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Very Large Telescope Paranal Science Operations VISIR User Manual

1. 1000 Sensitivity mJy 100 in 1h Dies re le ol re 12 80 12 90 13 00 13 10 Wavelength um 2 0x104 TOCATA TT 1 5x104 1 0x104 5 0x103 Sensitivity mJy 100 in 1h 0 E ao po e TR ae a A peje el 16 37516 38016 38516 39016 39516 40016 405 Wavelength um Figure 23 Observed sensitivity measured for the old DRS detector as a function of wavelength for high resolution mode IV Offered sensitivity is typically a factor of 2 larger and still valid for Period 95 Top Sensitivities over an extended region encompassing the observed wavelengtgh of Nell up to z 0 038 VISIR User Manual VLT MAN ESO 14300 3514 99 2 0x104 ET Tg AM 1 5x104 1 0x104 5 0x103 Sensitivity mJy 100 in 1h 0 L dlrs Ao o mr ld po Pepe Y TE Y 16 900 16 910 16 920 16 930 16 940 16 950 Wavelength um 1000002 Frage 8000 6000 4000 2000 Sensitivity mJy 100 in 1h a Re Be i 17 80 17 85 17 90 Wavelength um
2. OLS 7 9 9 10 1 Wavelenght um 10 3 10 4 Filter PAH2_ref2 VAN N pd P 11 2 11 8 12 2 Wavelenght um Filter SIV_ref2 12 8 10 6 10 8 11 0 Wavelenght um Figure 19 Transmission curves of VISIR imager filters manufactured by OCLI dashed is the atmospheric transmission at low resolution Only relative transmissions have been determined their values are normalized so that their peak transmission is equal to 1 Overplotted VISIR User Manual VLT MAN ESO 14300 3514 51 11 Appendix Sensitivities in various spectroscopic set tings 8000 6000 F al 4000 7 2000 a Sensitivity mJy 100 in 1h 9 00 9 09 9 10 O19 Wavelength um 8000 J 6000 F 4000 F 2000 Sensitivity mJy 100 in 1h Olia i 10 48 10 50 10 52 Wavelength um 10 54 Figure 20 Observed sensitivity measured for the old DRS detector as a function of wavelength for high resolution mode I Offered sensitivity is typically a factor of 2 larger and still valid for Period 95 52 VISIR User Manual VLT MAN ESO 14300 3514 4000 3000 2000 1000 f Sensitivity mJy 100 in 1h 0 AA A A li ee pe AH Ps 11 540 11 545 11 550 11 555 11 560 11 565 Wavelength um LOOOL Sa a Pe a E ze De Zu 3000 2000 1000 Sensitivity mJy 100 in 1h 0 A E A qe
3. Figure 5 Illustration of the chopping and nodding technique on observations of the blue compact galaxy He2 10 The galaxy only appears after chopping and nodding courtesy VISIR commissioning team June 2004 8 VISIR User Manual VLT MAN ESO 14300 3514 100E J E Q2 Q3 u u Q1 T 6 E B Nell 2 J7 9 sy 1 e E Nell 10 gt C Arill Se Je J E pl Pi a J E J9 8 el 7 E A R PAH2_2 7 c PAH1 B10 7 o _ u er Sa J8 9 VUN Pis hs pie ef een DU LIN 1 AA il on a bon 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 wavelength um Figure 6 Sensitivities for the VISIR imager in various filters in the N and Q bands deter mined with the new AQUARIUS detector during the re commissioning The atmospheric transparency spectrum for PWV 3mm at an airmass of 1 5 is plotted in the background During the re commissioning in November 2014 and January 2015 imaging sensitivities with the new AQUARIUS detector were measured for all filters that were offered in P95 For the time being these are based on relatively low statistics only a few measurements per filter and correspond to intermediate conditions clear or thin cirrus seeing 0 8 1 6arcsec PWV 1 5 3 0mm Further measurements during commissioning and science operations will continuously increase the statistics of the calibration database and allow for a statistical analysis of the sensitivity with respect to ins
4. VISIR_img_obs_GenericChopNod tsf To be specified Parameter Range Default Label INS FILTI NAME SEQ CATG SEQ NOFF SEQ OFFSET COORDS SEQ OFFSET1 LIST SEQ OFFSET2 LIST SEQ RETURN SEQ TIME TEL CHOP POSANG TEL CHOP THROW PAHI ARII SIV1 SIV SIV2 PAH PAH22 NEIL1 NEII NEIL2 Q1 Q2 Q3 B10 7 B11 7 J7 9 J8 9 J9 8 J12 2 NODEFAULT PRE IMAGE SCIENCE CALIB TEST SCIENCE 1 100 NODEFAULT SKY DETECTOR NODE FAULT NODEFAULT NODEFAULT FT T 30 3600 NODEFAULT 0 360 0 8 30 8 Imager Filter Observation Category Number of offset positions Offset coordinates List of offsets in RA or X List of offsets in DEC or Y Return to Origin Total integration time sec Chopping Position Angle deg Chopping Amplitude arc sec 43 A4 VISIR User Manual VLT MAN ESO 14300 3514 VISIR spec_obs LRAutoChopNod tsf To be specified Parameter Range Default Label SEQ CATG SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ NODNCYCLES SEQ RETURN SEQ TIME TEL CHOP POSANG TEL CHOP THROW SCIENCE CALIB TEST SCIENCE PARALLEL PERPENDIC ULAR PARALLEL 0 10 0 1 100 NODEFAULT FT N 180 3600 NODEFAULT 0 360 0 8 30 8 Observation Category Relative Chop Nod Direc tion Random Jitter Width arc sec Number of nodding cycles Return to Origin Total integration time sec Chopping Position Angle deg
