12 1 1 Observed source antenna temperature corrected for atmospheric attenuation radiative loss and rearward and forward scattering and spillover 72 2 13 T fss Source radiation temperature excluding any background emission like the cosmic microwave background emission Tog Ts LE C 1
8. UMOYS oq Jo uorgoung se 1018510165001 9 11814 ZH5 Kouenbeua 005 052 00 OST 001 0 120 e 0 gt 5 G0 2 7 VO d co E N 90 5 2 5 3 o i LO gt o 2 E 80 45 ea 60 os L I 20 2HD 522 4 AMd wwr ye uorssrussueJ oureudsouny 90 CHAPTER 6 CONTINUUM OBSERVING 6 3 1 Switching Modes In contrast to spectral line signals continuum signals are fundamentally indistinguishable from other broadband noise sources such as atmospheric emission spillover radiation from the ground and surrounding buildings and receiver gain fluctuations Such noise sources particularly atmospheric emission are in fact usually stronger than the celestial signal Furthermore atmospheric emission may change over very short time scales lt 1 second and may vary significantly at sky positions separated by as little as a few arc minutes To distinguish the celestial signal from other broadband signals the 12m systems pro vide various switching and subtraction techniques Most of the techniques employ beam switching of one type or another Beam switching is accomplished by a nutating subre flector also known as a chopping secondary The default subreflector switch rate is 4 Hz The subreflector throw can be varied from 0 to its physical limit of 4 5 The two subreflector positions
9. 5 31 log T A 7 8 PRIMARY POINTING EQUATIONS 115 Note that Ulich 1981 quotes a similar expression for log e 44 but does not give a refer ence The weather dependent term is calculated using the on site weather station and is recalculated every time the surface temperature changes by more than 1 5 the surface relative humidity changes by more than 5 or the surface barometric pressure changes by more than 1 Torr Ro usually remains in the range 48 to 60 A plot of Ro as a function of and relative humidity for a constant total surface barometric pressure is shown in Figure A 1 The elevation dependence of the refraction correction is given by 3 cos E ME tan SB This term is recomputed for each elevation of observation The 12m staff determines the pointing coefficients regularly To derive a new pointing model a campaign of optical and radio pointing sources measurements are made Both the optical and radio pointing sources are observed at as many azimuth and elevation positions as possible and the corrections necessary to receive peak flux from the sources are measured The corrections are then input to a fitting program and the coefficients are determined The total rms of the fit is typically 3 5 A 8 A 1 1 Secondary Pointing Corrections In addition to the primary pointing corrections a set of secondary pointing corrections are used at the 12m In azimuth these corrections
10. NS North top and south bottom focus translation stage offset position in mil limeters 5 9 SPECTRAL LINE STATUS MONITOR 83 EW East top and west bottom focus translation stage offset position in mil limeters Box 4 Azimuth and elevation position and pointing offset information COMMAND Commanded azimuth and elevation spherical coordinate conversion plus pointing corrections ACTUAL Actual azimuth and elevation encoder readings ERROR Difference between COMMAND and ACTUAL azimuth and elevation OFFSET Total azimuth and elevation offsets sum of pointing offsets plus subre flector beam throw POINT Azimuth and elevation pointing model corrections PSREF Reference position offset in azimuth and elevation Box 5 Receiver calibration and tipper information ATM Number of air masses toward current elevation RCVR Receiver name RBAY Receiver bay FO Axial focus zero position in millimeters TC Calibration scale factor for receiver channels 1 top and 2 bottom TAUO Actual top and 225 GHz bottom measured continuously using a tipping radiometer zenith opacity FWHM Beam FWHM top and HWHM bottom used for five point measure ments in mm ss TSYS Channel 1 top and 2 bottom system temperatures in MM_H20 Millimeters of water vapor at zenith based on the 225 GHz tipping radiometer measurement of 7 and a conversion of 1 millimeter H20 equals an opacity of 0 05 at 225 GHz Box
11. Spectrometer Configuration Filter bank spectrometers with several different resolu tions are available The observer must choose which to use and how they should be configured the parallel or series option The Millimeter Autocorrelator MAC which has numerous resolution and bandwidth options is also available Observing Mode Choices are m Total Power Scans ON and OFF total power spectra are recorded separately for later processing into final spectra The observer may take several ON scans for each OFF Position Switching The telescope moves between an offset or OFF position in relative or absolute coordinates and a source or ON position The spectrum is recorded as a ratio of ON OFF OFF Frequency Switching The local oscillator is shifted by several MHz at a rate of 1 25 2 5 or 5 0 Hz The spectrum is recorded as a ratio of SIG REF REF Beam Switching plus Position Switching The subreflector is chopped at a rate of 1 25 to 5 0 Hz and the telescope is repositioned at a prescribed rate typically every 30 90 seconds The spectrum is recorded as ON OFF a Grid Mapping Total power or absolute position or frequency switched data are acquired while the telescope moves to user specified spatial positions a On The Fly Mapping Total power data are acquired every 0 1 seconds while the telescope is driven continuously over a specified map field 95 56 CHAPTER 5 SPECTRAL LINE OBSERVING
12. scan_number nn s ave Gets displays and prints the average and rms flux of the continuum ON OFF sequence scan_number xx scan_number nn f Analyzes and plots a five point map of scan_number nn scan_number nn focalize Analyzes and plots a axial focus measurement of scan_number nn scan_number nn sptip Analyzes and plots a sky tip measurement Commands Specific to line Tells line that you wish to analyze filter bank data Tells line that you wish to analyze Millimeter Autocor relator MAC data Displays the most recent chopper wheel calibration array for filter bank number where is either 1 or 2 gget scan number Gets and displays the calibration scan associated with filter bank of scan number where is either 1 or 2 scan number f Gets and displays the first filter bank of scan number scan number s Gets and displays the second filter bank of scan number halves Displays the average of two filter bank polarizations ac quired in parallel mode gcopy Make a laser printer plot of the graphics screen 2 4 ALTERNATE DATA ANALYSIS PACKAGES 15 Table 2 3 Analysis Procedures in line badch Starts interactive bad channel flagging procedure bset Interactive procedure to set baseline fit parameters nfit n Sets the baseline order to n baseline xx Fits subtracts and plots the resulting baseline gset Interactive procedure to set gaussian fit parameters gauss Fits gaussians gparts Plots in
13. www tuc nrao edu Tucson html Three rooms are available in the dome for observer use During scheduled observing time you will normally want to sit in the control room at the observer s console so that you can communicate with the operator An adjacent breezeway room has an additional workstation and work area A third room called the observer lounge is available for work data reduction and private phone calls This room has a couch that can be used for naps If two observing teams are sharing time on the telescope the data reduction station in the observer lounge is reserved for the team not currently observing The team not currently observing should stay out of the control room if at all possible If more than two observing teams are sharing time at the telescope they should negotiate the use of the observer lounge 3 2 Telescope Optics The 12m employs bent Cassegrain optics for all of the receivers used by visiting observers A few test and special purpose receivers including the holography receiver are mounted at the prime focus A diagram of the optics is given in Figure 3 1 The primary mirror is a 12 0 meter paraboloid of 72 aluminum panels The position of each panel can be adjusted by stand off bolts The subreflector secondary mirror is mounted at the prime focus and is supported by a quadrapod feed leg structure The subreflector mounting box contains the nutation beam switching electronics and the sole
14. National Radio Astronomy Observatory Tucson Arizona User s Manual for the NRAO 12 Meter Millimeter Wave Telescope Kitt Peak Arizona J G Mangum January 18 2000 Preface Our intent with this manual is to provide a general reference book for the operation of the NRAO 12m millimeter wave telescope We have included basic material required for first time use of the telescope as well as more detailed information of possible interest to all observers In addition to this manual we recommend that first time users read the companion document Visitor s Guide to the NRAO 12m Telescope before coming to the telescope We are sure that there is room for improvement in this manual and ask users to bring to our attention any errors omissions or unclear passages Please note that we actively maintain this manual The most up to date version of this document can be found on the NRAO Tucson Home Page at http www tuc nrao edu Tucson html and at the telescope There are a number of companion documents which you should consult for more detailed information regarding On The F ly observing the current equipment and calibration status and the 1mm Array receiver The following list itemizes these supplementary documents most of which are accessible via the NRAO Tucson Home Page m The NRAO 12m Telescope Equipment and Calibration Status Information on current equipment characteristics such as receiver temperatures telescope effici
15. Observing Time Budget Prior to the start of observations you should make a rough budget of observing time requirements In addition to the integration time on the program sources you should allow time for overhead items such as telescope move ment and pointing and calibration tests Before beginning program observations check the telescope pointing and focus A few observations of test sources are also advisable These observations are discussed in Chapters 2 4 and 6 5 2 Sideband Choice All of the 12m receivers operate by default in a single sideband SSB mode and require upper sideband USB operation For the 2 and 3mm receivers it is possible to observe DSB Equipment constraints such as the tuning range of the local oscillator or the receiver may sometimes determine the sideband used in DSB mode Other times the presence of telluric lines steer the choice see Figure 6 1 When using DSB mode make the sideband choice with care The primary things to watch out for when choosing the sideband configuration for a DSB measurement is the presence of contaminating lines from the image sideband Consult a good tabulation of spectral lines such as the Lovas Catalog F J Lovas J Phys Chem Ref Data 15 251 1986 to see what spectral lines are present in both the signal and image sidebands If an image line is too close to the program line in the signal sideband a small local oscillator shift will usually cure the prob
16. 18 17 26 5 416 14 54 18 17 26 5 16 14 54 CAL 400 0 TS 417 FREQ 88631 85 SYN 1 93383272 VEL 19 4 DV 3 58 FR 1000 SB 2 Bs 5 4 OBSERVING MODES 0 8 0 6 0 4 0 2 Q0 e m EA 15 10 5 0 69 T T T Mesospheric CO 120 p o L I 1 Mesospheric CO 2 1 eee 4 1 1 4 72 2 1 Visr km s Figure 5 5 Frequency switched spectral line scans of the telluric CO J 1 0 and J 2 1 emission from the Earth s atmosphere 70 CHAPTER 5 SPECTRAL LINE OBSERVING 1 The magnitude sense and symmetry of the frequency shift lt 5 MHz 2 The switch rate the usual setting is 5 Hz 3 The total integration time of the scan Note that the time overhead for frequency switching is about 7 To reduce in band frequency switched data in the line data reduction system use the fold command 5 4 5 Beam Switching Spectral line beam switching can be useful when observing small angular diameter sources and when the best possible baselines are needed This observing mode involves the nutation chopping of the subreflector and a positional movement of the telescope and is thus
17. 2 3 Basic Data Reduction with UniPOPS 55 525 TCR RE emen 11 2 4 Alternate Data Analysis Packages 13 DAN SS ieu git aedi V AS at e SY a 13 2422 JORAWSD G x mu hae SEM eae ee ee ee Wa 15 2 5 Data Archiving and Export 17 3 Instrumentation 19 3 1 Telescope Site Layouts e voso tos Ble ta ee e we a a 19 m2 Telescope Optics xj og d pe eee wae 19 Bie J e eM See ir eus dot he iE ed 20 3 3 1 Imi Array Receiver 23 3311 1mm Array Rotator and Positioning Conventions 24 3 3 1 2 Pointing and Mapping Offsets with the 1mm Array Rotator 25 3 4 The Local Oscillator System 26 iii CONTENTS Bo CERO oe ure e le d d 27 107 Opectrom ters uu d oh b t S eem ESX SL 28 3 6 1 Filter RM ERE TERRESTRES LUSIT 28 3 6 2 Millimeter Autocorrelator 28 3 6 3 Continuum Backend 29 3 7 Computer 29 Tracking Pointing and Focus 31 4 1 Tracking Capabilities 31 4 1 Ephemeris Obj cts ke Gl eS 32 22 Tracking 5 es agde OE ek oi Rr a 33 42 1 LIMS ux ee Beet N
18. 87 90 91 96 98 110 width 4 9 10 22 23 34 38 42 51 83 110 blanking subreflector 29 87 93 calibration 84 antenna temperature 4 11 87 beam switched position switching BSP 80 continuum 29 94 95 103 105 107 hot cold 103 105 noise tube 19 94 95 106 107 relative 105 vane 103 gains see scan calibration see scan calibration see scan calibration 138 general 10 11 56 63 83 87 100 101 110 scan 11 12 14 57 60 source 108 sources 105 spectral line chopper wheel 79 spectral line 66 72 chopper wheel 70 79 direct 78 vane 66 75 77 79 119 vane 57 catalog ephemeris object 32 format 31 source 9 31 63 66 71 72 74 standard source 11 Cone of Silence see frequency switching FS spectral line observing modes see frequency switching FS spec tral line observing modes coordinates Cartesian 25 epoch 31 34 equatorial 9 15 31 34 64 topocentric 33 galactic 9 31 34 geocentric 33 distance 108 parallactic angle 24 25 66 reference position 66 source 2 55 data analysis 4 11 13 19 29 47 AIPS 99 condar 100 101 continuum 100 101 103 106 INDEX gpoint see pointing analysis data spectral line 57 64 70 UniPOPS see condar UniPOPS see line UniPOPS archiving 17 CLASS format 13 15 continuum 29 93 95 99 On The Fly OTF 99 OTF 99 Drawspec format 15 FITS format 15 raw see sdd Single Dish Data see sdd
19. FREQ 86243 44 SYN 1 96844368 VEL 9 5 DV 0 35 FR 100 SB 2 Figure 4 6 Display of each scan produced by the line procedure fivel USE OLD BASELINE INTEG REGIONS 1 YES 0 0 The observer must de fine baseline fitting and spectral line integration regions for this routine If these regions have already been accurately set as for example in a five point of the same source performed immediately prior to the present one you can answer yes and speed up the reduction process considerably If you answer no the spectrum at the center of the map is displayed on the screen and the question ENTER OF BL SEGS is printed This part of the froutine is the same as the bset procedure The observer must enter the number of baseline segments to be used to fit a least squares baseline NOTE The order of the fit is determined by whatever n fit is set to the default is 1 see the line data reduction manual for a description of the baseline fitting facilities When the number of segments to be fit is entered the graphics crosshairs appear and you must mark off the two ends of each segment with the crosshairs move the crosshairs to the desired position and strike any mouse button When this operation is finished the baseline is removed from the center spectrum and the spectrum is re plotted The prompt ENTER INTEG REGIONS is printed and the crosshairs reappear You should then mark off the two velocity frequency extremities
20. If the data were taken in parallel mode the two polarization channels in each filter bank are automatically averaged CHAPTER 4 TRACKING POINTING AND FOCUS 46 SCAN CH 5626 DATE TELESCOPE COMMANDED POI NTI NG OFF 09 18 96 Input AZ Pointing Correction 52 159 85 FIT W TH NO RESTRI CTI ONS EST 02 17 47 AZ 226 02 27 00 18 15 40 RMS of Hux Measurement SOURCE Soturn EL 47 23 55 0 03 118 99 19 67 SEC 5 TAMB HP 2275 0 30 7749 12 9 50 BEAM 02 00 00 00 2 00 00 00 Subreflector Beam Throw Input EL Pointing Correction 94 04 24 19 N 279 21 35 48 Best Fit Location for Pointing Source SYM DAZ 4 8 PEAK JY 285 29 B DEL 0 5 PEAK JY 285 29 FIT W TH AZBW ELBW 60 SY DAZ 4 9 268 09 DEL 0 5 PEAK JY 266 53 125 24 15 62 BOL N W 55 1 W 54 9 BOL F Fitted FWHP Beam Widths CORRECT AZ 00 13 ED OFFSETS EL 00 03 New Pointing Offsets Figure 4 5 Continuum Five Point Analysis Example 4 6 POINTING AT VELOCITY RADILSR 10 50 10 30 54 17 1 1 1 L 1 1 1 1 752 0 52 50 729 0 10 85 728 0 a U 12 4 4 12 FREQUENCY CHICYG 728 01 INT 00 00 30 DATE 06 SEP 98 1950RADC 19 48 38 5 32 47 10 19 48 38 5 32 47 10 CAL 400 0 TS 335
21. Single Dish Data reduction UniPOPS see condar UniPOPS sdd Single Dish Data 12 spectral line 64 66 subscan 12 13 45 51 139 coefficients 53 primary plate scale 53 137 frequency IF 27 56 81 84 111 LO 26 56 84 106 111 synthesizer 26 observing 2 4 9 11 20 22 26 32 38 56 60 63 66 83 87 89 92 100 103 105 108 110 111 123 124 offset see offset filter banks spec trometers see offset Millimeter Au tocorrelator MAC spectrometers offsets see offsets filter banks spec trometers phase lock loop 26 spatial 74 75 synthesizer 56 dormitories 7 limits efficiency elevation 2 aperture 122 beam 77 mirror beam switching 70 primary 19 main beam 120 main beam coupling 120 source coupling 120 122 spectrometer 77 133 135 switching 98 telescope 22 92 122 aperture 22 90 92 103 105 110 corrected main beam 22 forward spillover 22 76 78 120 radiative 120 rear spillover 22 76 78 99 103 120 focus axial 10 11 14 20 25 48 51 56 72 83 84 87 88 110 111 east west 20 51 53 83 109 coefficients 53 focal ratio 2 modulation 67 north south 20 51 53 82 109 quaternary 20 secondary see subreflector telescope tertiary 20 observing modes continuum east west focus 53 five point 10 38 42 45 48 51 83 87 96 110 focalize 10 grid mapping 87 98 north south focus 53 On The Fly OTF 37 87 90
22. The output of the mixer is an intermediate frequency that is the difference between the LO and RF signal frequencies The SIS junctions in the 3 and 2mm receivers have tunable backshorts which can be adjusted to resonantly cancel the unwanted sideband and are essentially single sideband SSB mixers A harmonic generator is switched into the optical path of the receiver to allow precise measurement of the sideband rejection in the 2 and 3mm receivers The image sideband of the 1mm receiver is rejected by inserting a Martin Pupplet filter into the LO path At most frequencies the image sideband can be rejected to gt 20 dB Table 3 1 lists the current tuning ranges and typical system temperatures for all of the facility 12m receivers while Table 3 2 lists representive telescope efficiencies 3 3 RECEIVERS 21 Nutating Subreflector Primary Reflector p 55 Mirror 1 of 4 Central Selection Mirror Rotatable to allow access to any one of four receiver bays Quaternary Mirror RX1 RX2 Primary Focal Length 5 08m f D of Final Beam 13 8 Figure 3 1 12m Telescope Optics CHAPTER 3 INSTRUMENTATION Table 3 1 12m Receiver Characteristics Tuning Range GHz Approximate 55 68 90 170 225 90 116 160 350 Table 3 2 12m Telescope Efficiencies a aperture efficiency bm rear spillover and scattering blockage and ohmic loss efficiency ness forward scattering and spillover
23. To start the program type gpoint from any login prompt gpoint is menu driven so you need only select the appropriate entries in each menu to configure the data selection and display parameters to suit your needs An example of a gpoint plot is given in Figure 4 8 4 7 Focus 4 7 1 Axial Focus To bring the incident radiation to a focus at the receiver feed horn the subreflector of the 12m may be moved in and out along the electrical axis of the telescope The focus exhibits an elevation dependence which is automatically corrected by the control system computer according to the equation F E Fy 2 8 sin E 4 1 where F E is the focus setting in millimeters E is the elevation and Fo the subreflector focus setting at 0 elevation is to be determined by the observer Fo is given in millime ters and larger values of Fo represent greater distances between the dish surface and the subreflector An automatic procedure called a focalize this word really is in the dictionary will determine the best value for Fg An analysis routine of the same name exists in condar and line for displaying and fitting the results Instructions for performing a focalize are given in 84 7 2 SCAN FB DATE LST SOURCE SEC NS FBR SB HP 730 100 9 06 98 20 32 52 CHICYG 15 2 334 100 2 35 AZ EL REST FREQ 86 243442 TELESCOPE COMMANDED 280 28 06 81 02 49 SYNTH 1 96844366 MAI N OFF O 15 00 09 VELOCI 9 5 3 45 2 84 Input
24. conversely it may converge to a nonsensical result Use stip with caution To analyze an sptip scan with stip in condar Condar gt scan number stip If you want to solve for set etafree 1 in condar etafree 0 by default Figure 6 4 shows what the stip output looks like 6 6 Calibration To accurately calibrate continuum data you must set the temperature scale and or flux density scale correct for atmospheric attenuation and correct for telescope systematics such as the gain elevation effect The method currently used to calibrate 12m continuum data is called vane calibration 6 6 1 Vane Calibration Calibration of the antenna temperature scale for continuum data is accomplished using the vane calibration technique see 5 6 1 In addition to a calibration of the antenna temperature scale absolute calibration of continuum data also requires a measurement of the aperture efficiency or a factor for converting from antenna temperature to flux density It may also be necessary to apply a correction for the gain elevation effect if it is important at the particular observing frequency To calibrate continuum data follow the steps outlined below 1 Perform a hot cold load calibration to determine the receiver noise temperature and to set the temperature scale The procedure for making a hot cold measurement is described in detail in Section 6 6 2 This measurement will produce a value for Te the calibration scale factor which is r
25. os Form us 312 39 40 CHAPTER 4 TRACKING POINTING AND FOCUS Table 4 1 Galactic and Extragalactic Continuum Pointing Sources continued X Oum 10 Const manas ma _ iaa 00 81 10 24 0 4 1044 719 71 59 27 1 5 10 55 55 3 B 1055 018 10 55 55 3 01 50 04 2 0 1116 128 11 16 20 8 12 51 07 0 3 1156 295 11 56 57 8 29 31 26 1 7 A 30273 12 26 33 2 02 19 43 25 2 VIRA 12 28 17 6 12 40 02 5 4 25 31 28 1 8 sanwas 15 352320 314 ezar senos eoan os _ Toer 15464 11 somos soo 312 312 4 6 POINTING Table 4 1 Galactic and Extragalactic Continuum Pointing Sources continued oa 05 Dess Tensa esma esra o OO cas eane esn 48 15 Fesser 10 250020 318 _ sa 34 Tazas pagas noces pases 22 ___ 10 2a Dese 25 O Cena 22826
26. section similar in design to that used at the Green Bank Telescope GBT The full band 3 7 COMPUTER EQUIPMENT 29 width for any given observation is pre filtered through a set of anti aliasing filters and then each IF is sampled in three levels and autocorrelated The 65536 spectral channels of the spectrometer can be divided among up to 8 inde pendent IF channels As such it matches well with the 1mm Array receiver as well as with the dual polarization channel single beam receivers The maximum useable bandwidth of the instrument is 1200 MHz Table 3 5 summarizes the currently available Millimeter Autocorrelator MAC observing modes 3 6 3 Continuum Backend Continuum data at the 12m is acquired by a two channel four phase digital backend DBE The DBE can record two switch phases and two calibration phases The calibration phases can be generated by the synchronous emission of a noise diode which is available at 3 mm wavelengths only The DBE also generates a signal reference pulse to move the subreflector at a default switching rate of 1 25 Hz so that each phase of a four phase switching cycle lasts 125 ms The DBE blanks the input signal i e stops taking data while the subreflector is in transition from one position to another The blanking properties are regularly adjusted by the 12m staff 3 7 Computer Equipment The 12 Meter Telescope computer system is composed of a network of Sun worksta tions The telescope control
27. 41 UT T Tig K B 8 B Appendix C Temperature Scales and Telescope Efficiencies The calibration mode used for essentially all spectral line observations at the 12m is the chopper wheel method see Ulich amp Haas 1976 ApJS 30 247 for a detailed description of this technique The chopper wheel technique corrects for atmospheric attenuation and several telescope losses In the following I describe the temperature scale used at the 12m and how it relates to temperature scales used at other millimeter wavelength observatories C 1 Definitions In the following I define the terms used in the subsequent temperature scale and telescope efficiency discussion I have tried to adopt a similar nomenclature to that used in Kutner amp Ulich 1981 ApJ 250 341 Note that throughout this discussion when I refer to a temperature I am actually referring to the effective source radiation temperature J v which is defined as J v T aoa C 1 General Terms Q Solid angle subtended by the source O4 Solid angle subtended by the central diffraction beam pattern of the telescope Solid angle on the sky VY Direction angle on the sky P Normalized antenna power pattern Normalized Gaussian antenna power pattern B Normalized source brightness distribution A Airmass toward which the measurement is made 119 120 70 Tr APPENDIX TEMPERATURE SCALES AND TELESCOPE EFFICIENCIES
28. 88 88 90 90 92 93 94 96 96 96 98 98 99 99 99 100 101 103 103 105 108 109 CONTENTS The Relationship Between Flux and Brightness Temperature B Uniform Disk Source B 2 Elliptical Gaussian Source Temperature Scales and Telescope Efficiencies Gol MCHA d s Ue C 2 Relations Between Temperature Scales Telescope Efficiency Measurements C 3 1 Corrected Main Beam C 3 2 Main Beam Efficiency Spectral Resolution and Sensitivity Bandwidth in Spectrometers D 1 Function Integrals 276 qme Rex BK M oe RO BO me me BS edi sot os reb eire to n SES c QE OK Ci ee vl Ld arai e dida Ip NES SENE MEME Sh ak teat RG potai c i ek ee Walsh Function Modulation The Radiometer Equation for Position Switched Measurements The 12m Telescope Primary Focus Plate Scale 117 117 118 119 119 121 122 122 122 125 126 126 126 127 127 129 133 137 Chapter 1 Introduction 1 1 The Observatory The National Radio Astronomy Observatory NRAO 12 Meter Telescope is a general purpose radio astronomical observatory that supports spectral line and continuum obser vations in the atmos