5. www eso org instruments visir inst 4 2 Spectrometer VISIR offers slit spectroscopy at three spectral resolutions with a pixel scale of 0 076 This is obtained by means of two arms one with a prism and low order gratings for the low and medium spectral resolution the other with large echelle gratings providing high spectral resolution The long slits have a length of 34 The short slits only used in high resolution cross dispersed mode have a length of 4 The all reflective optical design of the spectrometer uses two TMA systems in double pass pass 1 collimator pass 2 camera A schematic layout of the 12 VISIR User Manual VLT MAN ESO 14300 3514 VLT FOCAL DIAPHRAGM PLANE WHEEL y Y MOAI BIC I W N RE IMAGER ee Te o WHEEL Pp he WHEEL LMR EUA A HR 4 GRATING SCANNERS DUO ECHELLE 1 RETURN FLAT GRATING UNIT ESOLUTION SELECTION MECHANISM R COLLIMATOR CAMERA Figure 8 Schematic layout of the design of the VISIR spectrometer VISIR spectrometer design is shown in Fig 8 The 3 mirror system of the low and medium resolution arm gives a 53 mm diameter collimated beam the collimated beam diameter in the high resolution arm is 125 mm Both subsystems image the spectrum onto the same detector selection between the two spectrometer arms is done by two pairs of folding flat mirrors In front of the
6. Thus one read out has a size of 4096 kB The current version of the NGC by default provides output files in the format of fits extensions for each nodding half cycle each nodding position These are structured in the following way i a general long header ii averages of the images obtained in each of the two chopping positions chopping half cycles one on source and one off source as well as 111 a single intermediate result image for that given nodding position with the two chopping positions subtracted from each other Each of the three fits estensions have their own short headers For more details see 6 The number of useful individual exposures per chopping half cycle depends on DET DIT and on the chopping frequency fchop and is given by NDIT 2 DIT fenop NDITSKIP 1 Here NDISTSKIP specifies the number of read outs at the beginning of each chopping half VISIR User Manual VLT MAN ESO 14300 3514 19 Figure 13 Upper panel An example of the good cosmetics on the new AQUARIUS detector used for the acquisition of a Cohen standard in the 1 0 slit Note the central division of the detector due to the intrinsic central outward readout and how the target was placed slightly above Lower panel A sequence of chop nod reduced spectra obtained in the low resolution mode covering the entire N band with a single exposure The TEL CHOP THROW was set to 10 20 VISIR User Manual VLT MAN E
7. infrared MIR atmospheric windows the N band between 7 and 14 um and the Q band between 17 and 25 um In addition it offers a slit spectrometer with a range of spectral resolutions between 150 and 30000 The MIR provides invaluable information about the warm dust and gas phase of the Universe Micron sized particles such as silicates silicon carbide carbon coals aluminum oxides or polycyclic aromatic hydrocarbon PAH molecules are major contributors to the thermal MIR emission The gaseous phase emits through a large number of ionic and atomic lines Examples are Nell 12 8 um and the pure rotation lines of molecular hydrogen at 8 02 9 66 12 27 and 17 03 um Because of the very high background from the ambient atmosphere and telescope the sensitiv ity of ground based MIR instruments cannot compete with that of space born ones However ground based instruments mounted on large telescopes offer superior spatial resolution For example VISIR at the VLT provides diffraction limited images at 073 FWHM in the N band This is an order of magnitude better than what can be reached by the Spitzer Space Telescope SST The VISIR user manual is structured as follows Basic observing techniques of ground based MIR instruments are summarized in 3 4 provides a technical description of VISIR and its offered observing modes An overview on how to observe with VISIR at the VLT can be found in 5 A description of the structure of the imagin