29. 98 99 sequence 14 38 42 95 98 105 106 sky tip 79 92 96 101 spectral line five point 15 45 47 48 grid mapping 71 74 manual offsets 71 On The Fly OTF 23 71 74 140 phase lock digital 26 pointing 4 6 10 11 31 33 36 accuracy 2 34 37 model coefficients 48 114 115 offsets 10 refraction 84 111 114 115 position encoder 1mm Array 24 proposal deadlines 1 3 receiver 1mm 29 imm Array 24 25 27 offsets 25 pointing 25 26 bay 20 106 115 dewar 20 27 double sideband 55 77 81 88 106 feed horn 48 67 79 105 115 137 noise temperature see receiver noise temperature rockets 20 sideband rejection 9 11 20 28 56 57 67 imm receiver 20 system temperature see system temperature receivers 22 Sources asteroids 32 comets 31 32 ephemeris objects 31 32 36 spectrometers filter banks 10 13 27 28 45 55 57 60 63 70 77 81 83 84 125 multiplexer 28 67 80 84 offset 60 63 84 parallel series configuration 12 14 28 45 55 57 58 60 84 MAC see Millimeter Autocorrelator MAC spectrometers INDEX Millimeter Autocorrelator 28 Millimeter Autocorrelator MAC 10 12 23 29 55 57 60 offset 63 spectral resolution 10 28 55 57 60 125 126 subreflector 48 90 93 96 telescope drive 5 33 36 105 elevation 2 feed legs 6 19 Friend of 60 gain 51 53 103 108 illumination 74 75 137 latitude 2 longitude
30. AZPointing Correction 50 7 74 4 62 Azirtensty of Channel 2 Fitted FWHP Beam Widths in AZand EL FAG 5 60 FIT FI BW SYMBOL A NONRESTRI CTED FIT SYMBOL DAZ 4 1 9 34 DAZ 4 1 PEAK 9 27 BW 69 9 DEL 10 8 9 90 DEL 11 3 9 27 BW 77 9 FIT FIXED BW SYMBOL C NONRESTRI CTED FIT SYMBOL D 3 7 PEAKS 58 1 DAZ 3 7 PEAKS 8 14 Bw 72 0 DEL 8 9 PEAK 8 27 DEL 9 0 PEAK 8 14 BW 71 6 el FIT 2 AVG CORRECTED MAIN OFFSETS AZ 600 11 00 11 00 11 Figure 4 7 Spectral line five point analysis example 50004 ZF 6 CHAPTER 4 TRACKING POINTING AND FOCUS Elevation Pointing Corrections 3C454 5 2145 067 Delta El arcsec 0 02024149 0607 157 40 0 10 20 50 40 50 60 70 80 90 100 Elevation obs 24 4 1996 06 27 Figure 4 8 GPOINT display example Each pointing source is indicated by a unique symbol while the arrow on each symbol indicates whether the source was rising or setting 4 7 FOCUS 51 4 7 2 Determining the Axial Focus The axial focus of the telescope should be checked occasionally during an observing run It is often most convenient to make a focus check after a continuum five point pointing measurement since both measurements use the same kinds of sources strong continuum point sou
31. Sampling Figure F 1 Correlator efficiency for the MAC a 2 bit 3 level correlator as a function of oversampling Note that for the MAC which is oversampled by a factor of 5 over Nyquist the efficiency is 0 847 Appendix G The 12m Telescope Primary Focus Plate Scale The primary focus plate scale which determines the amount of beam displacement as a function of lateral displacement of the subreflector is given by f 0 _ ws sin 44 f f F r r dr 0 G 1 where 0 is the beam deviation on the sky F r is the receiver feed illumination pattern and f is the primary focal length The term in braces is often called the beam deviation factor For the 12m receivers F r is given by F r exp 2 57 0 5952r G 2 For the 12 Meter Telescope f 0 42xD which means that the beam deviation factor is 0 827 and that the plate scale for prime focus subreflector lateral movement 31 5 mm 137 Index atmospheric opacity see atmospheric at tenuation see atmospheric atten uation see atmospheric attenua tion attenuation 106 atmospheric 76 78 92 99 103 119 121 beam 20 22 88 93 98 119 1mm Array 23 beam switched position switching BSP 55 beam switched position switching BSP 38 70 80 position switching PS 66 sidelobes 76 77 switching 10 19 36 37 67 87 88 90 91 96 98 99 restoration algorithm 90 98 99 throw 36 37 70 83
32. amplified and filtered It is then split into two paths one for spectral line signals and one for continuum For continuum applications the 1 5 GHz signal is detected and passed directly to the backend continuum signal processors The spectral line signal must be mixed down to the baseband frequencies at which the filter banks operate The IF Processor Module performs this function The incoming 1500 MHz signal is first upconverted to 2442 MHz The mixer signal for this upconversion originates with a tunable Fluke synthesizer in the control room The frequency of the mixing can be changed by small amounts and the two IF channels can be controlled by separate Fluke synthesizers if desired This affords the observer some flexibility in setting up his observations For example the IF might be changed to get spectral lines in opposite 28 CHAPTER 3 INSTRUMENTATION Table 3 4 12m Filter Spectrometer Characteristics Filter Bandwidth Channels Filters per Bank Filter Banks Available NOTE This is the FWHM channel width See Appendix D for further details Series option only sidebands to fall within the bandpass One of the channels could also be offset in frequency relative to the other primary restriction to these changes is that the spectral line emission must fall within the 600 MHz bandpass of the first IF amplifier Chapter 5 Spectral Line Observing provides a more detailed description of these options 3 6 Spectr
33. and one for continuum describe the data reduction systems in use at the 12m These are available at the telescope and upon request The discussion below is intended only as a quick reference list to help the observer get started Raw data is accumulated into two data files one which contains filter bank data and a second which contains Millimeter Autocorrelator MAC data Both of these files are in the 12 CHAPTER 2 GETTING STARTED Table 2 1 Subscan Codes For 12m Data Filter Banks 01 IF1 in filter bank 1 series or first half of filter bank 1 parallel 02 IF1 in second half of filter bank 1 parallel 03 IF2 in filter bank 2 series or first half of filter bank 2 parallel 04 IF2 in second half of filter bank 2 parallel Millimeter Autocorrelator 11 12 18 IF1 IF2 IF8 for 1mm Array observations IF1 in 2IF mode or IF1 at frequency 1 in 4IF mode IF2 in 2IF mode or IF1 at frequency 2 in 4IF mode IF2 at frequency 1 in 4IF mode IF2 at frequency 2 in 4IF mode Single Beam Continuum imm Array Continuum 01 02 08 IF1 IF2 IF8 sdd or Single Dish Data format Each of these files has an associated gains or gsdd file which contains results for spectral line calibration scans In the following we give a brief introduction to the analysis of the spectral line and continuum data accumulated into these data files Before observations begin the operator will set up a subdirectory con
34. and primary data analysis functions are controlled by a dual processor Sparcstation Ultra The control program is a C language based inter face package called RAMBO The RAMBO interface handles all of the functions of data acquisition including communication with the VxWorks device drivers which run the spec trometers receivers and other devices associated with a particular measurement Most major analysis packages are available on any of the telescope workstations 30 800 800 400 400 200 200 100 100 800 800 400 400 200 200 100 1 00 00 8 800 400 400 200 200 100 100 CHAPTER 3 INSTRUMENTATION Table 3 5 Millimeter Autocorrelator MAC Configurations Bandwidth and Channels Useable Bandwidth and Channels Resolution um eum gt 512 1024 1024 2048 2048 4096 4096 8192 600 600 300 300 150 150 T5 79 600 600 300 300 150 150 T5 79 2 IF Modes 4 IF Modes 768 1536 1536 3072 3072 6144 6144 12288 8 IF Modes 384 768 768 1536 1536 3072 3072 6144 3 0 6 1 781 2 390 6 195 3 97 6 48 8 24 4 12 2 6 1 1 The useable bandwidth takes account of the 75 efficiency of the analog filters 2 NOTE This is the frequency sampling interval not the FWHM channel width for a given channel The FWHM channel width is 2 0 times this value See Appendix D for details All values in this table refer to each IF Modes tagged with a are produced by
35. are termed the BEAM and the With the analog phase sensitive detectors the source produces a positive deflection on the chart recorder when it is in the BEAM and a negative deflection when in the BEAM Two types of beam switching are employed single beam and dual beam For point source ON OFF scans a dual beam observation means that during the course of the scan the telescope is positioned so that the source is alternately in the BEAM and the BEAM For mapping observations the dual beam mode means that source emission is expected in each of the subreflector beam positions and that the dual beam restoration algorithm Emerson Klein amp Haslam 1979 A amp A 76 92 will be applied A single beam observation for either point source or mapping observations means that source is observed in only one of the subreflector beam positions The other switching mode is total power position switching for which the subreflector is locked to a single position the BEAM by convention Observers occasionally use this mode to make accurate aperture efficiency measurements so as to avoid any efficiency losses incurred during subreflector switching These observations usually use a planet whose strength is large compared to the emission differences of the atmosphere at the two sky positions of a position switched observation In the rare circumstance in which a source is so extended that the dual beam restoration mapping technique is inapplic
36. dv 2 6 0 54 0 46 cos 27v 128APPENDIX D SPECTRAL RESOLUTION AND SENSITIVITY BANDWIDTH IN SPECTROME 2 16 1 5896 2 935078 I Appendix E Walsh Function Modulation All observations which involve position switching or focus modulation which is usually only done with frequency switching measurements at the 12 Meter Telescope use a Walsh function cycle for each SIG REF measurement For a given measurement of duration P composed of n SIG REF measurement pairs a SIG REF pair is referred to a repeat in the 12m telescope lingo a switching sequence determined by the Walsh function of PALey order 2 1 is built up Building up a PALey order of 2 1 Walsh functions where n is the number of ON OFF pairs perfectly rejects polynomial drift terms of order up to and including n 1 This statement implies that 1 x si PAL 2 1 t dt 20 E 1 This sequence of SIG REF switching can be truncated at any even point Ideally it would be trucated after exactly 2 phases at which point it gives the maximum rejection of an 17 1 and lower order polynomial drift Although we recommend that observers try to truncate this way we don t insist on it The order of polynomial drift completely rejected is then a function of the biggest 2 sequence or the inverse of this sequence that has been repeated without truncation Since the PALey order is calculated in real time we don t need to know in ad
37. efficiency corrected main beam efficiency percent of power in the main diffraction beam relative to the outlying error beam 3 3 RECEIVERS 23 3 3 1 1mm Array Receiver The 1mm Array receiver is designed for rapid mapping of spectral lines in the 215 240 GHz range notably J 2 gt 1 CO and its isotopomers The receiver places 8 independent beams on the sky in a 2 x 4 array each beam is separated from its nearest neighbor by 87 see Figure 3 2 The FWHM of each beam at 230 GHz is 30 hence the telescope must be stepped in position to fill in the beams if Nyquist sampling is desired 180 8 7 Figure 3 2 1mm Array receiver layout Imm Array receiver is intended for use with the Millimeter Autocorrelator MAC and eight channel IF processor The Millimeter Autocorrelator MAC and IF processor can accept 8 IF channels of 600 MHz or narrower bandwidth or 4 IF channels of 600 MHz bandwidth A complete description on how to observe with the 1mm Array receiver is given in the companion document Observing with the NRAO 1mm Array Receiver 24 CHAPTER 3 INSTRUMENTATION 3 3 1 1 1mm Array Rotator and Positioning Conventions The entire 1mm Array receiver cryostat and optics assembly is housed in a precision rotation mechanism which can rotate the orientation of the array of beams to an arbitrary angle on the sky Thus the array may be positioned to take advantage of a particular source geometry Furthermore the
38. follow this procedure if you choose to use it 1 Perform an ordinary VANE cal to look for bad channels and give the operator a list of the bad channels 2 Tell the operator you will be using the no cal method He will first perform a dummy CALIBRATE to zero the bad channels You will not need to perform any more cali brates Tell the operator the value of T you wish to use It s up to you to decide how to calculate this number but you will probably want to measure the receiver noise temperature by a HOT COLD load in the least 4 Perform a tipping scan see 86 5 1 5 Repeat steps 3 and 4 at intervals short enough to catch any significant system or atmospheric changes 5 7 Signal Processing 5 7 1 Position and Frequency Switched Data 5 7 11 Vane Chopper Calibrates When a vane calibration is performed a calibration array called the gains array is computed according to the formula Ii i ec c Ta 5 11 2 Rai Di where C is the effective system temperature for channel i S is the calibration signal vane over the feed for channel i 80 CHAPTER 5 SPECTRAL LINE OBSERVING is the calibration reference cold sky for channel i Z is the zero value response of channel i with no input signal and is the calibration scale temperature The array of C elements is called the gains array The average effective system tem perature of all channels in the multiplexer Tys lt
39. generation program we have a copy of the TBEG4 two body ephemeris generator program written by Don Yeomans which you can use This program will produce a 12m format ephemeris file given a set of orbital elements Contact Jeff Mangum if you would like to use this program 4 2 Tracking Limits 4 2 1 Elevation Limits Mechanical limits constrain the movement of the telescope on both the azimuth and eleva tion axes Under normal source tracking operation the control system software prevents the telescope from moving into these limits The telescope is also equipped with fail safe hardware limit switches that will turn the drive motors off and apply the brakes before the telescope can be damaged When the telescope reaches a lower elevation of 15 0 it begins to depress a safety spring on the elevation stop Tracking can continue until the elevation drive motor begins drawing excessive current this usually occurs at an elevation near 14 5 A final limit switch is tripped at 13 8 elevation The control computer will allow a source to be tracked up to an elevation of 90 The pointing equations diverge at the zenith however and elevations greater than 88 should 34 CHAPTER 4 TRACKING POINTING AND FOCUS be strictly avoided during observations For routine observations we recommend that you avoid elevations gt 80 if possible since both tracking and pointing degrade in that range For maintenance purposes only the telescope can b
40. gt 5 12 is displayed and updated on the on line status monitor each time a CALIBRATE is per formed Note that Tys is an average of both receiver channels when a two channel receiver is in use The calibrated antenna temperatures T5 that are recorded on disk are calculated by the formula Si B R io peg On 5 13 where is the calibrated antenna temperature for channel i 5 is the Source or ON signal and R is the Reference or OFF signal 5 7 1 2 No Cal Signal Processing In the no cal mode the antenna temperatures are calculated from the relation Taj up where 79 is the zenith optical depth called TAUO in the control system and A is the airmass calculated as 1 sin elevation All other symbols are defined in 5 7 1 1 x Aro 5 14 5 7 2 Beam Switched Data Beam switched data are calibrated using the vane but the signal processing is different from position and frequency switching When a calibrate is performed during a beam switching session the gains array is defined as C 5 15 ET Sei zd where the terms are defined above The antenna temperature scale is given by T Si Ri x Ci 5 16 5 8 CHANGING THE INTERMEDIATE FREQUENCY 81 5 8 Changing the Intermediate Frequency Occasionally an observing situation may arise in which a change in the Intermediate Fre quency from the nominal 1 5 GHz is either necessary or desirable One example of th
41. harmonic If these two tests are passed the observer can be confident that he is locked to the correct frequency A final and conclusive method of checking LO tuning is to look for a strong astronomical spectral line in the band if one exists For continuum observations the precise frequency of phase lock is usually of little importance observers sometimes choose to run open loop for simplicity of operation Many of the receivers are more stable when phase locked however 3 5 The IF Section All mixer receivers at the 12m produce an intermediate frequency of 1 5 GHz The IF signal emerging from the receiver dewar must be further amplified and processed before detection by the spectral line and continuum backend devices A two channel IF system situated on the telescope performs this function All the mixer receivers except the 1mm Array system use this same processor the switch from one receiver to another is done remotely from the control room see 83 4 The incoming signal first passes into an automatic leveling module This device is used in spectral line observations to keep the input signal to the filter banks at a constant level thus improving the performance of the filter banks As this device will level out all continuum signals it is turned off by computer command when continuum observations are underway A manual switch in the control room can also turn off the device After leaving the Leveler Module the signal is further
42. if mask amp i bool 1 mask mask gt gt 1 return bool 2 T Aq UAS uonoung 943 Jo edooso o 1939 N ZI 943 3 pouoj3ras ur posn YSJEM AY 3s1g OT 2174 ANSIA a Ty1IVd 2 14 TVd a a TE TVd 132 APPENDIX E WALSH FUNCTION MODULATION Appendix F The Radiometer Equation for Position Switched Measurements Defining the following rms noise level for an ON OFF or GAIN scan measurment T 5 ee AU pos ON OFF AN AIN 2 SC G F 2 to ce F 3 ton Spectrometer efficiency see Table F 4 F 5 we can calculate 7 36 ON OFF 2 Ton 2 Tof f 2 Tof f 2 2 OFF ON OFF OFF GAIN GAIN Tom qoffy F 6 Ser rato F 6 where I have neglected terms quadratic in ON OFF Noting that ine qeu F 7 we see that Equation F 7 and Equation F 6 differ by only a constant which is the volt to temperature scaling factor Therefore 133 134APPENDIX THE RADIOMETER EQUATION FOR POSITION SWITCHED MEASUREMEN 1 7 1 p sean sys ES rms LIN bon tere 1o T spec V Av ton Aton 1 Leijs 1 1 F 8 NspecV Avo If one is taking multiple ON scans per OFF scan then the time spent to acquire scan is given b
43. of observers is declared the prime observer on the telescope schedule equipment changes needed for a program will be done at the beginning of that program s time 1 3 5 Observations Under Poor Weather Conditions There are a variety of weather conditions which can endanger the safety of the telescope It is the responsibility of the telescope operator to take appropriate action if any of the conditions listed below occur 1 3 5 1 High Winds e If the wind exceeds 15 mph observations will be restricted to those quadrants where the telescope drive motor currents are not excessive e If the steady wind or the average of gusty wind exceeds 35 mph the dome door must be closed Observations can be continued through the side of the dome e For winds above 45 mph the dome door must be positioned 180 from the direction of the wind and held fixed Observations can continue through the side of the dome but the dome cannot be moved e If the wind exceeds 55 mph operations must cease and the telescope must be placed in the service position with the stow pins in place 6 CHAPTER 1 INTRODUCTION 1 3 5 2 Moisture Accumulation In or On the Dome If there is fog in the dome or if moisture is condensing on the antenna or equipment the dome door will be closed Observations can continue through the side of the dome If there is a build up of snow ice on the dome the accumulated snow ice must be cleared from the dome door before observations
44. or interpolation works with a finite sized gridding or interpolation function A little oversampling may enable you to reduce the convolution interpolation function by a factor of a few saving a huge amount of computational overhead at the expense of a few per cent more data 5 5 Spectral Line Sensitivities The spectral line calibration technique generally used at the 12m is the chopper wheel or vane method The effective system temperature given by this technique includes corrections 76 CHAPTER 5 SPECTRAL LINE OBSERVING for atmospheric attenuation and antenna spillover blockage and ohmic losses The method does not include a correction for error pattern losses as the error beam will couple differently to different sources The effective system temperature T on the scale defined above is given by C De Ta sky Toys 5 5 MNfss where is the image sideband gain G is the signal sideband gain T is the receiver DSB noise temperature TA sky is the temperature of the sky definition given below is the rear spillover blockage scattering and ohmic efficiency Nfss is the forward spillover efficiency is the atmospheric optical depth at 1 airmass the zenith and A is the number of airmasses generally given by 1 sin elevation T4 sky is given by the equation Ta sky Tsky F Tant T Tembr mim 1 1 171 4 5 6 w
45. seconds per sample and 6 repeats the sequence takes 120 seconds 2 Whether the sequence will be made in beam switched mode or position switched PS mode If the observations are in the beam switched mode indicate whether the Dual Beam DBS or Single Beam SBS option is in effect the DBS mode is the normal case 3 The required telescope tracking tolerance TOL and the optimum pointing positions of the ON and OFF beams consult the pointing charts at the observer s console When you are satisfied that all is in order ask the operator to begin the scan These continuum sequences can be analyzed using the condar procedures described 86 3 5 6 4 2 Mapping Extended Sources Two options are available for mapping extended continuum sources a grid mapping pro cedure in which the telescope steps through a rectangular grid in the azimuth elevation frame and On The Fly OTF mapping For a detailed explanation regarding the pitfalls and perils of undersampling in mapping data see 5 4 6 4 6 4 2 1 Grid Mapping In continuum grid mapping a field is mapped in a rectangular grid in azimuth and eleva tion relative to the field center This observing procedure has been developed for use with the dual beam restoration algorithm of Emerson Klein amp Haslam 1979 Astr Ap 76 92 It will however work satisfactorily for either fixed beam total power mapping or the mapping of fields that are smaller than the subreflector throw via
46. 0 09 58 25 2 15 54 24 CAL 400 00 115271 20 CONTFOC SB 2 FREQ 52 Figure 4 9 Focalize display example 4 7 FOCUS 53 an appropriate correction to the telescope pointing in elevation and azimuth respectively This correction to elevation pointing is 31 5 mm which is the prime focus plate scale derived in Appendix G of movement in the north south focus or east west focus stages In the future the coefficients necessary to control the movement of the north south and east west translation stages will be installed in the computer and applied automatically At this time the north south and east west translation stages can be set to a given position but are usually left fixed in this position Automatic procedures for fitting for maximum north south or east west gain called nsfocal and ewfocal are available These routines operate analogously to the focalize procedure Normally only staff astronomers use these procedures we do not recommend that observers spend time trying to optimize this further 54 CHAPTER 4 TRACKING POINTING AND FOCUS Chapter 5 Spectral Line Observing 5 1 Startup Checklist Once the scientific goals of the observing session are clearly in mind you must decide upon the equipment and observing techniques to be used The decisions to be made and the options are listed below Sideband Choice For double sideband observations care must be taken in choosing the placement of the image sideband
47. 00 ms interval is not averaged with the rest of the data You can choose how much tracking error you are willing to accept This number is called the tracking tolerance and is displayed on the on line status monitor in the lower left hand corner under the heading TOL The 12m typically experiences 3 4 tracking errors under calm conditions The tolerance is usually not set to less than 5 for this reason Typical choices for the tolerance are 5 for 1 3 mm and shorter wavelength observations and 10 for 2 and 3 mm observations Figure 4 2 shows the amount of signal that is lost by a given pointing error expressed as fractions of a FWHP beamwidth 4 4 Sequence of Position Computation Operations The sequence of computer operations that is executed when seeking and then tracking a source is as follows 1 Input RA DEC 81950 J2000 or current epoch or galactic IILbIT 2 If B1950 position precess to current date If galactic coordinates first precess to epoch B1950 The precessed position of a source is computed only when the source is first accessed o21nos FUISTI e uorjmsuer yuze 09e e odooso o3 Jo c 8 sojeorpur 943 uoryeumoop pue o gue 207 p uqnurzy 081 Ole
48. 00 msec steps the monitor updates only every second 94 CHAPTER 6 CONTINUUM OBSERVING 6 3 5 Software Signal Processing of Digital Backend Data The data sent from the control computer and recorded in the sdd file contain the four phases integrated over the number of dumps for each sample or sky position Thus the raw data scan contains four times as many points as there were samples If the sample number is denoted by 5 and phases that sample as the data are recorded in the order 51 S2 0 3 51 2 52 5 51 52 53 51 By 5 5 By No calibration factors have been applied to the data as it is recorded the data are com pletely raw counts from the DBE The raw data are converted into meaningful signals by the analysis program condar The signals that can be computed from the phases are Switched Power SP 2 Total Power TP 4 Cal Signal 2 Zero Level Z Since the data stored in the analysis system are completely raw there are several calibration parameters used by condar which must be set in order to get proper temperature and flux density scaling Some of these can be set in the control system before you begin data acquisition This is the calibration scale temperature If the calibration is based on a noise tube T should be the temperature of the noi
49. 006 056 096 081 OST 061 06 09 06 0 _ _ 4 44 SEQUENCE OF POSITION COMPUTATION OPERATIONS 36 CHAPTER 4 TRACKING POINTING AND FOCUS Relative Gain ee i a 0 01 02 03 04 05 06 07 08 09 1 Fractional Beam Displacement Ax 0 Figure 4 2 Loss of signal due to pointing error 3 If source is an ephemeris object interpolate to current UT 4 Do spherical coordinate conversion from RA DEC to AZ EL 5 Add azimuth and elevation encoder corrections as computed from Equations A 1 A 2 A 9 and A 10 Appendix A 6 Add azimuth and elevation pointing corrections 7 Command telescope to the correct AZ EL position 8 Once every 10 seconds go back to spherical coordinate conversion Between loops extrapolate AZ and EL drive rates every 100 ms to compute positions 4 5 Subreflector Beam Throw The 12 Meter Telescope is equipped with a nutating chopping subreflector that is used for beam switched observations in both the spectral line and continuum modes Other observing modes such as position switching are made with the subreflector locked in 4 5 SUBREFLECTOR BEAM THROW 37 place Obviously the throw position of the subreflector whether it is chopping or locked in place affects the pointing of the telescope The current 12m subreflector switches only in azimuth although mount misalignments may produce a small component in the elevation dir
50. 1 The 4IF Observing Mode 60 bob SAS Mod s ac voe de Wate Ted 63 5 4 1 Total Power 63 5 4 2 Position Switching 64 5 4 3 Absolute Position Switching 66 5 4 4 Frequency Switching 66 DD Bean et SD nee eub ERGs oeni aps 70 nuo Mappings wee Awe ba wee pem 71 5 4 6 1 Manual Offsets 71 CONTENTS 46 27 Grid Mapping ade 5 4 6 3 When Should I OTF Instead of Grid Map 5 4 6 4 An Important Note About Spatial Sampling 5 5 Spectral Line Sensitivities DO Calibration a e decr eh e did a oh 5 6 1 Vane Calibration 5 6 2 Direct Calibration Dit Processing sorida mailaa di wanie US e e S te eS 5 7 1 Position and Frequency Switched Data 5 7 1 1 Chopper Calibrates 5 7 1 2 No Cal Signal Processing 5 7 2 Beam Switched Data a pomo o REA 5 8 Changing the Intermediate Frequency 5 9 Spectral Line Status Monitor 6 Continuum Observing 6 1 Startup Checklist 6 2 Selecting an Observing Frequency
51. 1mm Array rotator can then track parallactic angle so that as the source moves across the sky in hour angle the orientation of the array is the same in the RA DEC frame because the 12m is an AZ EL telescope parallactic angle rotation must be taken into account The rotation center of the array is defined to be its geometric center 1 the center of the box defined by beams 2 3 6 and 7 see Figure 3 2 Three different angles come into play in describing the positioning and movement of the rotator Source Parallactic Angle The parallactic angle is defined as the angle between lines of constant azimuth and hour angle Thus it is a function of the azimuth elevation hour angle and declination of the source The convention employed at the 12m is as follows When a source is on the Prime Meridian 0 Hour Angle in the south parallactic angle is defined to be 9 While the source is above the horizon in the south parallactic angle always increases with time When the source is on the Prime Meridian upper culmination in the north parallactic angle is defined to be 1809 In the north parallactic angle decreases with time Rotator Control Angle The angle describing the rotation of the 1mm Array rotator relative to its position encoders This angle is displayed on the Status monitor The convention is stated as follows When the telescope is pointing due south 180 azimuth and the long dimension of the array is aligned along the Pr
52. 2 slew rate 2 subreflector 19 20 29 surface accuracy 2 tracking tolerance 34 84 98 111 temperature ambient Tims 84 100 111 antenna T 121 121 190 191 T4 80 92 99 120 Ta sky 76 101 Tsky 78 105 brightness Tg planet 108 calibration 77 78 80 cold load Teora 106 107 cosmic background 99 cosmic microwave background Teg 76 hot load Thot 106 107 main beam brightness 121 mean atmospheric Tm 76 99 100 noise tube 107 of liquid nitrogen 105 radiation Tk 77 78 80 92 121 Tr 120 INDEX 141 Rayleigh Jeans equivalent 99 107 receiver noise Tpz 76 78 79 99 103 105 107 sky Ia 79 107 spillover 76 100 surface ambient T 114 system 75 80 83 95 110 warm spillover 99 time integration 56 63 66 70 76 77 84 111 sample 64 66 70 84 111 scan 72 sidereal 82 98 109 Universal UT 82 109 UniPOPS procedures badch 57 dbgauss 91 ewfocal 53 fbdata 14 five 14 fiveline 15 focalize 14 51 53 hcdata 14 nsfocal 53 velocity reference geocentric 32 topocentric 32 33 Walsh function modulation 129 weather wind gusts 34
53. 30 07 8 11 15 28 4 EW 0 000 ERROR 0 00 00 1 0 00 03 6 GALACTIC 133 94746 1 064321 0 000i OFFSET 0 00 00 0 0 00 00 0 i OFFSET 0 00 00 0 0 00 00 0 3 POINT 5 0 0718 1 POREF 0 00 00 0 0 00 00 0 ATM RCVR RBAY FO FWHM TSYS BRAM 1 00 0 00 0 6 30 5 1 14 4 43 3 400 0 0 223 0 30 76411 BEAM 1 00 0 00 QY 20 gt 2 24 400 0 0 112 0 15 135i LINE gt HCN BW CHANNEL CONF HS BW OBJ V ANT V FRAME TYPE SB 10 IF S FREQUENCY Fl 1000 1 256 SER 600 0 14 7 0 1 GEO RAD USB 3 100 265 886432 7 F2 2000 257 512 SER 2 CAL VANE USB 1500 265 886432 FB OFF 0 0 HS OFF 0 0 0 0 SPEC RUN HS RUN 1956 293040 BAD CHAN u 1956 293040 TOL FOCUS TORR 2RH T AMB MODE SCANS SAMPLES SEC TIME 8 INPUT 0 05 45 7 613 3 13 6 46 2 BSP 0 4 90 0 6 00 ACTUAL 0 04 45 7 WIND gt 113 18 46 2 2 90 0 3 001 9 Figure 5 7 Spectral line on line status monitor The numbered boxes indicate sections of information on the display which explained in 5 9 YOLINOW SALVLS ANIT IVH LOWHdS 6 oo 86 CHAPTER 5 SPECTRAL LINE OBSERVING Chapter 6 Continuum Observing 6 1 Startup Checklist Once the scientific goals of the observing program are in mind you must make the following decisions Precise Observing Frequency When choosing a precise frequency consider both the absorption spectrum of the atmosphere and the possibility of contamination by galactic spectral line em
54. 4 Ne Ta Source brightness temperature as measured by the main diffraction beam of the telescope Nem ATR 15 2 Relations Between Temperature Scales We can now combine the definitions above to derive the relations between the physical measurements and the temperature scale used at the 12m and the scales used at other telescopes Combining the equations above we can relate the source temperature corrected for atmospheric attenuation T4 to many of the antenna and source temperatures TA TR C 16 C 17 ql 18 C 19 can also be defined as the source brightness temperature corrected for atmospheric attenuation radiative loss and rearward and forward scattering and spillover if the source is equal to or larger than the main diffraction beam 122 APPENDIX TEMPERATURE SCALES AND TELESCOPE EFFICIENCIES C 3 Telescope Efficiency Measurements Telescope efficiencies are normally calculated using a measurement of the continuum bright ness of a planet for nm or the Moon for In the following I give the relations used to calculate several telescope efficiencies Since the source coupling between a disk source like the planets and a Gaussian telescope beam is given by 1 exp 2 2 20 I will use this term in the efficiency equation derivations given below C 3 1 Corrected Main Beam Efficiency The efficiency factor which converts t
55. 5 230 5 244 9 265 9 291 8 291 8 3 mb arcsec 31 4x35 0 12 2 12 2 8 5 8 4 16 9x15 3 22 1x22 1 43 4x 40 6 L9 2X 17 3 43 4x 40 6 19 2x17 3 13 4x13 4 5 5 5 5 35 5x33 2 19 1x17 2 3 9X3 5 2 2 2 2 17 4 15 7 18 0 16 1 19 5 17 6 3 63 5 7 5 7 5 5 5 5 5 7 1x7 1 17 8x16 1 1 04 0 13 0 66 0 08 April 1997 0 90 0 06 0 57 0 04 November 1996 0 83 0 07 0 53 0 04 June 1997 0 93 0 11 0 59 0 07 June 1997 0 97 0 04 0 62 0 02 August 1996 0 97 0 05 0 62 0 03 August 1996 0 97 0 06 0 62 0 04 August 1996 0 88 0 04 0 56 0 02 August 1996 0 97 0 06 0 62 0 04 August 1996 0 73 0 05 0 47 0 03 November 1996 0 85 0 07 0 54 0 04 November 1996 0 82 0 09 0 52 0 06 November 1996 0 92 0 09 0 59 0 06 November 1996 0 80 0 10 0 51 0 06 November 1996 0 76 0 10 0 48 0 06 November 1996 0 76 0 04 0 48 0 03 June 1996 0 58 0 03 0 37 0 02 November 1995 0 43 0 11 0 27 0 07 October 1996 0 46 0 15 0 29 0 09 October 1996 0 57 0 04 0 36 0 02 December 1996 0 48 0 03 0 31 0 02 November 1996 0 42 0 04 0 27 0 02 December 1996 0 37 0 04 0 24 0 02 December 1996 Jupiter Venus Mars Saturn Venus Jupiter Saturn Jupiter Saturn Venus Mars Jupiter Saturn Uranus Neptune Saturn Saturn Saturn Uranus Mars Mars Mars Saturn Appendix D Spectral Resolution and Sensitivity Bandwidth in Spectrometers For many of the
56. 6 Subreflector beam and quadrant detector position information BEAM Azimuth and elevation position for the Beam in mm ss BEAM Azimith and elevation position for the Beam in mm ss QX Quadrant detector x axis position in millimeters QY Quadrant detector y axis position in millimeters Box 7 Frequency velocity and calibration information LINE Spectral line name for current transition frequency BW First top and second bottom filter bank spectrometer channel width in kHz 84 CHAPTER 5 SPECTRAL LINE OBSERVING CHANNEL The multiplexer channels used for the first top and second bottom filter bank CONF A code indicating the whether the filter banks for configured for serial SER or parallel PAR operation BW Millimeter Autocorrelator MAC bandwidth in MHz OBJ V Source velocity in chosen reference frame in km s AN T V Antenna velocity in chosen reference frame in km s FRAME Velocity frame of reference code TYPE Velocity type SB Signal sideband upper or lower for channel 1 top and channel 2 bottom LO Local oscillator multiplication factor IF S Intermediate frequencies in MHz FREQUENCY Rest frequencies for channels 1 line 1 and 2 line 2 in GHz with their associated LO synthesizer frequencies lines 3 and 4 respectively in MHz CAL Calibration type FB OFF Filter bank offset in MHz MAC OFF Millimeter Autocorrelator MAC offset in MHz SPEC RUN Indi
57. 86 and several have been hit Please drive and walk carefully Please drive very slowly and carefully in all NOAO and NRAO parking or road areas Pedestrians including small children seem to leap out at cars on a regular basis A more complete list of safety rules and recommendations is available in the observers lounge and from the telescope operator You might find it interesting reading al though not required CHAPTER 1 INTRODUCTION Chapter 2 Getting Started 2 1 What to Bring to the Telescope Your observations will be more efficient and you will achieve better results if you have thoroughly prepared for your observing run before arriving at the telescope Most of this work should be done at the time the proposal is written see Chapter 1 For both spectral line and continuum observations you should prepare the following before coming to the telescope Source List A source list with epoch B1950 or J2000 RA and Dec or Galactic coordinates For spectral line observations you will also need the source velocities in the LSR optical or relativistic velocity reference frames see Chapter 4 Keep in mind that the beam sizes for the 12m can be quite small 20 at the higher frequencies so the positions should be appropriately accurate You can save time by composing your catalog prior to your observing run See Chapter 4 for the source catalog format Line Rest Frequencies You should have line rest frequencies wh
58. Banks Most of the analog filter banks have 256 channels each The filters are integrated mul tiplexed and recorded by the control computer every 100 milliseconds A total of 512 channels can be recorded at a time which means that two filter banks can be used for each scan The filter spectrometers available and the ways in which they can be configured are described below Table 5 1 lists the characteristics of the 12m filter banks 5 3 1 1 The Parallel Series Option Most of the 256 channel filter banks can be split into two 128 channel sections that can be fed with independent IF signals When a filter bank is split into two sections the bank is said to be in a parallel configuration Each polarization channel of the receiver feeds half of a filter bank in this configuration The advantage of this mode is that the two halves of the filter bank can be averaged to produce improved signal to noise in the final spectrum The disadvantage of this mode is that the total bandwidth is cut in half Observers of narrow Galactic lines typically choose the parallel option and use two filter banks with different resolutions Observations that require larger bandwidths such as CO from other galaxies usually choose the series mode In the series mode the two filter bank halves are used end to end to analyze a single input IF signal A typical observing mode might be to use one of the 1 filter banks for polarization 1 and the other for polarization 2 T
59. Configuration Option Rx2 V V Y Y 256 Channels 256 Channels Filter Bank 1 Filter Bank 2 Parallel Filter Bank Configuration Option Rx1 Rx2 Y Y Y 128 Channels 128 Channels 128 Channels 128 Channels Filter Bank 1 Filter Bank 2 Figure 5 1 The filter bank parallel and series configurations 60 CHAPTER 5 SPECTRAL LINE OBSERVING Give these numbers to the operator Up to 16 bad channels can be eliminated but rarely more than 5 channels out of 512 will be bad It is also possible to eliminate blocks of 16 channels but this is seldom necessary If blocks of 16 channels are bad notify the Friend of the Telescope Once the operator has entered the bad channels he will perform another vane calibra tion cycle to set the bad channels to 1 0 x 10720 which is the flagged channel value On all subsequent spectra those channels will be flagged These bad channels can be eliminated in reduction software by using the replace command see the line manual 5 3 1 3 Frequency Offsets Given the broad band tunability of the 12m receivers one can shift the rest frequency mea sured by the filter banks by specifying a frequency offset to be added to the rest frequency Offsets as large as the entire bandwidth of the receiver 300 MHz are possible Filter bank offsetting is most often used in conjunction with the 4IF Millimeter Autocorrelator MAC mode see 5 3 2 1 for further informa
60. D 45 7347 D 41 2452 J 50182 167387 0 00 55 97 D 45 7313 D 41 2300 7 50182 184053 0 00 55 97 4 2 TRACKING LIMITS 33 where the first three lines are descriptors for the object name velocity frame and velocity type The next three lines are comments Each column thereafter is defined as follows D 45 7745 the topocentric right ascension in degrees D 41 3975 the topocentric declination in degrees NOTE You can also use a geocentric right ascension and declination in the last two locations but if you do you must enter the correct horizontal parallax see below J 50182 000720 the Modified Julian Date mjd of this record in the TT Terrestrial Time which used to be called Terrestrial Dynamical Time or TDT system Note that TT UTC 63 184 sec after June 30 1997 0 00 the horizontal parallax the angle from you to the object to the center of the earth in arcseconds The horizontal parallax is zero in this case since we are using topocentric coordinates If one uses geocentric coordinates the horizontal parallax must be non zero and is given by 6378 137 HP arcsin mm p p geocentric distance of object 55 84 the topocentric velocity Non planets default to topocentric velocities Plan ets default to geocentric positions and velocities After you create the ephemeris put it in a file with the extension eph in the observers home directory home obs ini comet eph for example If you do not have an ephemeris
61. E can also be specified The default is 30 seconds with the vane and sky sampled 15 times each at 1 second per sample 3 Observe discrete astronomical sources to calibrate the telescope pointing and set a flux density scale for the data The measurement can be used to enter a value of the aperture efficiency The aperture efficiency is used by the data reduction routines to convert antenna temperature to flux density The observations of calibra tion sources should be repeated frequently during the course of the observing run A calibration curve plotted as a function of time can be used for the most accurate adjustment of the flux density scale in the final analysis Many observers choose to skip the measurement of the temperature scale by hot cold loads and calibrate their data strictly by comparisons with standard sources In this relative calibration method one merely observes a succession of standard sources forms a calibration curve of flux density conversion factors and through interpolation corrects the data after the run is over 6 6 2 Hot Cold Load Calibration A hot cold load calibration can be used to set the continuum temperature scale to measure the receiver noise temperature the noise tube temperature the sky temperature and the atmospheric optical depth At present we perform hot cold loads by manually inserting hot and cold loads into the beam and measuring the deflections through the computer on digital voltm
62. ENCY Total Integration Time FREQ 230537 99 SYN 1 95506818 V DR21 OH ZI 00 02 0 DAT 1950RADC 20 37 14 0 42 12 00 20 37 14 0 42 12 00 CAL EL 5 5 DV 1 30 Velocity Resolution km s E 22 96 400 0 TS 449 FR 1000 SB 2 7 Fitter Bank Resolution kHz Figure 5 3 Position switched spectral line scan analyzed in the unipops program SHGON DONIAHMSHO FS 66 CHAPTER 5 SPECTRAL LINE OBSERVING The number of ON OFF pairs repeats per scan can be specified to the operator as the total length of the scan a The number of scans to be performed for every calibration measurement 5 4 3 Absolute Position Switching Absolute position switching called the APS mode is useful when observing in complex emission regions where it is difficult to find an emission free reference position In such cases position switching with AZ EL offsets can be dangerous because rotation of the parallactic angle as the source is tracked across the sky may cause emission to rotate into the reference beam You will want to search for an emission free position as close to the source position as possible and use this as the reference position If you wish you can compute the RA DEC offsets to this position and use ordinary position switching Most observers find it most convenient particularly for future observations to enter the absolute RA DEC coord
63. G and REF refer respectively to the signal and reference positions BEAM and BEAM of the nutating subreflector CAL refers to the signal generated by the synchronous noise tube which is available for 3 mm observations only If the subreflector is fixed as in a single beam total power measurement the SIG and REF phases are the same apart from random noise The blanking and delay parameters for the subreflector are set by the Tucson staff regularly The default switch period is 0 25 seconds a 4 Hz switch rate so that the control computer can acquire DBE data in synchronism with its basic timing cycle The mechanical inertia of the subreflector imposes a limit of about 6 Hz on the maximum switch rate When the operator initiates a continuum scan the control computer issues a signal to the DBE to begin a cycle of four phases Each time a cycle is completed the control computer reads the contents of the four phase registers of the DBE Ideally this occurs every 500 msec of elapsed time The presence of error conditions loss of phase lock or pointing out of tolerance may cause the elapsed time to exceed the specified integration time If the control computer detects an error condition during phases 1 or 2 it sends a restart signal to the subreflector and the phases are retaken If the error condition occurs during phases 3 or 4 the computer rejects the entire 500 msec of data The control system time counter counts down monotonically in 1
64. ING POINTING AND FOCUS 4 6 Pointing You are responsible for determining the residual azimuth and elevation pointing offsets relative to the nominal telescope pointing These offsets are usually of the order 10 20 and often vary across the sky In particular the offsets often show an elevation dependence The pointing of the 12m may drift over a period of several months by 10 or so and you are cautioned not to assume that old pointing data are still valid If a project requires accurate pointing observers should budget 5 10 of their total observing time to pointing checks High frequency projects for which the FWHP of the main beam is 30 or less obviously require more attention to the pointing You may perform pointing checks in either continuum or spectral line mode Pointing sources must have an angular brightness distribution that is compact compared to the antenna beam In addition they must have well determined positions and must be strong enough to point on in a reasonable amount of time e g 10 minutes or less The best pointing sources are the major planets Venus Mars Jupiter Saturn and Uranus observed in continuum mode These sources have strong flux densities throughout the millimeter band although Uranus is faint at 3mm and longer wavelengths To obtain complete sky coverage particularly north of the zenith the planets must be supplemented with other pointing sources A tabulation of these sources and appro
65. Table 3 3 with RA corresponding to the Azimuth column and DEC to the Elevation column If you wish to convert arc seconds on the sky to seconds of time in RA remember to divide by 15 cos DEC 3 4 Local Oscillator System Mixer receivers require a local oscillator signal For spectral line work the LO signal must be phase and frequency stable to an accuracy of at least 1 part in 10 The purpose of the LO system is to phase lock the LO source which is otherwise a free running oscillator LO sources used at the 12m are solid state Gunn oscillators The power required of the LO source by present generation millimeter wave mixers precludes the direct use of a harmonic of a low frequency synthesizer At the 12m a precise synthesizer harmonic is used as a comparison frequency for the phase lock loop A 5 MHz rubidium oscillator is multiplied by 20 to give a frequency of 100 MHz This 100 MHz drives a comb generator thus enabling any multiple of the 100 MHz in the range 1 2 GHz to be selected by a filter Either the 18th or the 19th 1 8 or 1 9 GHz is usually selected a frequency between 50 and 150 MHz is then added and an oscillator is phase locked to the result In this way a spectrally pure signal in the range 1 85 to 2 05 GHz is generated This nominal 2 GHz signal is then fed up to the receiver and used to drive a harmonic mixer The nth harmonic of the 2 GHz signal n may be any integer from ten to seventy or higher then mixes with a
66. a grid built from offsets from a single central position FSM is another alternative for those cases where frequency switching can be used see 5 4 4 5 4 6 4 An Important Note About Spatial Sampling The following is due to Darrel Emerson When setting up your map observations it is important to keep in mind the following facts about sampling and aliasing in radio astronomical mapping data If you want to represent the full resolution of the telescope you have to sample the data often enough to represent all the spatial frequencies detected by the dish You can think of the extreme edges of the dish of diameter D as part of an interferometer of spacing D which has to be sampled at Depending on the illumination taper this corresponds typically to sampling at about 2 4 or 2 5 points per FWHM Of course if there is zero or by some definition negligible illumination at the edge of the dish you won t be sensitive to such high spatial frequencies It will be just the same as a smaller dish of diameter d equal to the diameter of that part of the dish that has significant illumination and the sampling will be calculated from A where X is defined as the diameter of the illuminated part of the surface It s useful to consider what happens if you undersample data Assume that the un dersampling happens on the sky rather than in later in the data processing Suppose you 5 5 SPECTRAL LINE SENSITIVITIES 79 have a 10m dish but you on
67. able total power observations may be required For most standard observing procedures observers prefer beam switching over total power position switching as it provides much better cancellation of the atmosphere 6 3 2 Checking the Subreflector Throw In the following we give two methods which can be used to measure the true subreflector beam throw 6 3 OBSERVING BASICS 91 Checking the Subreflector Throw with a Continuum Az El Grid Map or Continuum Raster Scan Suppose we are observing at 3mm and have set the subreflector for a nominal 4 beam throw 2 To confirm this have the operator setup to do either a Continuum Az Grid Map or a Continuum Raster Scan on a bright continuum source such as a planet I will describe both methods below Continuum Az El Grid Map Ask the operator to setup a Continuum Az El Grid Map with an integration time per sample of 5 seconds an azimuth grid spacing of 5 arcseconds 1 row 120 columns scan direction azimuth and all other parameters set to their default values This will give you an azimuth scan 10 arcminutes in length which will take approximately 10 minutes to acquire To process this measurement you can use the following commands in condar Note that you need to batch the dbgauss prc procedure only once per condar session Condar Condar batch home obs sys proc dbgauss prc bdrop 0 Condar gt get scan number Condar gt switched Condar gt dbgauss The proc
68. able flux density calibrators at 90 GHz are given in Table 6 4 6 7 CONTINUUM STATUS MONITOR 109 Table 6 4 Flux Density Calibrators W3 OH 3 8 0 3 3C274 Virgo 6 540 3 6 7 Continuum Status Monitor A sample continuum status monitor display is shown in Figure 6 5 Each numbered box in Figure 6 5 indicates a section of the display which describes a particular set of attributes of a continuum measurement Box 1 Scan number source name and timing information SCAN Current scan number SOURCE Source name HORIZON Time to 15 elevation rise or set LST Current local sidereal time UTC Current coordinated universal time DUT1 UTI1 UTC time correction date Current year top and date bottom OBS Current observer initials top and data file number bottom OPR Current operator initials Box 2 RA Dec and position information TOTAL Current total apparent plus offsets RA and Dec B1950 0 Current Equinox B1950 RA and Dec APPARENT Current apparent RA and Dec GALACTIC Current and bII OFFSET Current applied RA and Dec offsets Box 3 Apex position information NS North top and south bottom focus translation stage offset position in mil limeters EW East top and west bottom focus translation stage offset position in mil limeters Box 4 Azimuth and elevation position and pointing offset information 110 CHAPTER 6 CONTINUUM OBSERVING COMMAND Comm