8. 4 4 6 If applicable a note should be put into the README file specifically drawing the attention to this issue 38 VISIR User Manual VLT MAN ESO 14300 3514 4 Acquisition It is strongly recommended that a same guide star be selected and inserted in the acquisition template for all OBs of a same field in particular if e relatively good astrometric accuracy is required e the object is faint or diffuse and unlikely to be visible on short exposures e the object appears in the field of a bright nebula that saturates the digitized sky survey DSS used by the telescope and instrument operator The Guidecam tool see 5 4 can help in selecting appropriate guide stars Calibrations For calibration OBs use the appropriate VISIR_img_cal_AutoChopNod or VISIR spc_cal LR MR HR HRXAutoChopNod templates Position angle If the observations must be carried out at a position angle different from 0 check 85 2 1 and 85 2 2 In particular it is useful to clearly indicate in the README file if TEL CHOP POSANG is not equal to TEL ROT OFFANGLE to warn the in strument operator about the non standard configuration In particular in spectroscopy TEL CHOP POSANG must be equal to TEL ROT OFFANGLE in order to have the 3 beams along the slit VISIR User Manual VLT MAN ESO 14300 3514 9 Appendix VISIR template parameters 39 Note that the template parameters listed here only reflect the current state of the template defini
9. Chopping Amplitude arc sec VISIR spec obs HRAutoChopNod tsf To be specified Parameter Range Default Label INS FILT2 NAME INS GRAT1 WLEN SEQ CATG SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ NODNCYCLES SEQ RETURN SEQ TIME TEL CHOP POSANG TEL CHOP THROW NEI2 H251 NEIL2 7 5 28 0 NODEFAULT H25_4 SCIENCE CALIB TEST SCIENCE PARALLEL PERPENDIC ULAR PARALLEL 0 10 0 1 100 NODEFAULT FT N 180 3600 NODEFAULT 0 360 0 8 30 8 Spectrometer Filter Spectrometer Wavelength microns Observation Category Relative Chop Nod Direc tion Random Jitter Width arc sec Number of nodding cycles Return to Origin Total integration time sec Chopping Position Angle deg Chopping Amplitude arc sec VISIR User Manual VLT MAN ESO 14300 3514 VISIR spec_obs HRX AutoChopNod tsf To be specified Parameter Range Default Label INS GRAT1 WLEN 7 5 28 0 NODEFAULT Spectrometer Wavelength microns SEQ CATG SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ NODNCYCLES SEQ RETURN SEQ TIME TEL CHOP POSANG TEL CHOP THROW SCIENCE CALIB TEST SCIENCE PARALLEL PERPENDIC ULAR PARALLEL 0 10 0 1 100 NODEFAULT FT T 180 3600 NODEFAULT 0 360 0 8 30 8 Observation Category Relative Chop Nod Direc tion Random Jitter Width arc sec Number of nodding cycles Return to Origin Total inte
10. Furthermore at the moment of publishing the manual for phase 1 of Period 96 the commissioning of the high resolution and high resolution cross dispersed modes still have to be finalised Please consult http www eso org instruments visir for the latest update of the list of offered modes and slits and contact the User Support Department usd help eso org in case of questions or inconsistencies during proposal preparation 4 2 3 Low resolution offered wavelength ranges Since the upgrade of VISIR the N band 8 13um low resolution LR spectroscopy only requires one exposure to cover the entire 8 13um range This is achieved by replacing the former grisms by a new prism with 300 for a 073 slit The sensitivities reached with the new setup are 40 mJy at 100 1h in the clean regions of the spectrum which is slightly better than for the individual wavelengths settings of the old setup The largest gain however comes from the possibility to obtain the spectrum in the entire N band in one shot reducing the observing time by a factor of four The median and minimum low resolution sensitivities observed during commissioning are shown in Fig 9 Offered slits have widths of 0 4 0 75 and 1 4 2 4 Medium resolution offered wavelength ranges The medium resolution MR mode is pending to be re commissioned and is hence not offered yet 14 VISIR User Manual VLT MAN ESO 14300 3514 ANA AAA A ea ees tah