69. an start the Drawspec program and use the scanmstr utility in Drawspec to import the single dish FITS file See Harvey Liszt s home page at 16 CHAPTER 2 GETTING STARTED Table 2 4 Correspondence Between Subscan Codes and CLASS Telescope Identifiers CLASS Telescope Identifier Filter Bank Data 12M FB11 12M FB12 12M FB21 12M FB22 Millimeter Autocorrelator MAC Data 2 IF Mode 12 11 12 12 4 IF Mode 2 5 DATA ARCHIVING AND EXPORT 17 http www cv nrao edu hliszt programs html for more information Drawspec and its affiliated programs 2 5 Data Archiving and Export When you have finished your observations an archive tape of the observers files will be made and sent to the Tucson office If you submit a Data Tape Request Form the staff will create an export tape of your data and mail it to your home institution We offer two types of export tapes an ASCII tape following the FITS standard or a tar format copy of your sdd files 18 CHAPTER 2 GETTING STARTED Chapter 3 Instrumentation 3 1 Telescope Site Layout The 12m is located on the southwest ridge of Kitt Peak about two miles below the top of the mountain Other telescopes on the southwest ridge are the NRAO 25m VLBA antenna and the McGraw Hill Observatory 1 2m and 2 4m optical telescopes A drawing of the 12m site layout is given in the Visitor s Guide to the NRAO 12m Telescope and can be found on the NRAO Tucson Home Page http
70. anded azimuth and elevation spherical coordinate conversion plus pointing corrections ACTUAL Actual azimuth and elevation encoder readings ERROR Difference between COMMAND and ACTUAL azimuth and elevation OFFSET Total azimuth and elevation offsets sum of pointing offsets plus subre flector beam throw POINT Azimuth and elevation pointing model corrections PSREF Reference position offset in azimuth and elevation Box 5 Receiver calibration and tipper information ATM Number of air masses toward current elevation RCVR Receiver name RBAY Receiver bay FO Axial focus zero position in millimeters TC Calibration scale factor for receiver channels 1 top and 2 bottom TAUO Actual top and 225 GHz bottom measured continuously using a tipping radiometer zenith opacity FWHM Beam FWHM top and HWHM bottom used for five point measure ments in mm ss TSYS Channel 1 top and 2 bottom system temperatures in MM_H20 Millimeters of water vapor at zenith based on the 225 GHz tipping radiometer measurement of and a conversion of 1 millimeter H20 equals an opacity of 0 05 at 225 GHz Box 6 Subreflector beam and quadrant detector position information BEAM Azimuth and elevation position for the Beam in mm ss BEAM Azimith and elevation position for the Beam in mm ss QX Quadrant detector x axis position in millimeters QY Quadrant detector y axis position in millimeters Box 7 Fre
71. are given by the equation cos E cos E CA A 9 where the left hand side of the equation corresponds to the cross elevation azimuth correction on the sky given at a particular elevation by the observer applied azimuth correction In elevation the secondary correction is AE cos E IE A 10 These terms correspond to the linear in terms in the principal pointing equations given above is the supplemental azimuth encoder offset Ci is the supplemental elec tromagnetic collimation error correction B is the supplemental gravitational bend error and E is the supplemental elevation encoder offset Each of the four receiver bays have their own set of these secondary coefficients to compensate for slight differences between feed and mirror alignments in each bay The intent of these pointing coefficients is to keep the required pointing offsets as close to zero as possible The determination of these coefficients requires much less data than the main pointing equations and can thus be adjusted on a more frequent basis 116 APPENDIX A POINTING EQUATIONS FOR THE 12M TELESCOPE Radio Refraction Coefficient at P 610 Torr 100 T T T T T T 90 80 70 60 50 40 Relative Humidity 50 20 10 0 10 20 50 Air Temperature Figure R as a function of and relative humidity for a constant total barometric
72. ases is proportional to twice the source flux density The advantage of the ON OFF approach is that to first order it cancels imbalances between the two beams One can of course select the OFF position to be different from the position of the BEAM or the ON position different from the BEAM This is a single beam SBS observation The most frequent use of the SBS mode is to determine the position of the BEAM and the BEAM independently and thereby determine the subreflector throw and orientation To perform an ON OFF sequence you must give the operator the following setup parameters 1 The length of time to be spent on each ON or OFF sample and the number of repeats of the OFF ON ON OFF cycle The sample length is usually chosen to be 5 or 10 seconds This length is usually a good compromise between maximum switching 6 4 OBSERVING PROCEDURES 97 Source Subreflector Reference Subreflector Main Subreflector Reference Position Position Position Telescope in ON Position Telescope in OFF Position Continuum Scan Display Samples Figure 6 2 Position definitions for continuum sequences 98 CHAPTER 6 CONTINUUM OBSERVING efficiency of the telescope and the best compensation for atmospheric drifts the switching pattern corrects for linear drifts and will approximate higher order drifts if the sample time is sufficiently short The total integration time of a sequence is 4 seconds repeats Thus if you request 5
73. ates You can also define a grid to be acquired at an arbitrary rotation angle The number of rows and columns must be an odd number In the total power TPM and position switched modes you can choose between absolute Absolute Position Mapping or APM and relative reference position Position Switched Mapping or PSM measurements you can choose to observe several map positions ONs for each reference OFF position and you can observe several 72 CHAPTER 5 SPECTRAL LINE OBSERVING OFFs for each vane calibration scan Frequency switched mapping FSM requires no OFF position measurement Grid maps can be observed in three different ways standard grid which measures all positions on the grid sequentially spiral grid which measures all positions on the grid with a spiral pattern and cross grid which measures only the points which lie on the central row and column of the grid Figure 5 6 shows the observing sequence for each variety of grid map After defining the grid you can choose to map a subset by specifying the beginning and ending row numbers and the beginning and ending column numbers To use any of the spectral line grid mapping modes give the operator the following information 1 The catalog and name of the source map center position 2 Standard setup parameters including the AZ EL pointing offsets the pointing tolerance and the focus setting 3 The reference position offset relative to the map center in eithe
74. beam to one of the four quaternary mirrors over each receiver box The quaternary mirrors are oval flats and have one degree of freedom for position adjustment The optics following the quaternary mirrors are contained within the receiver boxes and are usually different for each receiver The alignment of the mirrors is done optically Small optical mirrors are fixed to the tertiary and quaternary mirrors A laser is mounted in the subreflector position and the mirrors are adjusted so that the laser beam spot is centered on the receiver lens The beam also may be autocollimated at the subreflector to achieve the most precise alignment 3 3 Receivers All of the receivers in use at the 12m employ heterodyne mixers sometimes called co herent detectors which use superconducting insulating superconducting SIS junctions The SIS junctions are housed in a dewar which is part of a closed cycle cryostat with tem perature stages at 20 K and 4 K The 20 K stage is cooled by a conventional compressed helium refrigerator system the 4 K stage is cooled by a separate Joule Thomson unit The two receivers are mounted in upright structures variously known as rockets or inserts The inserts are wholly self contained receiver units and may be removed independently albeit by warming the mother dewar A local oscillator LO signal provided by a Gunn oscillator is injected into the mixer or diplexed with the incoming radio frequency RF signal
75. ber hc The he procedure will prompt you for the hot load temperature in Celsius and the cold load temperature in Kelvins If the observation was made in double sideband mode give the frequency of the local oscillator The hot load temperature should be the temperature of the vane the vane temperature is displayed on the Chopper Control chassis in the control room After entering these data the crosshairs will appear on the graphics screen and you will be prompted to mark the zero offset the cold load and hot load samples The procedure will then prompt you to enter any sky and noise tube samples that may exist Ask the operator to update the value of calculated by hc for the type of calibration fixed or noise tube that you are using If you wish to reduce the data by hand you can get a table of the numbers in the scans by typing Condar gt scan_number table In addition to the computer scan you should reduce the data obtained from the volt meters or chart recorders for comparison The formulae for the quantities of interest are given below The receiver noise temperature is a function of the Y factor defined by Vha 2 Vosa where is the measured hot load voltage less the zero offset voltage and V is the measured cold load voltage less the zero offset voltage The receiver noise temperature is then given by 6 15 6 6 CALIBRATION 107 Thot Y Teota E EE 6 16 where Thot and are t
76. between the receiver feed and the center of the subreflector By placing an angled reflector the Cone of Silence at the center of the subreflector power incident at the center of the subreflector will be scattered onto the surface of the primary and scattered onto cold sky nearly eliminating the main component of reflected power between the subreflector and receiver feed If the lines are narrow and the frequency shift is small say lt 40 MHz good results can often be obtained One must also be careful when frequency switching in regions spatial or spectral which may contain multiple spectral lines Note that one common contaminant a frequency switched observation is mesospheric CO The CO J 1 0 emission from the mesosphere is rather weak but CO J 2 1 emission is quite strong see Figure 5 5 Frequency switching is effected by switching the phase lock loop offset frequency the Lock IF between two nearby settings usually generated by separate oscillators The oscillator settings must be set manually The frequency of switching is usually 5 Hz and is generated by the spectral line multiplexer when so instructed by the computer The phase lock circuitry must be able to lock at both the signal and reference frequencies This places a practical limit on the magnitude of the shift of typically lt 40 MHz Some receiver systems use the fundamental frequency of the Gunn oscillator as the LO frequency For these systems a
77. called the BSP beam switching plus position switching mode The technique is much the same as that used for continuum ON OFF s see 6 4 1 1 and Figure 6 2 With the subreflector nutating at a rate of typically 1 25 Hz the telescope is moved to place the source first in one of the beam positions and then in the other The beam position which for a positive source signal produces a positive response in the spectrometer is called the positive beam and a sample taken in this position is called an ON Conversely the beam position which produces a negative response in the spectrometer is called the negative beam and a sample taken there is an OFF A BSP scan always consists of four samples taken in the order OFF ON ON OFF The samples are taken in this order to get the best atmospheric rejection the best baselines and to reduce telescope movement The integration time of one of the individual ON or OFF samples controls the total integration time of the scan sample length times 4 The beam switching mode usually produces very good spectral baselines The subre flector switch rate is such that atmospheric changes and filter bank anomalies are most often subtracted out The primary restriction for beam switching is that the source angu lar diameter must be smaller than the subreflector throw The subreflector throw can be varied between 0 and 4 5 but beam throws larger than 3 arcminutes are inefficient The default swit
78. can resume 1 3 5 3 Sun on the Dish The pointing and focus of the dish can be seriously affected if the sun is allowed to shine on the surface of the dish or the feed support legs If accurate pointing is desired care must be taken to keep the sun off the dish To avoid excessive heating of the feed legs the prime focus regions and the cables to the prime focus the dish will not be pointed to within 15 degrees of the sun The projected distance between the sun and a position on the sky defined as Lo is given by the following equation cos Doy cos 90 E Lo cos 90 E Lo sin 90 E Lo sin 90 EL AZo 1 1 1 3 5 4 Observations Using Emergency Power Generators The telescope and dome have three sources of electric power the commercial source and two power generators Observations can continue as long as at least two of the sources are operational If only one source of power is available the dome door must be closed 1 3 5 5 Safety Rules The following safety rules obtain at the 12 Meter Telescope site We expect all observers and visitors to the site to read and abide by these rules 1 To drive a GSA car you must possess a valid driver s license 2 The Telescope Operator on duty is the only person allowed to operate the telescope 3 Observers are not to be on the telescope unless the duty operator has specifically authorized them to be there 4 Safety chains and rails have been installed at
79. cates that this is a spectroscopy measurement with the filter banks MAC_RUN Indicates that this is a spectroscopy measurement with the Millimeter Autocorrelator MAC BAD CHAN Filter bank channels flagged by the on line system Box 8 Telescope tracking and weather information TOL Input and actual tracking tolerance in m ss FOCUS Input and actual axial focus at the current elevation in millimeters TORR Barometric pressure in Torr RH Relative humidity T_AMB Ambient temperature in C REFRT Input and actual refraction constant for elevation refraction pointing cor rection in arcseconds Box 9 Current observation scan and integration time information MODE Observing mode SCANS Number of scans requested SAMPLES Number of continuum on off samples requested SEC Total top and remaining bottom sample time in seconds TIME Total top and remaining bottom integration time for this scan in mm ss SCAN SOURCE HORIZON LST UTC DUTI 1996 OBS OPR 1 1707 wars 6 41 22052925220 19 0 011 08 Nov sys VLG RNAME 13320019 289 R A DEC APEX AZIMUTH ELEVATION TOTAL 10 30 07 8 11915 28 1 1NS 0 500 COMMAND 132 10 58 2 61 37 20 0 2 1950 0 0 00 00 0 0 00 00 0 0 500 ACTUAL 132 10 58 2 61 37 23 6 APPARENT 10
80. ching rate is 1 25 Hz Switch rates of 2 5 and 5 0 Hz are also available The observing efficiencies are poorer at the faster rates but the cancellation of atmospheric drifts may be better You must decide upon the following parameters in a beam switched observation and give them to the operator 1 The subreflector throw Changes in the throw must be made manually the computer must be updated manually as to the new value of the throw 2 The switch rate of the subreflector 3 The integration time per sample ON or OFF The total length of the scan is the sample time x 4 4 The vane calibration method is available although it is applied in a different manner than for position switched data see 85 6 5 4 OBSERVING MODES 71 5 4 6 Mapping The 12m system offers three modes of spectral line mapping mapping by manual offsets automatic mapping of rectangular grids or catalogs in either the total power TPM position switched PSM or APM or frequency switched FSM modes and On The Fly mapping Mapping with manual offsets or rectangular grids is appropriate for small maps or maps with unevenly spaced points For most rectangular grid mapping we recommend the automatic position switched total power mode With the total power mode you can choose to observe several ONs per OFF and thereby increase your observing efficiency On the fly mapping is discussed in the separate manual On The Fly Observing at the 12m In the following we d
81. dar gt scan number sptiptti where 1 is either 1 or 2 to choose channel 1 or channel 2 The data and fit are displayed in Figure 6 3 6 5 1 2 The STIP Reduction Procedure The stip reduction procedure fits Equation 6 12 to the data from an sptip observation by way of nonlinear least squares This procedure is the most direct of the two tipping analysis options it makes no assumptions other than the accuracy of the basic model Equation 6 9 To give an accurate measure of 7 it does have several stringent requirements CHAPTER 6 CONTINUUM OBSERVING 102 TYPE SPTIP 7 50 TAU 0 2551 0 9978 7 40 Best Fit Zenith Atmospheric Optical Depth 7 30 Measured Values In Tot Ey 7 20 7 100 7 00 SAMPLES Service 7104 02 INT 110 00 DATE 08 NOV 96 1950RADC 00 04 0 0 22 57 12 CAL 400 00 FREQ 26589957 CONTSTIP 2 Figure 6 3 Sample SPTIP tipping scan 6 6 CALIBRATION 103 1 The temperature scale must be accurately calibrated which means that you must do a hot cold load measurement to determine the value for in the fixed calibration method see 6 6 2 2 You must supply several input parameters including 7 called ftm in condar and qum called ftsbr in condar The stip procedure can also be used to fit for m a basic efficiency factor Because it is a nonlinear hence iterative fitting routine it may not converge if the data are of poor quality or
82. data will be taken in total power fixed beam total power or beam switched mode 2 Give the operator the azimuth and elevation cell sizes the number of rows and columns and whether the map will be acquired in dual beam or single beam In a dual beam map the telescope is positioned so that the azimuth of the center of the map is at the mid point of the two beams In a single beam map the map is referenced to the BEAM in azimuth rather than the mid point between the BEAM and BEAM 6 4 2 2 Continuum On The Fly Mapping The observing setup for continuum OTF mapping is very much the same as it is for spectral line OTF mapping Continuum OTF data can be processed within The routines in AIPS use the Emerson Klein amp Haslam algorithm to restore the dual beam switched map data See the AIPS Cookbook chapter which describes continuum OTF data analysis for further information 6 5 Utility Observing Routines 6 5 1 Sky Tip Procedures To properly calibrate continuum data you must correct for the attenuation of the signal as it traverses the atmosphere We usually determine the atmospheric attenuation from the optical depth at the zenith The total power antenna temperature at a given airmass elevation is a function of 7 and is given by the equation T4 sky Try Tm 1 exp 79A 1 mT bg 0 4 6 9 where is the receiver noise temperature 7 is the warm spillover efficiency rear s
83. dividual gaussian fits for multiple gaussians and lists height FWHP width and position for each gaussian gdisplay Plots sum of gaussian fits and lists characteristics center scan number fivel Processes the spectra for a total power spectral line five point measurement from filter bank where is either 1 or 2 e CLASS and uni2class can access the data file at the same time This means that you can read the CLASS file at the same time as you are writing to it e uni2class will overwrite existing output files without warning Be careful e Only LSR and heliocentric velocities equatorial coordinates and the standard small field projection are handled correctly e No distinction is made between radio and optical velocity definitions e Continuum data are not handled e The sdd and CLASS headers do not map onto each other exactly but nearly all of the important parameters are mapped 2 4 2 Drawspec Drawspec is a PC based analysis program written by Harvey Liszt The public PC in the control room at the 12m has a current copy of Drawspec for observer use To port your data to Drawspec you must write a single dish FITS file of your data The program uni2fits will do this for you Just issue the command uni2fits at the unix prompt on any of the workstations at the 12m to start the program You will be asked a series of questions by uni2fits which should be self explanatory Once you have single dish FITS file you c
84. dropping the last half of the lags Chapter 4 Tracking Pointing and Focus 4 1 Tracking Capabilities The 12 Meter Telescope can track both stationary sidereal sources or fast moving sources such as the Sun Moon planets comets or satellites called ephemeris objects The positions of stationary sources may be entered into source catalogs by the observer or directly into the on line system by the operator Stationary source positions may be given as e Apparent equatorial positions already precessed e Equinox B1950 or J2000 equatorial positions or Galactic coordinate positions III An example of the 12m catalog format is the following 18 27 17 3 01 13 23 0 1950 SERMM1 8 0 LSR RAD 18 27 28 0 01 10 45 0 B1950 SERMM2 8 0 LSR RAD 18 27 27 3 01 11 55 0 1950 SERMM3 8 0 LSR RAD 18 27 24 7 01 11 10 0 1950 SERMM4 8 0 LSR RAD 18 27 18 9 01 12 02 0 1950 SERMM5 8 0 LSR RAD 18 27 25 4 01 12 02 0 1950 SERMM6 8 0 LSR RAD 18 27 22 0 01 12 30 0 1950 SERCTR 8 0 LSR RAD where column 1 right ascension 55 5 or galactic longitude DDD DDDDD column 2 declination DD MM SS 5 or galactic latitude DD DDDDD column 3 coordinate epoch B1950 J2000 APPARENT or GALACTIC column 4 source name column 5 source velocity 31 32 CHAPTER 4 TRACKING POINTING AND FOCUS column 6 source velocity reference frame LSR HEL GEO or TOP where LSR local standard of rest velocity referenc
85. dure five which can be abbreviated as f You can look at the results for each channel separately by using the command Condar gt scan_number f Channel 1 is selected by specifying subscan number 01 while channel 2 is selected with subscan 02 The display Figure 4 5 provides the observer with the best fit positional offsets in azimuth and elevation along with the estimated peak flux density of the source both for a constrained fit of Gaussian half width HP and an unconstrained solution The optimum pointing offsets labeled CORRECTED OFFSETS are also printed assuming the given position of the source to be correct 4 6 1 2 Spectral Line Five Point Analysis The display of the map and the fit for pointing offsets is somewhat more involved than for a continuum five point There is a standard line procedure which processes all types of spectral line five point measurements To process a measurement within line type Line center scan number fivel1 for channel 1 Line center scan number fivel2 for channel 2 You will be asked some questions by the routine AVERAGE THE TWO RECEIVERS 1 YES O NO This question is asked only if the data were taken in series mode If the question is answered yes the routine averages the first and second filter bank to improve sensitivity You should answer yes only if you are reducing the first filter bank you will usually not need to reduce the two filter banks separately if averaging the two
86. e driven to an elevation of 92 before tripping a final limit switch 4 2 2 Azimuth Limits To prevent the over wrapping of cables the telescope has hardware and software azimuth limits The control system will not allow the telescope to rotate through 66 8 azimuth to acquire or track a source As they rise sources with declinations between 27 and 38 75 will pass through 66 8 azimuth above 15 elevation When a rising source reaches an azimuth of 66 8 the telescope will rotate 360 to re acquire the source The region of the sky where azimuth transitions occur is displayed in Figure 4 1 If an integration is in progress when the transition azimuth is reached the integration is halted while the telescope rotates around to re acquire the source The integration is resumed when the source is reached again If it is undesirable for this to happen you should cease the integration before reaching the transition azimuth 4 3 Tracking Error Tolerance The 12 Meter Telescope is vulnerable to tracking errors produced by wind gusts The tracking software has a provision for rejecting from the integration any data sample that was taken with the telescope off source The basic timing cycle of the 12m control system is 100 milliseconds 0 1 sec After each 100 ms interval the system checks for error conditions such as tracking errors or a loss of frequency phase lock If an error condition is detected the data sample collected during the last 1
87. e frame HEL heliocentric velocity reference frame GEO geocentric velocity reference frame and TOP topocentric velocity reference frame column 7 source velocity type RAD OPT or REL where RAD radial velocity type OPT optical velocity type and REL relativistic velocity type The sky frequency equations corresponding to the above velocity types are given in Equa tions 5 4 4 1 1 Ephemeris Objects To track ephemeris objects comets asteroids etc the control system requires three positions centered upon the present time The tracking algorithm then makes a cubic spline interpolation to determine the source position at the specific time of observation The observer must prepare a catalog in the correct format in order to observe ephemeris objects other than the planets and the Moon The basic format needed by the control system is show below OBJ Hyakutake VELFRAME LSR VELTYPE RAD Ephemeris for Hyakutake 0 000 H 9 APR 1996 to 23 700 10 APR 1996 D 45 7745 D 41 3975 J 50182 000720 0 00 55 84 D 45 7705 D 41 3822 J 50182 017387 0 00 55 86 D 45 7662 D 41 3670 7 50182 034053 0 00 55 89 D 45 7620 D 41 3517 J 50182 050720 0 00 55 91 D 45 7580 D 41 3363 J 50182 067387 0 00 55 93 D 45 7540 D 41 3212 J 50182 084053 0 00 55 94 D 45 7500 D 41 3058 J 50182 100720 0 00 55 95 D 45 7463 D 41 2907 7 50182 117387 0 00 55 96 D 45 7422 D 41 2755 J 50182 134053 0 00 55 97 D 45 7385 D 41 2603 J 50182 150720 0 00 55 97