11. The re commissioning of VISIR will continue in March and April 2015 that is during phase 1 for P96 For this reason not all properties of the instrument have been fully characterised and similarly the operational procedures have not been finalised at the time of preparation of this manual Especially the following items are still pending e The default positioning of the source and the setting of the WCS world coordinate system as described in 5 3 have not been defined conclusively Therefore observers should assume that for imaging the source is by default centred in the upper left quadrant by the night astronomer or telescope operator because standard chopping to the North and nodding to the East is assumed If this is not the case this must be specifically mentioned in the readme during phase 2 e Sensitivities have not been measured for all instrument modes yet or are based on only a few measurements The preliminary values given in this manual are likely to change and with it the optimal observing parameters e g the chopping frequency e The spectrometer is not completely focussed due to a mechanical problem with the focussing mechanism This problem can only be solved during the next intervention VISIR User Manual VLT MAN ESO 14300 3514 3 2 Introduction The VLT spectrometer and imager for the mid infrared VISIR built by CEA DAPNIA SAP and NFRA ASTRON provides diffraction limited imaging at high sensitivity in two mid
12. Figure 12 Left panel displays the measured AQUARIUS detector linearity for both high and low gain read modes Right panel displays the non linearity as a function of the signal level as computed for the high gain setup flux of the source i e it has to be subtracted before doing e g aperture photometry Also it is recommended not to use the blind preset template VISIR_img_acq_Preset if looking for faint emission around a bright object so that the object can be manually centred by the night astronomer into the middle of one of the detector outputs Moreover due to the new spectroscopic imaging detectors readout structure from the center outwards it is now recommended that the scientific standard stars targets are not placed in the very center of the detectors but slightly offset by few arcseconds either above or below the central division of the detectors It is advised to observe only sources fainter than 500 Jy in N and 2500 Jy in Q Due to the low flux levels eventual detector artifacts are less important in spectroscopy 4 5 Data acquisition system The AQUARIUS detectors are controlled by the new NGC acquisition system In imaging the read out rate of the detector is high Up to 200 frames per second are read for a minimum detector integration time of DIT 5 ms Such a frame rate is too high to store all exposures for most observing programmes One VISIR image is of size 1024 x 1024 each pixel is coded with 4 bytes long integer
13. LOGUE 0 360 0 8 30 8 ra dec 0 360 0 0 ra dec 2000 3000 2000 0 2000 3000 2000 0 TEL TARG OFFSETALPHW TEL TARG OFFSETDEI TEL TARG PMA TEL TARG PMD FAO 10 10 0 0 10 10 0 0 Imager Filter Relative Chop Nod Direc tion Number of nodding cycles Total integration time sec Get Guide Star from Chopping Position Angle deg Chopping Amplitude arc sec Guide star RA Guide star DEC Rotator on Sky PA on Sky Alpha coordinate for the target Delta coordinate for the tar get Epoch Equinox RA blind offset DEC blind offset Proper Motion Alpha Proper Motion Delta VISIR User Manual VLT MAN ESO 14300 3514 VISIR_spec_acq_MoveTosSlit tsf To be specified Parameter Range Default Label INS FILT2 NAME INS SLIT1 TYPE INS SLIT1 WIDTH SEQ CHOPNOD DIR SEQ NODNCYCLES SEQ TIME TEL AG GUIDESTAR TEL CHOP POSANG TEL CHOP THROW TEL GS1 ALPHA TEL GS1 DELTA TEL ROT OFFANGLE TEL TARG ALPHA TEL TARG DELTA TEL TARG EPOCH TEL TARG EQUINOX ARII NEIL2 FAULT LONG SHORT LONG NODE 0 40 0 75 1 00 NODE FAULT PARALLEL PERPENDIC ULAR PARALLEL 1 100 NODEFAULT 30 3600 NODEFAULT CATALOGUE SETUP FILE NONE CATA LOGUE 0 360 0 8 30 8 ra dec 0 360 0 0 ra dec 2000 3000 2000 0 2000 3000 2000 0 TEL TARG OFFSETALPHA TEL TARG OFFSETDEI TEL TARG
14. Number of nodding cycles Return to Origin Total integration time sec Chopping Position Angle deg Chopping Amplitude arc sec 47 48 VISIR User Manual VLT MAN ESO 14300 3514 10 Appendix Filter transmission curves The filter transmission has been measured using a Fourier Transform Spectrometer at a temperature of 35 K for filters manufactured by the company READING Their absolute transmission curves are displayed in Fig 20 The other filters manufactured by OCLI have been measured using the WCU and wavelength scans with the monochromator Note that for these filters the transmission curves are normalized to 1 see Fig 19 Filter Arlll Filter Nell_ref2 120 120 amp 1008 _ 4 1008 E DR SAA A ig fe aop 1 en I ON Rn of J S eof Wa E 40F 1 E 405 J S 20 e 20H E C E ja 0 L L 1 0 L 8 80 8 90 9 00 9 10 9 20 12 6 12 8 3 0 1552 13 4 Wavelenght um Wavelenght um Filter Nell Filter PAH1_ARlllref 1 120 i 120F i 1007 i 3 amp 100 PNA Le J V A got po TR N Bop STA E c lt al amp eof 5 60 fy 1 E aof 40f N i N Cc y E tol Ext Y J 0 L 0 L 1 N 12 4 12 6 12 8 13 0 12 LS 8 0 8 5 9 0 g5 Wavelenght um Wavelenght um Filter Pah2 Filter QO 120 120 1002 A 7 100 J c OOF y A NV pa ur FA E 00F Fe WH lt sok E N S sof S 60 i
15. PMA TEL TARG PMD 10 10 0 0 10 10 0 0 Acquisition Filter Spectrometer Slit Type long or short Spectrometer Slit Width arcsec Relative Chop Nod Direc tion Number of nodding cycles Total integration time sec Get Guide Star from Chopping Position Angle deg Chopping Amplitude arc sec Guide star RA Guide star DEC Rotator on Sky PA on Sky Alpha coordinate for the target Delta coordinate for the tar get Epoch Equinox RA blind offset DEC blind offset Proper Motion Alpha Proper Motion Delta 41 42 VISIR User Manual VLT MAN ESO 14300 3514 VISIR spec_acq_ImgMoveTosSlit tsf To be specified Parameter Range Default Label INS FILTI NAME INS FILT2 NAME INS SLIT1 TYPE INS SLIT1 WIDTH SEQ CHOPNOD DIR SEQ NODNCYCLES SEQ TIME TEL AG GUIDESTAR TEL CHOP POSANG TEL CHOP THROW TEL GS1 ALPHA TEL GS1 DELTA TEL ROT OFFANGLE TEL TARG ALPHA TEL TARG DELTA TEL TARG EPOCH TEL TARG EQUINOX PAHI ARII SIV1 SIV SIV2 PAH PAH22 NEII_1 NEI NEIL2 Q1 Q2 Q3 B10 7 B11 7 J7 9 J8 9 J9 8 J12 2 NODEFAULT ARII NEIL2 NODE FAULT LONG SHORT LONG 0 40 0 75 1 00 NODE FAULT PARALLEL PERPENDIC ULAR PARALLEL 1 100 NODEFAULT 30 3600 NODEFAULT CATALOGUE SETUP FILE NONE CATA LOGUE 0 360 0 8 30 8 ra dec 0 360 0 0 ra dec 2000 3000 2000 0 2000 3000 2000 0 TEL TARG OFFSETALPH
16. TARG OFFSETALPHA and TEL TARG OFFSETDELTA for a blind offset Here the object A is a bright star used to center the target the faint object B at the center of the field The telescope will first point at the object A The instrument operator centers it properly Once done the telescope is offset so that object B is now properly centered and the observation templates can be executed Following the convention described in the text and since the target object B is at the east of the offset star TEL TARG OFFSETALPHA is negative on the other hand the target is at the south of the offset star so TEL TARG OFFSETDELTA is positive 5 3 2 Acquisition Templates There are two acquisition templates for imaging VISIR_img_acq_Preset and VISIR_img_acq MoveToPixel Two acquisition templates are also available for spectroscopy VISIR_spec_acq_ MoveToSlit and VISIR_spec_acq_ ImgMoveToSlit The latter one allows to perform spectroscopic acquisition with the imager detector and therefore offers the possibility to acquire fainter objects in a larger variety of filters The observing parameters are described in 89 The effect of all acquisition templates is first to point the telescope so that the coordinates at the center of rotation located at x y on the detector match e the target coordinates if no blind offset is used e the offset star coordinates otherwise within the accuracy of the VLT pointing see below For VISIR_spec_acq_MoveToSl