88. e used Below I describe the most popular alternate analysis packages in use at the 12m 2 4 1 CLASS Conversion of sdd format data to CLASS format is currently done on line All scans except continuum and OTF measurements are automatically converted to CLASS format and put in a file called class 12m in the observer s directory One can also convert other sdd format data files to CLASS format using a utility program called uni2class To convert all data in the sdd file sdd jgm_001 issue the following command from any Unix prompt on any of the mountain workstations uni2class home data jgm sdd jgm 001 sdd jgm 001 class for the filter bank data and uni2class home data jgm sdd_hc jgm_001 sdd hc jgm 001 class for the Millimeter Autocorrelator MAC data The second parameter in uni2class is the output file name The current features and limitations of this conversion are e Each spectrum is assigned a unique observation number in the CLASS file according to the perscription described in Table 2 4 For filter bank data the nomenclature used for the CLASS telescope field identifier is FB for filterbank followed by a two number identifier indicating which filter bank there can be only two and which polarization from a filter bank this scan represents For Millimeter Autocorrelator MAC data the nomenclature used is MAC followed by a two number identifier indicating which rest frequency there can be two and which polarization from a g
89. ecified weighting 6 4 Observing Procedures 6 4 1 Point Source ON OFF Observing Procedures The 12m has available several observing procedures for making ON OFF measurements i e measurements of the difference between the output powers at a defined point ON and a nearby reference position OFF These procedures work with the subreflector nutating or not The ON OFF procedures are often used for measuring the flux density of weak point sources 6 4 1 1 The ON OFF Sequence The standard point source ON OFF observing procedure is called a sequence in the control system terminology Using this procedure the telescope moves between the ON and OFF source positions in the pattern OFF ON ON OFF A sequence is made up of one or more repeats of this basic cycle This order of samples eliminates the effects of linear drifts in atmospheric noise or receiver gain on the measurements You can specify integration time per position Procedures that are simply ONs OFFs or an ON OFF pattern are described below The standard set up for an ON OFF sequence in beam switched mode is double beam switching DBS i e with the ON and OFF positions separated by the subreflector throw In this way the radio source to be studied can be cycled between the positive and negative beams by the movement of the telescope Figure 6 2 displays the positional relationships between the OFF and ON samples For the DBS mode the output power difference between the ON and OFF ph
90. ection usually these are less than 1 2 The pointing equations for the 12m are set up for 0 offset in subreflector position i e with the subreflector axis aligned with the electrical axis of the primary reflector When the subreflector is offset so that the telescope must be moved in the positive azimuth direction to bring the source into the beam the subreflector is said to be in the BEAM When the telescope must be moved in the negative azimuth direction to acquire the source at least with respect to the BEAM the subreflector is in the BEAM Figure 4 3 illustrates this convention Both the spectral line and continuum backends are configured so that the BEAM signal is positive and the BEAM signal negative Pointing and Electrical Axis BEAM BEAM AZIMUTH Figure 4 3 Subreflector Beam Beam conventions The control system software takes account of the subreflector throw setting when posi tioning the telescope The computer does not read the subreflector setting automatically however it depends upon the operator to manually set the throw value The subreflector is physically set by turning two dials on the subreflector control chassis Default values of subreflector throw are stored in the computer for example 2 for 3 mm and 1 for 1 mm You can check the beam throw by making a continuum cross scan on a strong source Continuum mapping rows are explained in detail in Chapter 6 38 CHAPTER 4 TRACK
91. ed from its null control angle of 0 the position of the beams are given by the equations for a Cartesian coordinate system rotation In the AZ EL frame the position of an individual beam is given by AA 0 05 E sin 0 06 AE Eo 0 09 Ajsin 0 6 3 1 where and are the azimuth and elevation pointing correction for the center of the array o is the hardware control angle of the array in its null position 0 0 is the rotator control angle and A and E are the azimuth and elevation offsets of the individual beams relative to the geometric center and are given by Table 3 3 If you are not tracking parallactic angle e g for a pointing check then you will be observing in an AZ EL frame and all positioning offsets should be in AZ and EL If you are tracking parallactic angle you will be observing in an RA DEC or IILbIT frame and mapping and beam offsets should be given in RA and DEC Relations similar to Equations 3 1 exist for RA DEC or IILbIT offsets when the array is rotated by a user position angle The equations describing these offsets are ARA RAicos 0 DEC sin 0 3 2 ADEC 0 RA sin 0 3 3 where 0 is the position angle and RA and DEC are the RA and DEC offsets of each beam relative to the rotation center when the array is at 0 position angle These offsets 26 CHAPTER 3 INSTRUMENTATION are the same as in
92. edure dbgauss will do a two Gaussian fit to your measurement and report the measured beam separation gt gt Condar gt edrop 0 gt gt Continuum OTF Scan Ask the operator to setup for a Continuum OTF Scan with a ramp up distance of 30 arcseconds a scan distance of 10 arcminutes a scan rate of 10 arcseconds second 1 row and all other parameters set to their default values To process this measurement you can use the following commands in condar Condar gt batch home obs sys proc dbgauss prc Condar gt get scan_number Condar gt bdrop 0 Condar gt edrop h0 noint 136 8 Condar gt scan number m1 to process channel 1 m2 for channel 2 Condar gt dbgauss As with the Continuum Az El Grid Map dbgauss will do a two Gaussian fit to your OTF measurement and report the measured beam separation The beam separation you measure should be within 5 of the commanded value 92 CHAPTER 6 CONTINUUM OBSERVING 6 3 3 Continuum Sensitivity Continuum observations at the 12m are usually calibrated by a direct conversion of the measured antenna temperature into flux density Janskys The scaling requires that the observer determine the atmospheric zenith optical depth usually done with a tipping measurement For a point source the conversion is given by the standard equation 2kT 2kT 4 exp 7A i AKT 58 6 1 ma A I 6 1 Sy where k is Boltzmann s consta
93. encies etc m On The Fly Observing at the 12m Telescope On The Fly data acquisition and analysis a Visitors Guide to the NRAO 12m Telescope General guide for prospective observers m Observing with the NRAO 1mm Array Receiver Observers guide to the 1mm Array receiver Jeff Mangum ii Contents 1 Introduction 1 LL TheObservatory bate eee dae ee db aite pue d 1 1 2 Observing Proposals 2 1 2 1 1 2 1 2 2 Proposal Submission and Refereeing 3 1 2 8 Proposal and Observing 4 1 3 Observatory Policy we BSR 4 1 3 1 Staff Responsibilities 4 1 8 2 Observer s 4 1 3 3 Maintenance and 8 5 1 3 4 Sharing Telescope Facilities with Other Observing Teams 5 1 3 5 Observations Under Poor Weather Conditions 5 Tx gh AV mds ar i oi gee ate Ee Wu 5 1 3 5 2 Moisture Accumulation In or On the Dome 6 1 3 5 un ou dhe Dish pag eee Sid UE Se a 6 1 3 5 4 Observations Using Emergency Power Generators 6 1 3 5 5 6 2 Getting Started 9 2 1 What to Bring to the Telescope 9 2 2 COUACUPASNOORIISU 52 e ROE 10
94. equired by the data analysis routines to scale the data 2 Perform a calibration cycle called a CALIBRATE at intervals which you prescribe The interval should be small enough that the atmospheric transmission during the interval does not change appreciably This depends on the atmospheric optical depth and its stability and the elevation During stable conditions of low optical depth a CALIBRATE every 10 15 minutes is usually sufficient If the atmosphere is choppy or the elevation is changing rapidly as when the source is rising or setting more frequent CALIBRATEs may be necessary The operator can command the system to perform CHAPTER 6 CONTINUUM OBSERVING 104 TYPE STIP TVANE 1 000000 FIM Best Fit Zenith Atmospheric Optical Depth 220 200 180 160 140 120 Service 0 9500000 5 0 9500000 TAU 0 2771 0 0053 ETAL 0 9427 0 0089 CHISQR 2 05 Best Fit TAU 0 0 2550 Measured Values Tor FR 1 Starting Value for Fit ETAFREE 1 Solve for 1 ETAFREE 0 Hold 1 Fixed 1 5 3 7 SAMPLES 7704 02 INT 110 00 DATE 08 NOV 96 1950RADC 00 04 0 0 22 37 12 CAL 400 00 CONTSTIP SB 2 FREQ 265899 67 Figure 6 4 Sample STIP tipping scan 6 6 CALIBRATION 105 a CALIBRATE before each continuum scan or before a specified block of scans The length and number of repeats of a given CALIBRAT
95. es ues 33 PAS see ees 34 Zu Tracking Error Tol eratic e 6i ecd ate dog ree A RC HR AE Oe 34 4 4 Sequence of Position Computation Operations 34 4 5 Subreflector Beam 36 ZU ha ee Reed S but a dere ION 6 38 4 6 1 Continuum and Spectral Line Five Point Measurements 38 4 611 Continuum Five Point Analysis 45 4 6 1 2 Spectral Line Five Point Analysis 45 4 6 2 Pointing Model Equations 48 4 6 3 Pointing Data Analysis Program 48 cae uic dec Hae qe Dee 48 AN Te HORUS eh ae ais he pat 48 4 7 2 Determining the Axial 51 a Hee eed 51 Spectral Line Observing 55 5 1 Startup Checklist accrued deus Sea SA oe eet 55 52 Sie bard eas a de EC ATE S 56 5 3 57 e eee Be ox s 57 5 3 1 1 The Parallel Series Option 57 5 3 1 2 Bad Channel Elimination 57 Frequency Offsets NU EIE 60 5 3 2 Millimeter Autocorrelator 60 5 3 2
96. escribe each of the mapping modes 5 4 6 1 Manual Offsets Often an observer will want to make a simple source map consisting of only a few points In such cases manual offsets in RA DEC from the center position are the easiest way to proceed Follow these steps 1 Enter the center position into a source catalog or tell the operator the name and position so that he can enter it manually 2 Compute the offsets in angular units For declination offsets this is unambiguous For right ascension two cases exist a You want true angular offsets in the RA direction i e with the cos DEC correction made Tell the operator the magnitude of the offset in minutes and seconds of arc and whether you want to go East RA or West RA Tell him that these are true angular offsets b You want an RA offset in units of time Tell the operator the magnitude of the offset in units of minutes and seconds of time and whether to go East or West Tell him that these are offsets in time i e no cos DEC correction applied 3 After each integration loop to Step 2 and select a new point in the map The header information on the spectrum displayed by the line data reduction program will show any offsets that have been entered The offsets are given in real angular units 5 4 6 2 Grid Mapping In the grid mapping modes you can define a rectangular RA DEC or grid with different grid spacings in the horizontal and vertical coordin
97. eters and sometimes on the analog chart recorders The cold load usually consists of a styrofoam box holding a square of microwave absorber immersed in liquid nitrogen The vaporization temperature of liquid nitrogen at the elevation of the 12 Meter Telescope is 80 K For frequencies below about 230 GHz we recommend that you use this temperature For higher frequencies the styrofoam may begin to absorb the radiation and will add to the temperature of the load At these high frequencies we recommend that you use another type of cold load known as a dipper The dipper consists of a funnel shaped box lined with absorber on the end of a wooden handle The dipper is dipped into liquid nitrogen and held over the receiver feed during cold load cycles A hot cold measurement requires the active participation of the operator and at least one observer To perform a hot cold load calibration with a coherent receiver proceed as follows 1 The operator must drive the telescope to the zenith position and open the dome door to 90 2 Set the observing parameters for a continuum on off sequence Use a 1 second sample time with a scan time of 300 seconds 5 minutes which is equivalent to 75 repeat cycles This should be an adequate amount of time to make all of the necessary measurements 3 Set the calibration mode to fixed with a calibration temperature value of 1 0 106 CHAPTER 6 CONTINUUM OBSERVING 4 The operator should then fill the co
98. f are available for recording total power spectral line scans The two procedures are identical except that tpon tracks the ON source position and tpoff tracks the OFF reference position You must execute the procedures manually one scan at a time As such these procedures are mainly used for diagnostic purposes To use tpoff and tpon follow this prescription Provide the operator with the following setup information 1 The source catalog and the source name the ON position 2 The AZ EL pointing corrections and the reference offset position OFF The offset may be specified in AZ and EL or in RA and DEC 3 The integration time of the scan in seconds the scan will have only one sample i e repeats does not apply to TPN or TPF scans Have the operator perform a vane CALIBRATE TPF the OFF scan and a TPN the ON scan in that order The scan numbers of the CALIBRATE and TPF will be stored in the header of the TPN scan for use in data processing m To look at either the TPF or TPN scans type Line scant f Line scant s to choose either the first or second filter bank respectively The displayed scan will bea total power bandpass Unless the band contains a very strong spectral line you 64 CHAPTER 5 SPECTRAL LINE OBSERVING will probably not be able to see any lines To display a final spectrum formed from the ratio ON OFF OFF CAL type Line gt install ton Line gt scan ton Note that you o
99. f the sinc leaves that function unchanged we are left with a hanning function response Therefore 125 126APPENDIX D SPECTRAL RESOLUTION AND SENSITIVITY BANDWIDTH IN SPECTROME Table D 1 Spectral Resolution and m Bandwidth Rose TAE gaussian 2000 1 505 ranning 2000 2667 1520 2555 Av is the frequency sampling interval Avil yp Av D 2 DAD D 3 Av Av D 4 Avg 2 667Av D 5 D 1 Function Integrals 0 1 1 Sinc Avsg PS Pree sepa D 6 SB pam dv 2 al PES 82122 dv 2 _ i0 x 1 D 1 2 Gaussian exp nya tt Pee nna i 2 exp In 2 v dv 25 exo meno dv D 1 FUNCTION INTEGRALS 127 T 21n 2 1 50538 I 0 1 3 Hanning The Hanning function is non zero only from N 1 to N 1 where N is the number of channels which are being smoothed Uca i cos 27v a D 8 2 dv Normally we take channels and give them weights 0 25 0 5 0 25 Therefore becomes Avsz 5 cos 27v 7 D 9 2 sh cos 2zv dv 2 a 2 dv 2 hii 27 2 i cos 27v wI CO A D 1 4 Hamming The Hamming function is just the Hanning function with different weighting 0 54 0 46 27 di D 10 2 Las fosi 0 46 dv 2 7 2 2 0 46 costano
100. h Jeans equivalent temperature of the vane generally the ambi ent temperature The difference between vane and sky temperatures is thus Dus nidis 1 exp 1 Jd nL bg exp 7 A 6 12 The SPTIP analysis procedure makes the following assumptions dune din Joi Tamb l 0 6 13 where Tomo is Rayleigh Jeans equivalent ambient temperature Under these assump tions one then finds that In T 6 14 From the measured array of as a function of A the procedure fits for by linear least squares Among the assumptions 6 13 the assumptions that Jm Jam and that 1 are the most inaccurate In addition because the calibration scale is not always in calibrated in Kelvins the term In 775 5 in Equation 6 14 is fit as a free parameter This gives more freedom than it ought to have Experience with the sptip routine has shown that although it is the easiest of the available routines to use it suffers from two deficiencies 1 it tends to underestimate especially for large values of 70 and 2 it often appears to give a misleadingly good fit i e it may indicate that the atmosphere is more stable than it really is Nevertheless under moderately low 79 s say 0 3 the procedure usually gives an acceptably good measure of the atmospheric optical depth To use the sptip analysis procedure in condar type Con
101. he 12m T scale to the Tm scale is given by MN _ 1 Tmo T C 21 1 exp In 2 amp can also calculate 7 using the Ruze equation EM ET E 22 m je E given that DECR C 23 E n 2 Bos 1 222 C 24 m 1225 1 26 E exp amp 1 C 25 where is the wavelength of observation c is the correlation scale size of the surface deviations 280 mm mao is the zero wavelength aperture efficiency 0 55 and 6 is the surface accuracy 75 Figure C 1 shows this relation with the actual measurements given in Table C 1 C 3 2 Main Beam Efficiency The efficiency factor which converts any source antenna measurement to the Tma scale is given by 123 C 3 TELESCOPE EFFICIENCY MEASUREMENTS jo uorjoung se l Jo pue PaINsRe 2170 ZH5 Kouenbeuj 00 006 091 001 124 APPENDIX TEMPERATURE SCALES AND TELESCOPE EFFICIENCIES T mbd Toz 15 Tig 1 exp In 2 26 Table lists the most recent measurements of r7 and 7 for the planets and frequencies given Table C 1 12m Telescope Efficiencies Date Measured Frequency Source Source Size m GHz 72 0 98 0 98 0 98 0 109 8 109 8 109 8 115 3 115 3 140 8 140 8 140 8 140 8 140 8 140 8 144 6 218 2 230
102. he equivalent Rayleigh Jeans effective temperatures of the hot and cold loads respectively given by Equation 6 10 The Kelvins per volt KPV of the continuum system is given by Thot KPV 6 17 Veola eg The Rayleigh Jeans equivalent sky temperature is given by T sky Very x cold x Tol 6 18 where is the equivalent Rayleigh Jeans temperature of the cold load given by Equation 6 10 If a noise tube was measured its temperature is given by Vnt Vsky x KPV 6 19 where Tp is the Rayleigh Jeans equivalent temperature of the noise tube and V is the measured voltage of the noise tube The optical depth of the atmosphere at the zenith is from Equation 6 9 1 sky Jos MIm 1 A M Tog 2 Tm 6 20 where A 1 if the observation is at the zenith Equations 6 15 through 6 20 are used in the condar procedure hc and can be used to reduce the computer data manually if you so choose If you are using a noise tube use the value of given by Equation 6 19 for the calibration scale factor entered into the control system If you are not using a noise tube the value of to use is given by Thot Teold Toe 22 TA hot T old value 6 21 where Thot and Toota are the Rayleigh Jeans equivalent hot and cold load temperatures T A hot and T4 cold are the apparent antenna temperatures of the ho
103. he two banks can be averaged in software to improve the signal to noise of the final spectrum Figure 5 1 is a diagram showing how the parallel series option works 5 3 1 2 Bad Channel Elimination At times certain channels in the filter banks are defective You can identify these bad channels by examining the output of a vane calibration cycle which is called the gains Vane calibration is described in detail in 5 6 1 Bad filter channels usually appear as spikes in the gains and will often cause scaling problems in the final spectrum if not eliminated You can display the gains with the line data reduction program To find the bad channels perform a calibration scan and type g1 to display the first filter bank and g2 to display the second bank If bad channels exist type the command badch You will be asked to enter the number of bad channels After doing so the crosshairs will appear on the screen Move the vertical crosshair to each bad channel and select any mouse button When all the bad channels have been entered the program will give a report of the channel numbers CHAPTER 5 SPECTRAL LINE OBSERVING 58 Filter Resolution NOTE This is the FWHM channel w Table 5 1 12m Filter Bank Characteristics Channels per Bank Filter Banks Available Parallel Series Option Hardware Center Channel Par Ser 64 5 128 5 64 um 5 idth See Appendix D for further details 5 3 SPECTROMETERS 59 Series Filter Bank
104. here Tm is the mean atmospheric temperature Tspiu 15 the spillover temperature and is the cosmic background temperature Note that Tm and are actually equivalent Rayleigh Jeans temperatures of the point on the Planck blackbody curve corresponding to the same frequency This correc tion factor is given by Equation 6 10 For simplicity we will retain the symbol T for temperatures but in calculations T should be replaced by J v T The rms noise level for a given integration time assuming equal integration time at the ON source and OFF source reference positions is given by the radiometer equation see Appendix F for a general derivation of the radiometer equation for non equal ON and OFF source integration times 21 5 7 V NUE adn where 5 6 CALIBRATION TT Av is the bandwidth of an individual channel in the spectrometer in Hz Nspec 18 the efficiency associated with the spectrometer used 5 54 0 81 for the Millime ter Autocorrelator MAC and 1 0 for the filter banks See Appendix D for more information is the total integration time including ON and OFF source time in seconds Note that for observations of unpolarized signals with receivers that have two polarization channels the two channels can be averaged to reduce the effective system temperature In this case is the average of the effective system temperatures for the two polarizati
105. iated OFF measurement for total power spectral line measurements The five sequences or ON measurements are positioned in azimuth and elevation relative to the nominal source po sition as shown in Figure 4 4 The numbers identifying the positions give the time order in which the sequences are taken The offset for each position in Figure 4 4 is selected offset in arcsec that is usually chosen to be close to half the HPBW of the tele scope at the observing frequency HP should be somewhat larger if the source is extended for a planet that is resolved by the beam chose HP to be approximately the semi diameter 4 6 POINTING Table 4 1 Galactic and Extragalactic Continuum Pointing Sources wor aos 094524 18 Fousson masa 312 ossa oras aas 28 masazr 72 699 313 Foams a7 wom 38 os foras em NRaono roso s Fosse vera 2292 19 353047 Fosos oss 15420 oso aia meni orasaso risas
106. ich are accurate to at least 10 kHz If emission lines are weak test line frequencies should be included For continuum measurements choose your observing frequencies so that no strong spectral lines lie in either of the receiver sidebands Observing Mode The available observing modes for spectral line observations are Total Power Measurements Relative and Absolute Position Switching m Frequency Switching Beam Switching a Grid Mapping a On The Fly Mapping while the available observing modes for continuum observations are 9 10 CHAPTER 2 GETTING STARTED Switched or Total Power ON OFF s a Grid Mapping a On The Fly Mapping Reference Positions The reference offset position in angle or frequency should also be considered carefully If you are using beam switching you should consider carefully the optimum beam separation The default beam throws are 2 at 3 and 2mm wavelengths and 1 at 1mm wavelengths Spectrometer Configuration For spectral line observations you need to determine how you will configure the filter banks and Millimeter Autocorrelator MAC in cluding the resolution and the mode of operation This decision hinges on the resolu tion and total bandwidth required No firm rules exist but the minimum resolution acceptable should probably give 3 5 channels across the line and the minimum bandwidth should have 10 20 of the band on each side of the line 2 2 Startup Checklist A ge
107. ilter Bank Offset EANA penne ee eba 4 v 2 MAC Spectrometer 1 MAC Spectrometer V2 Offsets Vs V2 Offsets V3 n n MAC Spectrometer MAC Spectrometer Filter Bank MAC Spectrometer MAC Spectrometer IF 11 IF 12 IF 13 IF 14 Figure 5 2 Millimeter Autocorrelator MAC 4IF mode configuration 5 4 OBSERVING MODES 63 1 Tune the receiver to a frequency midway between the two rest frequencies which in this case is 218348 912 MHz 2 Set the filter bank offset so that I can measure the 393 202 transition which would be 218348 912 218222 192 126 720 MHz 3 Set the two Millimeter Autocorrelator MAC offsets so that IF s 11 and 13 receive the 393 202 transition and IF s 12 and 14 receive the 392 29 transition Since the Millimeter Autocorrelator MAC offsets must be in 5 MHz steps I would set these offsets to 4125 MHz The center frequency for IF s 11 and 13 would then be 218348 912 125 0 218223 912 MHz while the center frequency for IF s 12 and 14 would be 218348 912 125 0 218473 912 MHz 5 4 Observing Modes There are six primary spectral line observing modes are available at the 12m The at tributes and applications of each are described in detail below Signal processing and calibration for each mode are described in 85 6 5 4 1 Total Power ONs and OFFs Two observing procedures called tpon and tpof
108. ime Meridian up and down in the elevation direction with beams 4 and 5 at high elevation and high declination the array has a rotator control angle of 0 If the rotator is turned so that beams 4 and 5 move toward larger azimuth the rotator control angle increases User Position Angle The user defined position angle is available to allow the user to position the rectangular array of beams in the RA DEC frame to achieve optimal mapping efficiency according to the geometry of the source The most common example of this would be to position the long dimension of the array of beams along the major axis of a galaxy The convention for defining User Position Angle is as follows The user defined position angle follows the usual astronomical position angle conven tion Position angle increases toward the east increasing Right Ascension Note that this convention is independent of whether the source is north or south of the zenith 3 3 RECEIVERS 25 Table 3 3 1mm Array Azimuth and Elevation Offsets Relative to Array Center 3 o pL ms __ 3 3 1 2 Pointing and Mapping Offsets with the 1mm Array Rotator The rotation and tracking center of the array is at its geometric center where no beam exists We take account of this fact in pointing and focus measurements by allowing the user to specify a beam to point or focus on and then calculating the pointing center of the array based on this measurement When the array is rotat
109. inates of the reference position and use the absolute position switching observing mode APS is identical to ordinary position switching except that the switching is done be tween two positions absolutely specified by their celestial coordinates The reference OFF position should be given a different name from the signal ON position and is best placed in a different source catalog from the signal position Data taking and calibration options are the same as for ordinary position switching The parameters of an APS scan that you must give to the operator are The name of the source ON position and catalog which contains it m The name of the reference OFF position and its catalog m The integration time for each sample ON or OFF 30 seconds is the default m The number of repeats of an ON OFF pair or the total integration time of the scan m The number of scans between each vane calibration if that calibration method is chosen 5 4 4 Frequency Switching The frequency switching observing mode has two primary uses to increase the on source integration time through in band switching and to alleviate the problem of finding an emission free reference position when observing in a spatially complex emission region It also entails less system overhead than most other observing modes In this mode called an 5 observation a reference spectrum is obtained by shifting the center frequency of the signal spectrum In principle this ca