17. cross dispersed HRX mode Total integration time SEQ TIME the slit width INS SLIT1 WIDTH and SEQ CHOPNOD DIR are specified as usual 7 2 7 4 Calibration Specific templates exist for the observations of photometric and spectro photometric standard stars They offer the same functionality as the corresponding science templates but allow to monitor the sensitivity and image quality by observing calibration standard stars Their use is recommended to be properly recognized by the VISIR pipeline VISIR User Manual VLT MAN ESO 14300 3514 37 8 Checklist This section provides advice for the preparation of the proposal phase I and of the Observing blocks phase IT 8 1 Phase 1 It is very important that the time justification Box 9 of the proposal contains enough infor mation so that its feasibility can be correctly assessed The following points must be respected 1 2 the expected S N for each object and modes must be given in particular for extended sources does the reported S N refer to an area of 1 arcsec as given by the imaging ETC to an extent of 1 arcsec in the spatial direction as given by the spectroscopy ETC or to the whole spatial extent of the object in spectroscopy does the S N refer to one pixel in the dispersion direction as given by the ETC or to one resolution element in case of large throw does the S N take into account the fact that some beams would fall outside the
18. detector does the overhead calculation include the time required for each preset given that OBs should in general not be longer than 1 hour is there a guide star brighter than V 13 mag within a radius of 7 5 arcmin around the object The PWV constraint under which the observations need to be executed needs to be specified as a comment in the Target List of the proposal Phase 2 Acquisition Are the coordinates accurate in the equinox J2000 0 reference frame For high proper motion objects are they valid for the epoch of the observations For solar system objects are they in the topocentric ICRF or FK5 J2000 0 reference frame at the epoch of the observations Acquisition If the VISIR_img_acq_Preset is used and the following templates have SEQ CHOPNOD DIR PERPENDICULAR the target will appear at the center of the detector by default with the risk of losing 3 beams that would appear outside of the field Either slightly change the coordinates of TEL TARG ALPHA and TEL TARG DELTA or use TEL TARG OFFSETALPHA and TEL TARG OFFSETDELTA see 5 3 Acquisition Rather use VISIR_img_acq MoveToPixel instead of VISIR_img_acq_Preset if the goal of the observations is to obtain accurate photometry of to identify faint struc tures around a bright object This is to allow an accurate positioning of the source into the centre of one of the detector outputs and to avoid the bleeding effect of the detec tor see
19. e counted positively from north to east within the range 0 to 360 VISIR User Manual VLT MAN ESO 14300 3514 23 North AS on ES pos Pointing position East Figure 16 Definition of chopping parameters from the telescope point of view If the position angle PA is measured counter clockwise from North to East with PA between 0 and 360 then TEL CHOP POSANG is 360 PA The positive beam is obtained when the M2 is at Chopping Position and corresponds to the pointing position of the telescope as given in the FITS header idle position The negative beam is obtained by moving the M2 so that it points to a position angle on the sky given by PA and a throw of TEL CHOP THROW from the telescope pointing position Chopping Position B If TEL CHOP POSANG TEL ROT OFFANGLE 360 PA the resulting image on the detector will appear as in one of the nodding position images illustrated in Fig 18 then setting TEL ROT OFFANGLE 360 PA allows one to have both A and B objects on the slit 5 2 2 Chopping parameters The chopping technique as described in 83 4 is based on beam switching using the moving secondary mirror of the telescope It allows to alternatively observe a field then another field offset from the first by a chopping distance or throw called TEL CHOP THROW see Fig 16 This parameter can be set by the user To avoid chopping inside the object it is recommended to use