110. ip the system makes total power observations of both the sky and the ambient temperature absorbing vane at each position in Table 6 2 Both sky and vane temperatures are recorded for analysis To perform an sptip observation simply ask the operator to slew the telescope to an azimuth near your program source position and start the measurement The condar data analysis package contains two methods of analyzing an sptip If you wish on line atmospheric corrections to be applied to continuum data displays ask the operator to enter the new value of 7 Remember that the atmospheric absorption changes with time and if the highest accuracy is required a small correction should be applied off line using all the opacity data for the run to interpolate the most probable value for each scan 6 5 1 1 SPTIP Analysis Procedure The sptip data reduction procedure uses the difference between vane and sky temperatures to derive 7 This is the simplest of all the tip analysis routines it requires no input parameters does not require accurate calibration of the temperature scale nearly always 6 5 UTILITY OBSERVING ROUTINES 101 produces at least a first order approximation to the optical depth but makes a number of simplifying assumptions that may diminish its accuracy The antenna temperature of the sky is given in Equation 6 9 while the antenna tem perature of the vane is given by Ta vane Tre Trane 6 11 where is the Rayleig
111. is is when the program line is just outside the tuning range of the local oscillator Another example occurs when for double sideband observations it is advantageous to observe lines from both the signal and image sidebands for pointing calibration or simultaneous line search purposes and the image line is just outside the normal spectral bandpass These observing situations can be accommodated provided the needs to be changed by only a small amount The IF system has a bandpass filter centered at 1500 MHz with a bandwidth of 300 MHz Any change of the IF must thus fit within the 600 MHz IF bandwidth leaving room to include the filter bank bandwidth Hence the minimum IF can be 1200 MHz plus 5 the filter bank bandwidth and the maximum IF be 1800 MHz less 5 the filter bank bandwidth Lines opposite sidebands can be separated by at most 3600 MHz less 5 the filter bank bandwidth or in the least by 2400 plus i the filter bank bandwidth To adjust the IF bandwidth the first IF synthesizer which is normally set to 109 50000 MHz must be changed The equation for determining the synthesizer frequency is syn 5 17 where frr is the desired IF If the lines are in opposite sidebands fir z Si 5 18 where f is the upper sideband center frequency and is the lower sideband center fre quency This intermediate frequency must be entered into the control computer so that the local oscillator synthesizer frequency
112. is rising or setting more frequent CALIBRATEs may be necessary The operator can command the system to perform a CALIBRATE before each position switched scan or before a specified block of scans The length and number of repeats of a given CALIBRATE can also be specified The default is 30 seconds with the vane and sky sampled 15 times each at 1 second per sample 3 The result of a CALIBRATE is loaded into the gains array To examine the gains type 61 for receiver channel 1 and g2 for channel 2 Exact definitions of the signal processing are given in Section 5 7 4 During position and frequency switched observations the telescope moves to the OFF position to take the CALIBRATE 5 6 2 Direct Calibration Note The staff does not recommend this calibration procedure We describe it here for completeness In the direct calibration mode the data are scaled by the system temperature and the atmospheric attenuation according to the relation T SIG REF REF where SIG is the signal array REF the reference array T the system temperature the zenith optical depth and A the airmass The exact expressions for no cal signal process ing are discussed below The observer is responsible for the computation of and the measurement of The value for can include efficiency factors or the antenna tem perature can be scaled up in the data reduction and post processing stage One reasonable definition of T is Toys
113. ission Observing Mode Select the appropriate observing mode from those available and decide upon the setup parameters Observing modes supported are ON OFF s single beam and dual beam on the fly and grid mapping and utility programs such as five point maps sky tips and focus checks Beam Throw A particularly important setup parameter for beam switched observa tions is the beam throw The throw can be varied from 0 to 4 5 a throw larger than 3 may be inefficient in terms of the blanking necessary Observing Time Budget As with all observations you should carefully plan the observing program to include time not only for observations of the program sources but for pointing focusing and calibration When beginning a continuum observing run we recommend that you follow these steps 1 Have the operator tune the receiver to the frequency of your program observations and make any other necessary changes 2 Perform a HOT COLD load observation to measure the receiver noise temperature and to set calibration scale factors 3 Check the telescope pointing and focus on a standard source preferably in the same part of the sky as your first program source 4 Make a short ON OFF observation of a strong standard source such as a planet to see that the telescope and receiver systems are functional at least in gross terms 8T 88 CHAPTER 6 CONTINUUM OBSERVING 5 Observe a standard source using the system configuration and
114. iven frequency this scan represents CLASS does not support subscan numbers The original scan number is preserved in the header e The UniPOPS backend descriptor is converted to a more useful identifier which is inserted into the CLASS telescope field to avoid ambiguities between subscans for a particular scan e Map offsets are carried over There seem to be small 1 discrepancies in times and coordinates between the sdd and CLASS files which are perhaps due to roundoff error CHAPTER 2 GETTING STARTED Table 2 2 Analysis Commands in UniPOPS Commands Common to Both condar and line get scan_number nn Retrieves scan number nn into the work area Alias for page show which plots scan_number nn fol lowing a get header Displays the header from scan number following a get scan number add Adds scan number to the stack beg scan end scan add Adds all of the scans between beg scan and end scan to the stack Averages all of the scans in the stack and displays the average for channel where is 1 or 2 cb Averages all of the scans in the stack and displays the average for channels 1 and 2 combined tell stack Lists scans in the stack empty Empties the stack yrange ymin ymax Sets the vertical scale to the range ymin to ymax freey Resets automatic scaling for the y axis Commands Specific to condar scan_number nn s Gets and displays the continuum ON OFF sequence scan_number nn
115. ld load box with liquid nitrogen and carry it to the receiver bay The ambient temperature vane can be used as the hot load 5 On the continuum receiver control rack switch the leftmost thumbweels on the IF attenuators of both channels to 1 giving gt 100 dB of attenuation This will provide the zero point measurement at the start of the scan 6 Start the sequence Measure the zero point levels for about 10 seconds Don t forget to take this attenuation out when you are done doing your zero point measurement Then by speaking over the intercom to the telescope have the operator insert the cold load and leave it in place for about 20 seconds Note the voltages on the digital voltmeters on the continuum IF chassis Ask the operator to remove the cold load and switch the ambient vane in for about 20 seconds and note the voltages The vane is controlled from the Chopper Control chassis in the control room Perform 3 cycles of HOT COLD measurements 7 After the HOT COLD measurements ask the operator to move away from the beam and measure the sky voltage If the noise tube is not in use let the scan run out with the telescope looking at blank sky If you wish to measure the noise tube emission switch the tube on using the modulate switch on the subreflector control chassis and note the voltages You may wish to make several OFF CAL measurements before the scan ends To process the HOT COLD measurement in condar Condar gt scan_num
116. led spectral line forest We advise continuum observers to research the sources they wish to observe to find if any of these molecular line sources are either associated with the program source or along the line of sight You should then consult a molecular line listing such as the F J Lovas Catalog 1986 J Phys Chem Ref Data 15 251 to pick the optimum observing frequency For continuum measurements the 12m receivers operate in the double sideband mode with 600 MHz of bandwidth in each sideband and a sideband separation of 3 0 GHz 6 3 Observing Basics You can perform several types of continuum observations at the 12m The most common types are ON OFF s With the subreflector chopping the telescope moves first to one beam position and then the other Appropriate for point source observations Grid Mapping The telescope steps through a rectangular azimuth elevation grid a On The Fly Mapping Beam switched data are acquired every 0 125 seconds while the telescope is driven continuously over a specified map field This section describes the basic considerations terminology and techniques for making these observations Detailed descriptions of the observing procedures are given in Section 6 4 Suorjepo eo asey ur pesn c JAVY o qe3idioo1d Jo yoyo ue suonrpuoo oreudsoure SI9AI9291 YHA o qrsso2o Fey
117. lem the frequency axes run oppositely for the upper and lower sidebands One can also make small adjustments in the IF to change the placement of lines from the two sidebands Sometimes lines from the image sideband can be used to advantage for calibrations or system checks With a slight rearrangement of Equation 3 4 we can write the expression for the sky frequency as a function of the LO frequency settings Jd v m N foun frock 5 1 where fsyn is the synthesizer frequency is the sky frequency of the emission the rest frequency with Doppler corrections j 1 for lower sideband and 1 for upper sideband frr is the IF frequency 1 5 GHz by default m is the factor by which the LO frequency is multiplied before injection into the mixer flock is the the phase lock loop offset frequency 100 MHz and N is the synthesizer harmonic The Doppler correction is determined by the choice of velocity type see Chapter 4 RAD 2s falia eos ay y C frest VobjecttVantenna 1 1 object antenna Fa REL frest AS 5 4 Vobject T antenna 1 E 5 3 SPECTROMETERS 57 where Vobject Vantenna are the object and antenna velocities relative to the local standard of rest and c is the speed of light 5 3 Spectrometers Two spectrometer systems are available at the 12m a suite of analog filter banks and a Millimeter Autocorrelator MAC which is a digital correlation spectrometer 5 31 Filter
118. ly sample at rather than the qu that you should That means that the spatial frequencies present from the dish baselines of 8m to 10m get reflected back into the spatial frequencies of 8m down to 6m Not only have spatial frequencies from the 8m to 10m baselines been lost but valid spatial frequencies from baselines of 6m to 8m have been corrupted You can t tell if structure in your map with a spatial wavelength of ES is genuine or was really structure at z which has been aliased on top of any genuine spatial wavelength signal In this sense undersampling the sky is really twice as bad as you might have thought How important this undersampling is depends on exactly what the illumination taper is how important it is that you retain the maximum possible resolution of the telescope how good a dynamic range you want in the observations and at some level how much fine scale structure there is in the source itself If you only sample at 0 8 Nyquist e g rather than what matters is the energy in the data at spatial wavelengths shorter than oT So in a sense you need to ask what the illumination taper is at a radius of 0 4 x D on the dish surface The spatial frequency response of a single dish is the autocorrelation function of the voltage illumination pattern So you need to calculate how much area there is under the 2 D autocorrelation function beyond spatial frequencies of 0 8 x D compared to the area within 0 8 x D This ratio is
119. ments by hand using UniPOPS see 2 3 If the fit to the five point is poor repeat the measurement with updated pointing 4 Ask the operator to perform a focalize on the chosen continuum source This checks for the best value of the axial focus The focalize measurement requires that you specify a first guess for the focus position called F0 and the spacing between the 2 3 BASIC DATA REDUCTION WITH UNIPOPS 11 focus settings called WL The system will automatically set FO to a reasonable starting value usually around 45 mm WL is usually chosen to be 5 the observing wavelength When the focalize measurement is complete a fit will be made to the measurement on line and displayed on the on line dataserver You can also reduce these measurements with the condar analysis program See Chapter 4 for more focus information and 2 3 for data reduction commands If the fit to the focalize is poor repeat the observation with an updated value for FO NOTE Pointing and focus may change as the temperature of the dish or parts of the dish changes Pointing and focus should be checked at least after nightfall and daybreak and more frequently if the dish is illuminated by the sun 5 For spectral line observations you should perform the following checks to insure that the receiver is tuned correctly the spectrometer is properly configured and the calibration scale is correct a If the program line is weak and no other strong lines are in
120. n Status document At millimeter wavelengths the flux densities of most extragalactic sources are variable and we recommend the use of the planets or compact HII regions for calibration At least at Imm the brightest planets are usually significantly resolved The peak flux densities of the planets should be computed using the planets utility program available on any of the mountain workstations This program needs to know the effective observing frequency GHz the telescope HPBW arcsec the planetary unit semi diameter in arcseconds i e the semi diameter of the planet as seen from a distance of 1 AU available from the Astronomical Almanac or Table 6 3 the geocentric distance of the planet AU available from the Astronomical Almanac and the brightness temperature of the planet at this frequency The result is given in Jy beam Table 6 3 gives the recommended brightness temperatures of the planets at 90 150 and 227 GHz Most of these are taken from the work of Ulich et al Griffin et al and Hildebrand et al The brightness temperature of Mars depends on solar distance At 90 GHz Ulich 1981 AJ 86 1619 suggests an effective temperature for Mars of 1 524 T amp 90G Hz 206 8 6 22 where is the Mars Sun distance AU Some other radio sources are expected to be non variable and in the case of HII regions unpolarized Peak flux densities for sources measured with the 12 Meter Telescope that make suit
121. n be done by switching the frequency of the LO or an IF oscillator at the 12m the former is generally used If the frequency shift is small enough the spectral line will appear in both the signal and reference spectra When the resultant spectrum is formed the line will appear twice once in emission and once in absorption The spectrum can be folded to obtain a v2 improvement in signal to noise With this technique which is called in band or overlapped frequency switching you 5 4 OBSERVING MODES 67 are effectively observing on source all the time Figure 5 4 shows an example of a spectrum produced by a FS scan The primary drawback of frequency switching is that the spectral baselines are generally not as good i e flat as with position or beam switching This is because the two fre quency positions each have their own spectral bandpass shapes which do not cancel in the computation of the final spectrum which leaves a residual standing wave in the overlapped spectrum We have nearly eliminated this standing wave by using two techniques Focus Modulation Focus modulation is a technique whereby the axial focus is modu lated by 4 and during alternate integrations usually 30 seconds long When these alternate scans are averaged the individual standing waves will be 90 out of phase thus their amplitudes will cancel Beam Peak Scattering The primary component of the standing wave comes from the reflective path
122. n spectrometers like the 12m MAC Cooper 1976 Methods of Experimental Physics volume 12 part page 284 discusses this correlator efficiency and quotes the 7 values noted in Table F 1 Note that the quoted values assume Nyquist sampling Many correlators including the MAC oversample by some amount so the correlator efficiency is actually somewhat higher than that for Nyquist sampled data The theoretical calculations are always predicated on having rectangular passbands of exactly the nominal bandwidth whereas the effective bandwidth with practical filters is always smaller This and the fact that there are various non ideal properties of digitizers will tend to reduce the efficiency Thus for the purpose of estimating the required observing time to reach a given sensitivity users should use the theoretical efficiency for Nyquist sampling since the benefit of oversampling will be cancelled by miscellaneous other losses Formulas for calculating the correlator efficiency for arbitrary oversampling factors can be found in Thompson Moran and Swenson 1986 and are listed in Table F 1 For the MAC which is a 2 bit 3 level correlator the correlator efficiency is plotted in Figure F 1 136APPENDIX THE RADIOMETER EQUATION FOR POSITION SWITCHED MEASUREMEN 2 Bit 3 Level Correlator Efficiency 0 9 Correlator Efficiency jpeg I 1 pa Gs Leg 1 1 1 5 2 2 5 3 Oversampling Factor Relative to Nyquist
123. ncise scientific justification for the project Do not exceed 1000 words 2 An estimate of the observing time required 3 Frequencies and source coordinates to be observed As a proposer you should insure that the project is within the capabilities of the tele scope both in terms of available equipment and the sensitivities and integration times required The telescope and receiver parameters and system sensitivities given 53 3 will be of use in estimating the required integration times The most up to date in formation on these parameters can be found in the companion document The NRAO 12m Telescope Equipment and Calibration Status and on the NRAO Tucson Home Page http www tuc nrao edu Tucson html The 12m management imposes no hard rules as to the maximum or minimum lengths of observing programs A typical 12m observing run lasts 3 or 4 days of either partial or 1 2 OBSERVING PROPOSALS 3 Table 1 2 Proposal Submission Deadlines Deadline Observing Period January 1 April to mid July July 1 mid September to December 31 October 1 January 1 to March 31 around the clock time Requests for more than 5 days of time usually receive close scrutiny by the referees and scheduling committee If only a specific LST range is required you should request only that range For any proposal period the Scheduling Committee always receives more proposals than can be scheduled the requested time often exceeds the available time by fac
124. neral startup checklist for both spectral line and continuum observations is given below Although the Observatory staff tries to provide a fully functional system and advice about calibration constants and procedures the responsibility for the integrity of the data rests with the observer This checklist will help insure that the system is configured properly and that variable quantities such as pointing and focus are properly set Completion of this checklist may take an hour or more but the time will be well spent 1 Have the operator tune the receiver to the desired frequency including sideband and harmonic checks 2 Select a strong continuum source from the list of standard sources see Chapter 4 A bright planet 7 e Venus Mars Saturn or Jupiter is preferable Pick one whose position is near the first program source if possible 3 Ask the operator to perform a five point map of the source to check for pointing offsets Records of recent pointing offsets are kept on graphs near the observer s console and can be used to estimate an initial value for the pointing The operator will need to know the map grid spacing the default value is 5 the beam FWHM and the integration time per point A detailed discussion of telescope pointing characteristics is given in Chapter 4 When the five point measurement is complete a fit will be made to the measurement on line and displayed on the on line data server You can also reduce these measure
125. nly need to install the procedure once per session The ton procedure will use the last CALIBRATE and TPF scan to form the spectrum 5 4 2 Position Switching Position switching called the PS mode is the most common and reliable observing mode at the 12m for general spectral line observations It involves considerable overhead in telescope movement and requires that equal time be spent in the ON and OFF source positions but the data quality is usually good In this mode the telescope moves between the ON position and a relative OFF position which may be specified in either azimuth and elevation or right ascension and declination offsets Usually the offset is in azimuth so that the ON and OFF positions are taken at about the same airmass The best rejection of the atmosphere and the best spectral baselines are achieved with small angular switches Choose the smallest switch possible so long as you are confident that the OFF position is free of source emission PS data recorded on disk is a final spectrum formed from the ratio ON OFF OFF where the ON and OFF data are total power samples In contrast to the total power observing modes TPON and TPOFF the ON and OFF samples are not saved as separate scans for independent processing Although the PS mode offers less flexibility in processing data than do the total power modes it also reduces the total volume of data and makes processing easier To reduce telescope movement and provide the bes
126. noid drivers for the switching The box also contains a gas discharge noise source and associated electronics The feed horn of the noise source protrudes from a hole in the center of the machined aluminum hyperboloid subreflector Under normal operation the noise tube is covered with a cone reflector called the Cone of Silence to minimize standing waves in the IF passband If you would like 19 20 CHAPTER 3 INSTRUMENTATION to use the noise tube for calibration you must let the operator know so that he can remove the cone reflector The subreflector box is located in a focus translation mount with three degrees of freedom of movement The subreflector can be moved in and out along the radio axis to adjust for axial focus changes it can be moved in an up down or north south direction to compensate for north south focus changes or east west to adjust for optimum azimuth position The tertiary or central mirror is a rectangular flat mirror with azimuth and elevation position adjustments The elevation position of the mirror is periodically measured and then clamped down The azimuth position can be rotated to direct the radio beam to any of the four receiver bays located behind the main reflector The central mirror positioning is motorized and under servo control from the control room making it possible to use more than one receiver during a single observing run though not simultaneously The central mirror directs the
127. nt 1 380662 x 10 7 7 is the measured antenna temperature with no efficiency or atmospheric corrections applied is the measured antenna temperature corrected for atmospheric attenuation is the measured antenna temperature corrected for atmospheric attenuation radiative loss and rearward and forward scattering and spillover na is the aperture efficiency is the physical aperture 113 10 m for the 12m is the zenith optical depth and A is the number of airmasses The quantity a 24 4155 Jy K for the 12m A convenient measure of sensitivity for continuum observations is the rms flux density per root integration time outside the Earth s atmosphere 50 The sensitivity achieved in a given integration time t and under an atmosphere with zenith optical depth 79 is given by S Syt V 6 2 A table of 50 values for key frequencies is given in Table 6 1 6 3 OBSERVING BASICS 93 Table 6 1 12m Receiver Continuum Sensitivities Frequency GHz Sp per channel 5 17 a a a 145 6 3 4 The Digital Backend The data from all standard continuum observations are processed through the digital backend DBE In 3 6 3 we gave a description of the hardware configuration of the DBE It processes two independent channels and accumulates the signals from four switching phases defined as REF CAL SIG REF where SI
128. observing techniques you intend to use for program observations 6 Commence the program Remember to check pointing and focus and to observe flux density standards frequently We strongly recommend that you invest the time to perform these checks Remember that the responsibility for the integrity of the data rests with the observer 6 2 Selecting an Observing Frequency You should take some care in the precise choice of a continuum observing frequency In addition to the scientific goals that determine the approximate frequency you should also consider two other factors what frequency gives the best atmospheric transmission and will the observations be affected by contamination from spectral lines in the bandpass In the 3 mm band maximum transmission occurs near 90 GHz while in the 1 3 mm band maximum transmission occurs near 230 GHz Figure 6 1 is a plot of the atmospheric transmission as a function of frequency for the bands covered by the 12m receivers If the observations are near molecular clouds spectral line contamination can seriously affect continuum measurements This is particularly true if the continuum bandpass con tains strong molecular lines such as those from CO HCN or HCO but even a series of weak lines when integrated over the band can be a problem As receiver sensitivities have improved observers have discovered that some molecular line sources contain what is nearly a continuum of spectral lines the so cal
129. of the spectral line or whatever region you wish to integrate over DISPLAY EACH SCAN 1 5 O NO If you answer the map and the fit for new offsets are printed out If you answer yes each scan of the map with baselines removed is displayed on a single plot one spectrum on top of the other see Figure 4 6 48 CHAPTER 4 TRACKING POINTING AND FOCUS 1 TO CONTINUE OR 0 TO STOP If you approve of the fits answer 1 and the map and fit for new offsets will be displayed An example of the five point map with explanatory annotations is given in Figure 4 7 4 6 2 Pointing Model Equations In addition to coordinate conversions the telescope tracking program incorporates pointing corrections The pointing corrections take into account resolver zero offsets differences between electrical and mechanical axes axis tilt etc The coefficients of the pointing model equations are determined during dedicated optical and radio pointing source measurements several times during the observing season The pointing models used for the 12m are described in detail in Appendix A 4 6 3 Pointing Data Analysis Program Each time a continuum five point measurement is made the on line data server performs a fit to the results and writes them to a permanent file for later analysis A program called gpoint is available on any of the mountain workstations for display and analysis of the recent pointing history of the telescope
130. ometers 3 6 1 Filter Banks The 12m has eight filter spectrometers available for spectral line work The multiplexer will provide two spectra with a total number of spectral channels not in excess of 512 Thus it is always possible to record simultaneously the output of two filter banks Except for the 30 kHz bank all of the filter banks have two independent 128 channel sections You can configure these banks in one of two ways In the series option the two sections are placed end to end in frequency space i e the 256 channels are sequential in frequency In the parallel mode the two sections are used independently to accept different receiver polarization channels The series mode is appropriate for observations requiring a large bandwidth The parallel mode is useful for narrow band observations in which two different spectral resolutions are needed Additional discussion on the use of the filter banks is given in Chapter 5 Table 3 4 provides a list of the spectrometers The resolutions listed are the filter half power widths and their separations The 2 MHz filter banks work at a center frequency of 342 MHz and thus directly accept the output of the IF processor discussed above The other filter banks work at a center frequency of 150 MHz and require that the 342 MHz input signal be further down converted 3 6 2 Millimeter Autocorrelator MAC The Millimeter Autocorrelator MAC MAC is a correlation spectrometer with a digital
131. on channels 5 6 Calibration 5 6 1 Vane Calibration The calibration mode used for almost all spectral line observations at the 12m is the vane or chopper wheel method In this method a calibration signal is generated by differencing the signals recorded first on cold sky and then on an ambient temperature absorber In 12m vernacular a vane is a paddle covered with microwave absorber that is switched in and out of the beam at a rate of about 1 Hz A chopper wheel is in this case a chopping blade whose solid portions are covered with absorber and which rotates at a rate of typically 10 50 Hz The calibration technique is identical with the two devices The receivers currently in use at the 12m all use a common central vane Chopper wheel calibration has been discussed extensively in the literature see Ulich amp Haas 1976 ApJS 30 247 and Kutner amp Ulich 1981 ApJ 250 341 The technique corrects for atmospheric attenuation and several telescope losses At the 12m the tem peratures resulting from this technique are on the T scale Kutner amp Ulich which means that the temperatures are corrected for the atmosphere and all telescope losses except for coupling of the source and beam The beam is defined here to include the central diffraction lobe all near in sidelobes and the error pattern error pattern losses are often the largest of the uncorrected losses Observers should be aware that other observatories using the chopper
132. pheric windows at 3 mm 2 mm and 1 2 mm wavelengths The facility is located on Kitt Peak Arizona approximately 50 miles southwest of Tucson The Ob servatory was constructed in 1967 with an original surface diameter of 36 feet 11 meters In 1982 the surface and backup structure were replaced with a 12m diameter reflector Table 1 1 lists basic information on the observatory site and telescope The NRAO operates the telescope as a visitor facility open to use by competent ob servers without regard to affiliation or nationality Proposals are accepted before three deadlines each year and are evaluated by a panel of anonymous referees see 1 2 for more information on proposal submission The telescope is open for visitor use from ap proximately September 15th to July 15th each year From late July through the middle of September the prevailing weather pattern precludes observations at millimeter wave lengths and the telescope operation is shut down Extensive overhauls telescope upgrades and major maintenance are done during the summer shutdown period The 12 Meter Telescope is one of five observatory units operated by the NRAO The NRAO is administered by Associated Universities Inc AUI under cooperative agree ment with the National Science Foundation Operations of the 12 Meter Telescope are managed by the Assistant Director who with the Assistant Scientists also handles the scheduling of the instrument The Assistant Director or Depu
133. pillover scattering blockage and ohmic loss efficiency Tm is the mean atmospheric temperature is the atmospheric optical depth at the zenith A is the airmass temperature of the warm spillover and is the temperature of the cosmic background radiation Note that Zm and are actually equivalent Rayleigh Jeans temperatures of the point on the Planck blackbody curve corresponding to the same frequency This correction factor is given by hv J v T exp amp 1 6 10 100 CHAPTER 6 CONTINUUM OBSERVING Table 6 2 Sky Tip Antenna Positions sss Cs s T 3m where v is the observing frequency T is the kinetic temperature and h and k are the Planck and Boltzmann constants respectively For simplicity we will retain the symbol T for temperatures but in calculations T should be replaced by J v T The opacity of the atmosphere at the zenith 70 is usually determined by measuring the 4 5 at several elevations and fitting the results with some form of Equation 6 9 The mean atmospheric temperature and the spillover temperature are usually 0 95 0 97 of the ambient temperature The basic observing procedure for measuring is called sptip This procedure steps the telescope through a series of elevation angles moving from low to high elevation and then back to low The precise elevations in steps of 0 3 airmasses are listed in Table 6 2 In an spt
134. portion of the receiver LO frequency to produce the lock IF frequency For the Gunn oscillator systems F2 100 MHz The phase lock of the Gunn is completed by phase detecting this beat frequency with a synthesized loop offset frequency as a reference The loop offset frequency is generated by a tunable Fluke synthesizer The phase loop will lock when the LO frequency differs from the nth harmonic of the 2 GHz source by the loop offset frequency This means of course two lock points one with the LO above the nth harmonic and the other with the LO below These two points will be separated by twice the loop offset frequency 200 MHz The computer tests the loop for lock while taking data and stops taking data if the loop is found to be unlocked The synthesizer frequency is computed from the following equation fskyti frr k flock where is the synthesizer frequency is the sky frequency of the emission the rest frequency with Doppler corrections j 1 for lower sideband and 1 for upper sideband fir is the IF frequency indexfrequency IF m is the factor by which the LO frequency is multiplied before injection into the mixer k 1 for the lower lock sideband and 1 for the upper lock sideband flock is the the phase lock loop offset frequency and N is the synthesizer harmonic The four permutations of j and k are given by a parameter SB for sideband that is entered into the control computer The control compu
135. pressure of 610 Torr Appendix B The Relationship Between Flux and Brightness Temperature In the following I derive the relationship between the flux of a source and its brightness temperature for a gaussian beam measuring two different source geometries uniform disk and gaussian The general relation between flux and brightness temperature is fT B 1 Note that throughout this discussion when I refer to a temperature I am actually referring to the effective source radiation temperature J v which is defined by Equation C 1 B 1 Uniform Disk Source CE 2 IE 2 7 Ty ox f S 2 E 2 where I have used the fact that Sv OR B 4 Plugging in some constants into Equation B 2 yields the following 117 118APPENDIX B THE RELATIONSHIP BETWEEN FLUX AND BRIGHTNESS TEMPERATURE 8 179 x arcsec exp In 2 E lm T K B 6 The peak flux densities of the planets are calculated by the planets program using Equation B 6 B 2 Elliptical Gaussian Source x IT VE fel Aln 2 5 An 462 02 2 16 In 2 NE kr 67 7 Ty 02 in i O az i 2 In 2 A2 io 07 9 Plugging in some constants into Equation 7 yields the following 5 2k 2 109 xv GHz 63 arcsec 02 in 07 23 B 1 min max ae 4In 2 Ti B T gee 2 Bin T P a 8 179 10 v GHz 0s arcsec
136. quency velocity and calibration information WL Focus step size used for focus measurements in millimeters OBJ V Source velocity in chosen reference frame in km s AN T V Antenna velocity in chosen reference frame in km s CAL Calibration type EFF Aperture efficiency SB Signal sideband upper or lower for channel 1 top and channel 2 bottom 6 7 CONTINUUM STATUS MONITOR 111 LO Local oscillator multiplication factor IF S Intermediate frequencies in MHz FREQUENCY Rest frequencies for channels 1 line 1 and 2 line 2 in GHz with their associated LO synthesizer frequencies lines 3 and 4 respectively in MHz DBE RUN Current observing mode Box 8 Telescope tracking and weather information TOL Input and actual tracking tolerance in m ss FOCUS Input and actual axial focus at the current elevation in millimeters TORR Barometric pressure in Torr RH Relative humidity T_AMB Ambient temperature in C REFRT Input and actual refraction constant for elevation refraction pointing cor rection in arcseconds Box 9 Current observation scan and integration time information MODE Observing mode SCANS Number of scans requested SAMPLES Number of continuum on off samples requested SEC Total top and remaining bottom sample time in seconds TIME Total top and remaining bottom integration time for this scan in mm ss SCAN SOURCE HORIZON LST UTC DUTI 1996 OBS 1
137. r the AZ EL RA DEC or III bII frames if you are doing total power of position switched map ping 4 The frequency switch offsets in MHz if you are doing frequency switched mapping 5 The horizontal coordinate grid spacing in seconds of arc real angle 6 The vertical coordinate grid spacing in seconds of arc 7 The number of rows in the map Must be an odd number 8 The number of columns in the map Must be an odd number 9 The beginning and ending rows of the map optional 10 The beginning and ending columns of the map optional 11 The grid rotation angle optional 12 For position or frequency switched maps you must also specify a The scan time in seconds a b c d The number of scans per center position measurement The number of ON OFF pairs per point The number of scans per calibration measurement 13 For total power mapping you must also specify a The integration time per ON and OFF in seconds b The number of map positions ONs to be observed for each reference position OFF Dec or III GRID SPIRAL CROSS ajs efe s ols fo RA Figure 5 6 Grid map observing sequence for all 12m grid mapping modes SHGON DONIAHMSHO FS ee 74 CHAPTER 5 SPECTRAL LINE OBSERVING c The number of OFFs for each vane CALIBRATE Display of position and frequency switched map measurements is the same a
138. rces It is particularly important to check the focus after nightfall and daybreak Focusing is performed via the focalize procedure This steps the radial distance of the subreflector from the telescope vertex through seven positions centered on the nominal best setting F0 Larger values of F0 represent greater distances between the dish surface and the subreflector The incremental movement between positions WL is set to half of the observing wavelength by default but can be set by the observer Perform a focalize as follows 1 Choose as bright a continuum source as possible The planets Jupiter Venus and Saturn are ideal If the source is extended relative to the telescope main beam the accuracy of the focalize will be diminished however 2 Make a five point map of the source to optimize the antenna pointing see 4 6 1 3 Reasonable default values for the axial subreflector position 0 have been assigned to each receiver but if you find it necessary you can give the operator a value for FO 4 The default integration time per sample is 5 seconds but if you are performing a focalize measurement on a weak source you may need to increase this to 10 15 seconds 5 If you are satisfied with the setup ask the operator to start the measurement Focalize measurements are processed automatically by the on line dataserver the re sults of which are sent automatically to the operator for entry into the control system To red
139. rver s Comment Sheet available at the telescope and on the NRAO Tucson Home Page 1 3 OBSERVATORY POLICY 5 1 3 3 Maintenance Repairs One period approximately every 10 days is assigned to preventive maintenance and routine system tests If during a scheduled observing period a catastrophic failure of the instrument occurs which results in a loss of data observations will be stopped and the NRAO technical staff will attempt to repair the equipment In less serious cases where data taking continues but where the quality of the data is not optimal it is your responsibility to decide whether or not you wish to give up telescope time so that repairs can be made Only the Tucson Assistant Director can make the decision to interrupt scheduled operations to make non essential repairs 1 3 4 Sharing Telescope Facilities with Other Observing Teams Since living quarters and work spaces at the telescope are limited you should leave the mountain as soon as possible at the end of your run allowing of course for a reasonable period of rest If you wish to continue the reduction of your data sets you should do so at the NRAO Tucson office When two or more observing teams are sharing observing time the team currently observing has priority to all telescope facilities including computer usage The other observing teams should endeavor to stay out of the control room and not interfere in any way with the ongoing observations Unless one group
140. s x 10 103 56 95 5 Iw 4995002 PUE and is given similarly by Crane Methods of Experimental Physics volume 12 part page 186 equation 2 5 1 as Py Py No 96 0 A 4 where I have ignored the partial pressure contribution due to carbon dioxide only 0 03 of the total pressure n is the complex index of refraction and the pressures and temperatures are as defined below A similar expression to the two given above can also be found in Allen 1963 Astrophysical Quantities page 120 Taking the Liebe amp Hopponen relation Equation for No and solving for Rp in arcseconds yields P 21 36 1 66 29 27 Ry 21 3677 106777 103029 5 where P is the partial pressure of dry gases in the atmosphere in Torr is the partial pressure of water vapor at the surface in Torr P is the total surface barometric pressure in Torr which is equal to P Py and is the surface ambient air temperature in Kelvin P must be measured by a barometer at the telescope site P may be calculated from the expression Esat P RH A 6 100 where RH is the surface relative humidity in percent and is the surface saturated water vapor pressure in Torr and is given by Crane in Methods of Experimental Physics volume 12 part B page 187 equation 2 5 3 as 2990 14 log sat 24 67
141. s that used to display regular PS or FS measurements The spectrum of a single total power map point is displayed in line using the ton procedure Line gt install ton Line gt scan ton where scan is that of the ON position To display a contour map of peak or integrated intensity versus position it is best to use the CLASS program See 2 4 1 for information on how to port your 12m data to CLASS format Manuals for the CLASS program are also available 5 4 6 3 When Should I OTF Instead of Grid Map As a general rule of thumb it is best to use OTF mapping instead of the step and integrate mapping described in this section when your map field is larger than about 6 in either RA or DEC and your target spectral line is expected to be reasonably strong For those rare cases when OTF is not suitable one must decide which step and integrate mapping technique to use There is no hard rule to determine whether one should use PSM or TPM the total power mapping mode TPM is more efficient in the sense that you can use one OFF scan with several ONs However PSM may produce better baselines because of the switching pattern As a general recommendation we suggest that you use TPM for maps of strong lines and large mapping grids use PSM if the lines are weak and baseline stability is critical is also an alternative to PSM it uses the same OFF ON ON OFF pattern but the mapping positions are taken from discrete catalog entries rather than
142. se tube emission If there is no noise tube should be set to the value that gives the proper antenna temperature difference between hot and cold loads has the following standard values 400 0 for single sideband vane calibration 800 0 for double sideband vane calibration 6 0 for noise tube calibration value based on a hot cold load measurement for fixed calibration 6 3 This is the optical depth of the atmosphere at the zenith in nepers The condar verbs that manipulate and calibrate DBE data are SWITCHED reorders the data array to contain only calibrated switched power data 6 3 OBSERVING BASICS 95 TOTALPWR reorders the data array to contain only calibrated total power data CALDBE reorders the data array to contain only calibrated calibration values ZERO calculates the calibrated rms of the zero values and stores the rms in the condar array vrms 1 AVG computes the mean temperature and standard error of the mean of an ON OFF sequence In general you will not need to use these verbs explicitly as resident condar procedures see below are available for standard displays of switched and total power data The way in which the data are calibrated depends upon the type of calibration chosen when you acquired the data For the three types of calibration available vane fixed and noise tube the data scaling factors are given by T yane gt A wane cay 4
143. shift in the loop offset frequency will produce exactly the same shift in the spectrum Other receiver systems particularly the high frequency receivers use a harmonic multiple of the oscillator source as the LO frequency The desired frequency shift must be divided by the appropriate multiple before setting the loop offset frequencies You will usually want to know whether the reference frequency is higher or lower than the signal frequency or in other words where the apparent absorption and emission features appear in the band This is dependent upon which sideband upper or lower is being used For upper sideband operation the signal frequency will be higher than the reference frequency so that the emission line will appear to the right of the reference signal in spectra The reverse is true for lower sideband operation When performing frequency switched observations you must decide upon the following parameters and give them to the operator CHAPTER 5 SPECTRAL LINE OBSERVING 68 VELOCITY 300 100 RADILSR 100 500 2 80 0 40 0 80 100 Rest Frequency MHz Figure 5 4 Frequency switched spectral line scan analyzed in the unipops program The frequency switch throw for these measurements was 25 MHz 20 LO Synthesizer Frequency kHz 1489 01 FREQUENCY 60 Total Integration Time M17SW INT 00 06 0 DATE 30 AUG 97 1950RADC
144. sideration it will be in competition with new proposals received for that period If a proposal is not selected on its second consideration it will be declared inactive and generally will not receive any further consideration for telescope time The 12m Scheduling Committee will notify proposers as to the disposition of their active proposals after each selection process After any evaluation of a proposal the authors may submit an amended version of the proposal to address referees remarks or to otherwise strengthen the proposal The proposal will be re refereed for the next available period Investigators are also free to withdraw a proposal and resubmit it as a different proposal CHAPTER 1 INTRODUCTION 1 2 3 Proposal and Observing Propriety Observers are expected to confine their observations to those described in their refereed proposal It is absolutely essential that observers consult with the Assistant Director or Deputy Assistant Director and obtain his approval before altering scheduled observing programs Approval for changes can be granted under those circumstances that do not lead to an infringement on work proposed by others and when the changes are in keeping with the spirit of the original refereed proposal These rules are fundamental to the integrity of the observing system at NRAO and are taken very seriously by the management 1 3 Observatory Policy 1 3 1 Staff Responsibilities The following is the responsibility of
145. some measure of the dynamic range A better definition of dynamic range might take into account the spatial frequency structure of the source If the source has no structure on scales smaller than Tue then you don t need to sample at the full m anyway This is one circumstance where it is perfectly rigorous to undersample the data If say you have a 10m dish and you are taking data to compare with other observations using a 1m dish at the same wavelength or the equivalent number of wavelengths at some other frequency then you only need to sample the data at TL Or This is so because when sampling a 10m dish as if it were a 5 5m dish the spatial frequency components from baselines of 5 5m out to 10m will be reflected back into the data as if corresponding to baselines of 5 5m down to 1m So the spatial frequency terms of 1m baseline and below will not have been corrupted The data analysis of this undersampled data would apply a spatial frequency cutoff at 1m and there will have been no corruption in this smoothed data caused by the undersampling Putting it in more general terms if you are going to be smoothing a dish of diameter D to simulate observations made with a smaller dish of diameter d then the sampling interval only needs to be rather than gt There are other aspects that make it desirable to cumple more often iban the Nyquist rate as we recommend for OTF observing These are practical points like how well grid ding
146. spectrometers used at millimeter wavelength observatories filter bank spectrometers correlation spectrometers etc the true bandwidth of a single spectrometer channel is a convolution of the original gain response of the channel and any smoothing functions applied This convolution of the response and smoothing functions affects two important factors Avpwgp The full width to half power or 3dB bandwidth which is the true spectral resolution and The sensitivity bandwidth which is the channel bandwidth used in calculations of the rms sensitivity in a spectrum The sensitivity bandwidth is defined as follows Hs F v dv Avsp ee Pa doe Ff dv D 1 co Kraus Radio Astronomy 2nd Edition page 7 8 defines a similar term Table D 1 describes the relationship between Avpwg p and the frequency sam pling Av for various smoothing functions F v assuming Nyquist sampling at Av At the end of this Appendix I show the calculation for each of these integrals For the filter bank spectrometers at the 12m the channel response is a second order Chebyshev bandpass filter which approximates a pill box function there is no smoothing and they are not Nyquist sampled A single channel in the Millimeter Autocorrelator MAC on the other hand has a sinc frequency response which is hanning smoothed Since the convolution of a sinc with any function that is already band limited within the frequency response o
147. t and cold loads respectively and T old value is the old value of that was in the computer during the hot cold measurement For each channel there should be good consistency between the values computed via the computer and the voltmeter If you are using a noise tube to calibrate the data the calibration scale should stay fairly accurate even if the gain of the receiver changes slightly If you are not using a noise tube the 7 s and noise temperatures are accurate only at the moment they are measured receiver gain and tuning drifts will change these parameters Depending on your choice of calibration methods you may need to repeat the HOT COLD measurements frequently 108 CHAPTER 6 CONTINUUM OBSERVING Table 6 3 Planetary Flux Density Standards Planet 7 90 GHz T amp 150 GHz 227 GHz Unit Semi Diameter K K K arcsec 172441 1734 1 17144 95 20 1494 4 137411 1404 14 78 15 135 44 11245 98 5 35 02 130 5 107 5 93 5 33 50 6 6 3 Calibration of the Flux Density Scale For most continuum observations the flux density scale is calibrated by observations of standard radio sources In doing this it should be remembered that in addition to correc tions for receiver and atmospheric effects you should allow for the gain elevation properties of the telescope if the observations cover a significant range of elevations Current gain elevation curves are given in The NRAO 12m Telescope Equipment and Calibratio
148. t compensation for polynomial drifts in atmospheric emission choose the number of OFF ON pairs to be a multiple of 2 The observing cycle will then be repeats of an OFF ON ON OFF pattern characterized by a Walsh function see Appendix E Each ON or OFF is called a sample and each OFF ON pair is called a repeat The observer must tell the operator how long to integrate for each sample the default is 30 seconds and how many repeats per scan or alternatively the total length of the scan in minutes A typical scan might be 6 minutes long with 30 second samples meaning 6 repeats You can of course vary the length of the scan to suit your own needs Each ON OFF pair can be edited individually with the line program see Record Editing in the supplement to the line manual The operator can issue the command to take scans one at a time or can set the system into an automatic data taking loop Figure 5 3 shows an example of a spectrum produced by a PS scan The parameters of a PS scan that you must give to the operator are m The relative offset position which may be specified in either AZ EL or RA DEC coordinates The integration time for an individual sample ON or OFF 30 seconds is the default VELOCITY X RADILSR 60 20 20 60 23 1 n 1 1 18 oe 13 8 PG CEN 5 _ 60 20 20 60 Rest Frequency MHz 1 Synesiter Frequency dH FREQU
149. taining data files This subdirectory is private to each observing team and is denoted by obs ini where ini are the 3 letter initials of the lead observer The data files in this subdirectory are also labeled with the same initials Log into the system as username obs the operator can tell you the current password for this account You will then be prompted for your initials discussed above After the login process is complete you will be in your obs ini subdirectory The spectral line system always records data from two 256 channel filter banks and up to 65536 channels from the Millimeter Autocorrelator MAC Each scan is composed of a number of subscans indexdata subscan which individually contain one filter bank or Millimeter Autocorrelator MAC measurement Table 2 1 lists the subscan codes with their associated polarizations and backends To start the continuum analysis program from the Unix prompt type condar 2 4 ALTERNATE DATA ANALYSIS PACKAGES 13 while to start the spectral line analysis program at the Unix prompt type line Table 2 2 lists a number of condar and line commands and their function while Table 2 3 lists a number of basic data analysis procedures 2 4 Alternate Data Analysis Packages There are a number of other data analysis packages available at the 12m which can be used to analyze 12m data All of these packages require a conversion of the 12m sdd format to the resident format of the analysis package to b
150. ter calculates two synthesizer settings corresponding to different harmonic numbers N for a given rest frequency source Doppler velocity and SB value The operator can switch between these two settings by turning a knob on the synthesizer 3 5 THE IF SECTION 27 control chassis The computer chooses the synthesizer setting so that one is usually slightly above 1 9 GHz and the other slightly below Both of these synthesizer settings are updated by the computer to reflect changing Doppler velocity as a result of the LSR reference frame or diurnal variations A manual synthesizer setting can be entered from this chassis so that the receiver can be tuned without the aid of the computer if that is desired When tuning the receiver the operator and observer must take care that the LO is locked to the correct harmonic and loop sideband Two tests can be performed to assure that these conditions are met First if you try to lock to the wrong lock sideband a comb of frequency spikes will appear on the spectrum analyzer If this happens turn the tune dial until the main spike moves off the edge of the screen and then returns You must then perform a harmonic check This is done by opening the phase lock loop i e turning the phase lock circuitry off and switching to the other synthesizer harmonic on the synthesizer control chassis If the tuning is correct the beat signal on the loop spectrum analyzer will appear at the same frequency for either
151. that described by Stumpff 1972 Kleinheubacher Berichte 15 431 and Ulich 1981 Int J Infrared amp Millimeter Waves 2 293 The azimuth and elevation terms used to correct the nominal encoder positions are given by the equations CAsec E NPAEtan E AN tan E sin A AW tan E cos A Aos sec E A 1 AE IE ECEC cos E AN cos A AW sin A Eg Rof E 2 where A and E are the azimuth and elevation of the source and AI CA NPAE AN AW A obs AE IE ECEC is the azimuth encoder zero offset is the collimation error of the electromagnetic axis Non perpendicularity between the mount azimuth and elevation axes is the azimuth axis offset misalignment north south is the azimuth axis offset misalignment east west is the observer applied azimuth correction is the total elevation encoder correction is the elevation encoder zero offset is the gravitational flexure correction at the horizon is the observer applied elevation correction 113 114 APPENDIX A POINTING EQUATIONS FOR THE 12M TELESCOPE Ro is the weather dependent term to the atmospheric refraction correction is the elevation dependence of the atmospheric refraction correction The weather dependent refraction coefficient which is practically independent of wave length is given by Liebe amp Hopponen 1977 IEEE Trans Antennas Propagation AP 25 336 equation 9 as 1 x 10 R radian
152. the BEAM Note that continuum OTF mapping is functionally the same as continuum grid mapping for most applications Given the fact that we currently have no analysis software for continuum grid mapping but have analysis software for continuum OTF we recommend that ob servers use continuum OTF mapping for all continuum mapping experiments The telescope builds a two dimensional grid of observations by scanning rows at con stant elevation relative to the source position The telescope moves along a row in discrete steps performing an integration at each position The grid points are separated in azimuth by real angle If a map has M columns and N rows the requested field center will lie at the central grid point INT M 1 2 INT N 1 2 if N and M are odd numbers where INT denotes an integer truncation If N and M are even the requested field center will fall at grid point 2 1 N 2 1 A scan represents row of the map in this observing mode The mean sidereal time of each point is stored in the scan array Analysis programs have been prepared to combine the scans into two dimensional maps You can process these maps using the dual beam restoration algorithm then transform them into celestial 6 5 UTILITY OBSERVING ROUTINES 99 coordinates and stack them Unfortunately we do not currently have an implementation of this analysis system at the telescope To observe such a map proceed as follows 1 Tell the operator whether the
153. the NRAO staff To insure that the equipment needed for your observations is available and installed at the telescope To tune the receiver to the desired frequency To provide sound telescope pointing To provide fundamental telescope calibration parameters efficiencies beamwidths gain curves at standard observing frequencies To provide advice on observing strategies if requested 1 3 2 Observer s Responsibilities As the visiting observer you have the responsibility for proper supervision of all aspects of the observing program This includes Providing to the NRAO staff well in advance of the time scheduled a full description of the equipment needed for the observations as well as a complete list of frequencies to be observed Usually this information is included on the proposal cover sheet To verify the telescope pointing and fine tune it as needed To obtain all calibration and other receiver telescope parameters necessary for data reduction This can be done either by adopting or scaling the NRAO provided infor mation from standard frequencies and or by making the appropriate measurements In either case proper data calibration is your responsibility not the NRAO s To inform the NRAO staff before the observing period has ended about the types of data to be written on an export tape and the format of the export tape In addition you are requested to provide feedback on the observing run via the Obse
154. the bandpass tune first to a strong test line that is as close by in frequency as possible Strong means any line that will produce a good signal to noise spectrum in a 5 10 minute integration for example Standard sources are listed in the NRAO standard catalog called standard cat If possible use the same observing setup same spectrometer mode and observing mode as will be used for the program observations If the observations are of a common species such as CO there is no need to tune to another line b Perform a calibration scan and check for bad channels in the filter banks Report the bad channels to the operator who will flag those channels in the control system software c Observe a test line in a strong source Observers may wish to verify the sense of the velocity frequency scale by shifting the rest frequency or center velocity by a small amount and seeing if the line moves in the correct direction for the sideband choice d Check that the line temperature calibration is correct This can be done by observing a standard source presuming that the test line has known strength Measurements of many of the sources in the NRAO standard catalog stan dard cat have been made in the CO and J 1 gt 0 and J 2 1 transitions Plots of these spectra can be found at the telescope and on the NRAO Tucson Home Page 2 3 Basic Data Reduction with UniPOPS Two other manuals one for spectral line
155. the entrance to the observing rooms They are there to prevent you from walking into any possible pinch points or dan gerous areas 5 Do not stand in the red areas because parts of the telescope and dome that move in those areas could injure you severely 6 Do not touch the yellow curtains around the inside wall of the dome Behind them are exposed 480 volt power lines 1 3 10 11 12 13 14 15 OBSERVATORY POLICY T Please abide by all printed and posted safety rules such as No Smoking and Do Not Enter This Area posters etc Only the telescope operator or other qualified Arizona employees are allowed to operate the cherry picker Observers may ride in the cherry picker if authorized to do so by the duty operator Hard hats are required for all persons in the dome area if someone is working above or in the cherry picker The hats are located on the wall just outside of the observing room door When walking outside to the dormitories or the lab at night please be sure to carry a flashlight You may encounter steps drop offs or hungry wild animals The consumption of alcoholic beverages or illegal drugs is absolutely forbidden in the lab and telescope control room areas All employees and observers are required to wear seat belts while riding in government vehicles Ice rocks and rock slides are frequently a hazard on the roads and walkways Cattle and horses cross Highway 3
156. the measurement Spectral Line Five Point 1 Select an appropriate source The source listing in Table 4 2 is by no means all inclusive but if another source is used take care that it is really suitable for use as a pointing source In particular it should have an angular size that is smaller than or at least comparable to the antenna beam size and possess an accurately known position 43 4 6 POINTING ees 1 r oem _ e sr 2o on 9 1 0 0 0 an 1 so or Jeorsez o 90 T Ln p qq p uw 80 0 80 or o 0 20 68 0 n oc c o 260 a s ox ws ro or 11 arorec L ux 90 ve 50500 Lor ee po om fermo Pp me Tr ame an 2 2 r osr seerr0z 940 A 86 87 61 Py 11 66 81 0 0 amp 505081 195 XA 0 0 6 21 9060050 PHA 9 M recocer 61 67 01 910 81 67 60 9IZOT OUL 60 LY 0620 1 10 90 99815 6066650 02 66 60 8 66 66 0 81979 LEOS 06 9750 6 0 Psu
157. tion 5 3 2 Millimeter Autocorrelator The Millimeter Autocorrelator MAC offers a number of bandwidth and resolution modes listed in Table 5 2 Most general single beam dual polarization measurements are done us ing the configurations with 2 IF s The 8 IF modes are designed for use with the Imm Array receiver The 4IF Millimeter Autocorrelator MAC mode is a special purpose observing mode allowing the measurement of two frequencies each at two polarizations simultane ously 5 3 2 1 The Observing Mode One of the more efficient observing modes available at the 12m involves the use of the configuration of the Millimeter Autocorrelator MAC in conjunction with the frequency offsetting capabilities of the filter banks Within the IF modules which feed the receiver signals to the Millimeter Autocorrelator MAC there are oscillators which can shift the input receiver signals by 300 MHz in 5 MHz steps Since these oscillators are independent of the Fluke synthesizers which provide the frequency offsetting capabilities of the filter banks it is possible to measure spectra at three separate frequencies within the 600 MHz bandwidth of the receiver assuming both mixers are tuned to the same frequency A graphical description of this mode is shown in Figure 5 2 To given a practical example say I want to simultaneously measure the H5CO emis sion from the 393 292 and 322 25 transitions at 218222 192 and 218475 632 MHz respecti
158. tors of 2 4 For this reason you should prepare proposals with care 1 2 2 Proposal Submission and Refereeing Twelve meter telescope scheduling operates on a trimester system with proposal submis sion deadlines and their corresponding observing periods listed in Table 1 2 intention of the 12m proposal system is to insure that the projects granted telescope time are of current interest and that all proposals receive a prompt scheduling decision Proposals should be sent to the Director of the NRAO in Charlottesville Virginia In formation regarding electronic submission of proposals can be found on the NRAO Tucson Home Page http www tuc nrao edu Tucson html After receipt by the Director s office the proposals are assigned a reference number and are sent to a panel of five referees who are anonymous to the proposer and to each other The referees rank the proposal as to scientific merit and feasibility of achieving the scientific goal recommend what percent age of the requested observing time should be granted and make any comments they feel are pertinent On the basis of the referees rankings and comments the 12m Scheduling Committee selects the proposals to be scheduled A report of referees comments and the disposition of the proposal is sent to the proposal s contact authors usually within 6 8 weeks after the deadline Proposals are considered for two consecutive trimester periods On a proposal s second con
159. ty Assistant Director should be contacted with regard to general matters of operations policy In addition more general questions or comments pertaining to Observatory wide activities scheduling procedures scientific or instrumentation priorities inter site relations or specific criticisms of or sug gestions for the Tucson operation may be addressed to the Director of the NRAO located at the Charlottesville Virginia office Additional visitor information including maps lodging fees travel reimbursement poli cies and names of specific staff members responsible for operation of the telescope can be found on the NRAO Tucson Home Page at http www tuc nrao edu Tucson html This information can also be found in the companion document Visitors Guide to the NRAO 2 CHAPTER 1 INTRODUCTION Table 1 1 Telescope and Site Characteristics East Longitude 111 36 53 475 North Latitude 31 577 12 0 Elevation 1894 5 meters 6215 8 feet Telescope Primary Reflector Diameter 12 0 meters Focal Ratio f D Prime Focus 0 42 Cassegrain Focus 13 8 Surface Accuracy 75 pm rms Mount Elevation over Azimuth Slew Rate 68 minute Pointing Accuracy 5 rms Elevation Limit 15 Enclosure Tracking astrodome with movable door 12m Telescope 1 2 Observing Proposals 1 2 1 Proposal Preparation All proposals should include a completed 12m Observing Application Cover Sheet The body of the proposal must include 1 A co
160. uce the focalize in condar use the commands Condar gt scan_number focalize where each channel is selected by specifying either subscan 01 for channel 1 or 02 for channel 2 The output from the focalize procedure is shown in Figure 4 9 The fit should be smooth and quasi Gaussian If not repeat the focalize Increase the value of WL if you think this is necessary The display will give a best fit value for F0 Where appropriate you should average the values for the two channels 4 7 3 Lateral Focus The position of the subreflector for optimum gain shifts in the elevation direction with changing elevation angle The change in antenna gain produced by this effect is not significant at 3mm wavelengths but could be as much as 20 or more at 1 3mm and shorter wavelengths The 12m is equipped with lateral north south and east west translation stages at the prime focus to eliminate this loss in antenna gain The computer is able to automatically control the positioning of these stages simultaneously applying 1 20 gt x e lt Z T gt 0 80 E E 0 60 E amp E m 0 40 0 20 Mars Input Radial Foc us Value FO mm 45 000 44 75589 BEST FO TO 74800000 IAN FIT Focus Step Size Between Measurements 0 8000000 Measurements at Discrete Foci are indicated by a 1 SA e 7 SAMPLES 5758 01 INT 70 00 DATE 15 OCT 96 200
161. vance how many terms there will be The algorithm is simply the following After every complete set of m 2 or more but always a power of 2 phases the next m phases will be a repeat of the first m with 0 and 1 reversed This is equivalent to adding another order of Rademacher function to the R products generating a Walsh function of order 2 1 PAL 2 1 t sign 2x0 2 129 130 APPENDIX E WALSH FUNCTION MODULATION This assures we build up a PALey order of 2 1 6 with n being increased by 1 after every complete set and which means that polynomial drift terms up to order n 1 are perfectly rejected In practice each phase usually lasts 30 seconds although phases as short as 10 seconds and as long as a minute are occasionally used How long a sequence we have really depends on how long the observer wants to integrate The 0 1 1 0 is normally the shortest sequence with the 2 pairs of phases We probably rarely use more than 8 pairs which would be an 8 minute in total time integration at 30 seconds per SIG and REF integration phase In other words typical functions are usually PAL 3 T PAL 7 T or PAL 15 T Figure E 1 shows the Walsh functions of order 2 1 for n 1 5 The following C code fragment written by Jeff Hagen is used in the 12 Meter Telescope control program to calculate the Walsh function walsh i int i 1 int bool 0 int mask 0x8000 while mask 4
162. vely These two frequencies are separated by 253 440 MHz which would restrict my choice of backends to the 2MHz filter banks in parallel and the 600 MHz Millimeter Autocorrelator MAC mode Since I want to look at galactic sources with narrow lines this wide band modes are not acceptable To use the 4IF mode to measure both transitions simultaneously I would 5 3 SPECTROMETERS 61 Table 5 2 Millimeter Autocorrelator MAC Configurations Bandwidth and Channels Useable Bandwidth and Channels Resolution um eum gt 2 IF Modes 800 600 800 600 400 300 400 300 200 150 200 150 100 75 100 75 3 0 6 1 4 IF Modes 800 600 768 781 2 800 600 1536 390 6 400 300 1536 195 3 400 300 3072 97 6 200 150 3072 48 8 200 150 6144 24 4 100 75 6144 12 2 100 75 12288 6 1 8 IF Modes 800 512 384 800 1024 768 400 1024 768 400 2048 1536 200 2048 1536 200 4096 3072 100 4096 3072 100 8192 6144 1 The useable bandwidth takes account of the 75 efficiency of the analog filters 2 NOTE This is the frequency sampling interval not the FWHM channel width for a given channel The FWHM channel width is 2 0 times this value See Appendix D for details All values in this table refer to each IF Modes tagged with a are produced by dropping the last half of the lags CHAPTER 5 SPECTRAL LINE OBSERVING 62 Vi mum Filter Bank lt Spectrometer Mode Configuration Filter Bank Offset F
163. wheel method have different definitions of the basic temperature scale Exercise care when comparing data An essential part of the chopper wheel calibration method is the specification of the calibration scale temperature which is given by Thot Ta sky ie ee MIN fss EXP To A where T A sky is given by Equation 6 9 For observations under typical atmospheric con ditions 400 for single sideband operation and 800 for double sideband operation Note that varies with elevation particularly for double sideband observations on the wings of an atmospheric line such as at 115 GHz The procedure for performing a vane CALIBRATE is the following 5 8 78 CHAPTER 5 SPECTRAL LINE OBSERVING 1 Set the calibration scale temperature As explained above is approximately 400 for SSB receivers and 800 for DSB receivers The precise value of is dependent upon efficiency factors temperature atmospheric optical depth and elevation or airmass 2 Perform a calibration cycle called a CALIBRATE at intervals which you prescribe The interval should be small enough that the atmospheric transmission during the interval does not change appreciably This depends on the atmospheric optical depth and its stability and the elevation During stable conditions of low optical depth a CALIBRATE every 10 15 minutes is usually sufficient If the atmosphere is choppy or the elevation is changing rapidly as when the source
164. will be correctly calculated Note that if the two target lines are at the center of their respective sidebands they will fall on top of each other in the resultant spectrum To keep this from happening offset the line rest frequencies from the band center If the signal sideband center frequency is offset by an amount the signal and image sideband lines will be separated by 26 and will be symmetrically displaced about the center of the final spectrum To observe with a non standard IF follow this procedure 1 Discuss your intentions with a staff member well in advance of the observations 2 Calculate the new synthesizer frequency according to the equations above Make sure that you are not exceeding the 600 MHz bandwidth of the IF system 3 Ask the operator to dial the synthesizer frequency into the IF synthesizer that is nominally set to 109 5 MHz 4 Ask the operator to set the new IF into the control computer 5 If appropriate offset the signal band center frequency from the line rest frequency to prevent overlap of signal and image lines 82 CHAPTER 5 SPECTRAL LINE OBSERVING 6 Remember to have the synthesizer set back to 109 50000 MHz when the special observations are finished Step 6 above is especially important to remember as the computer does not check the IF synthesizer for a correct setting If you or the next observer resume standard observations and the synthesizer is not reset to 109 5 MHz the band center
165. will be offset from what you intend In addition we highly recommend that you check your special observing configuration on a strong test line before conducting program observations on weak lines 5 9 Spectral Line Status Monitor The spectral line status monitor provides basic information about the status of the tele scope and the observations that are underway The observer should check the monitor frequently to see that the telescope and control system are configured as they should be Figure 5 7 shows a sample monitor screen divided into several boxes Each numbered box indicates a section of the display which describes a particular set of attributes of a spectral line measurement Only box 7 differs from the continuum status display see 6 7 Box 1 Scan number source name and timing information SCAN Current scan number SOURCE Source name HORIZON Time to 15 elevation rise or set LST Current local sidereal time UTC Current coordinated universal time DUT1 UT1 UTC time correction date Current year top and date bottom OBS Current observer initials top and data file number bottom OPR Current operator initials Box 2 RA Dec and position information TOTAL Current total apparent plus offsets RA and Dec B1950 0 Current Equinox B1950 RA and Dec APPARENT Current apparent RA and Dec GALACTIC Current and bII OFFSET Current applied RA and Dec offsets Box 3 Apex position information
166. ximate flux densities at 3 and 1 mm is given in Table 4 1 This table includes the sources traditionally used at the 12m together with sources found useful at the IRAM 30m telescope The sources with no listed flux density are on the order of 1 Jy in strength It should be noted that the flux densities of many of the extragalactic sources in the table can be highly time variable For an up to date listing of measured 3mm fluxes see http dopey haystack edu cmva quasar list The spectral line emission from a number of sources can be used for pointing purposes Emission from rotational transitions of CO HCN and SiO are often strong enough and angularly compact enough to provide good pointing results A list of spectral line pointing sources and their strong molecular emission lines are listed in Table 4 2 4 6 1 Continuum and Spectral Line Five Point Measurements The five point mapping option is the standard observing technique for determining the antenna pointing of the 12 Meter Telescope Continuum five point measurements can also be used to measure the flux density of a strong source with the highest precision or the flux density of a source whose position is not exactly known The five point map con sists of five successive ON OFF sequences for continuum and beam switched spectral line measurements or five successive ON and OFF measurements for position switched spectral line measurements or five successive ON measurements with one assoc
167. y lof f N F 9 where is the number of ON source measurements per OFF source measurement There fore T2757 becomes tscean ton Tavs i tscan a 1 56 T spec Tus Q 1 2 mm a 1 1 10 V DSU dn E l To determine the optimum OFF source integration time time when one is acquiring multiple ON source measurements per OFF source measurement we find the minimum of Equation F 10 with respect to a dT son Tos m E id j 2 ma E da AVtscan 2N N 2a N 0 F 11 which reduces to ET CUN gt See P sc gt 2 Ne GO S l ii Mae GS gt Sen 227 Se tel ieee F 12 135 Table F 1 2 Bit Correlator Efficiencies Number of Levels fpe TMS Equation and Page optimal where I have used Equation Inserting this value for tpp Equation F 10 leads to the following relation for the optimal scan rms when one observes N ON source mea surements per OFF source measurement T 1 optimal pscan Sys 14 FER T spec v AV scan V J 13 See Ball 1976 Methods of Experimental Physics volume 12 part page 52 for a parallel discussion on noise considerations for position switched measurements The factor takes account of the efficiency for quantized correlatio

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