20. eso org instruments visir These stars are distributed as uni formly as possible in Right Ascension with spectral types as similar as possible In addition their flux in the N band of the order of 10Jy is bright enough to be observable in the Q band without reaching non linearity levels in the N band even in non ideal background conditions At least one star in this reduced catalogue will be observed every night VISIR is in use Note that this list could be modified without previous notice A PSF can be derived from these photometric standard star observations However it is not guaranteed that its S N is sufficient for deconvolution purposes If the observer requires a specific PSF measurement s he has to provide the corresponding PSF OB Observations of photometric standards provided by the observatory are taken using the VISIR_img_cal_AutoChopNod template 7 with the following settings SEQ TIME 180 sec for N and 360 sec for Q band TEL CHOP POSANG 0 TEL CHOP THROW 13 SEQ CHOPNOD DIR PERPENDICULAR Filter INS FILT1 NAME will be set according to the science observations In spectroscopy the observatory will provide spectro photometric observations of a telluric K or M type standard star in the low resolution mode based on the same catalog as for imaging 8Cohen et al 1999 AJ 117 1864 30 VISIR User Manual VLT MAN ESO 14300 3514 with an airmass difference no larger than 0 2 with respect to the science t
21. generating raster maps with VISIR_img_obs_GenericChopNod An illustration of generating an raster map can be found in Fig 19 The following parameters correspond to this setting SEQ NOFF 3 SEQ OFFSET1 LIST 50 15 15 SEQ OFFSET2 LIST 50 15 15 SEQ OFFSET COORDS SKY 36 VISIR User Manual VLT MAN ESO 14300 3514 Note that for the time being images obtained with the VISIR_img_obs_GenericChopNod are not reduced by the ESO VISIR pipeline Pre imaging observations Since Period 76 the observatory supports a fast data release for VISIR pre imaging observations Pre imaging images must be obtained either with the VISIR_img_obs_AutoChopNod or VISIR_img_obs_GenericChopNod templates The SEQ CATG key word must be set t0 PRE IMAGE In addition the name of the OB must start with the prefix PRE 7 3 Observing with the spectrometer Conceptually the same observing techniques apply for spectroscopy as well as for imaging The default slit orientation is in the North South direction The length of the slit is selected by the keyword INS SLIT1 TYPE only for cross dispersed high resolution observations SHORT must be used otherwise LONG is the default setting A preferred observing strategy is called nodding on the slit where the chopping and nod ding amplitudes are small SEQ CHOPNOD DIR PARALLEL Note that nodding on the slit requires to set the telescope rotator offset angle and the M2 chopping position angle to the same value w
22. org observing etc is recommended to estimate the on source integration time 4 Instrument description and offered observing modes VISIR now offers one spatial scale in imaging and several spectral resolution modes in slit spectroscopy The imager and spectrometer are two sub instruments They have independent light paths optics and detectors The cryogenic optical bench is enclosed in a vacuum vessel The vessel is a cylinder 1 2m long and 1 5m in diameter Standard Gifford McMahon closed cycle coolers are used to maintain the required temperatures 29K for most of the structure and optics and lt 15K for the parts near the detector The detectors are cooled down to OK 4 1 Imager The imager is based on an all reflective design The optical design is shown in Fig 7 It consists of two parts e A collimator which provides an 18mm diameter cold stop pupil in parallel light As generally designed for IR instruments the pupil of the telescope is imaged on a cold stop mask to avoid straylight and excessive background emission The collimator mirror M1 is a concave aspherical mirror It is followed by a folding flat mirror M2 which eases the mechanical implementation e A set of three objectives mounted on a wheel Each objective is based on a three mirror anastigmatic TMA system Each of the TMA s is made of three conic mirrors With the new detectors only the small field SF is used The pixel scale of 0 045 of the
23. rer best median 150 100 sensitivity mJy 100 h Vs wavelength um Figure 9 Median and best low resolution sensitivities mJy 100 h observed during comis sioning 2 under mediocre atmospheric conditions 4 2 5 High resolution offered wavelength ranges The VISIR spectrometer offers a high resolution long slit HR mode for 3 passbands centered in the wavelengths of the H2_S4 Nell and H2_S1 lines using the respective order sorting filters Table 2 lists the details of wavelength ranges covered as well as sensitivities It is foreseen that further filters will be commissioned for the high resolution long slit mode in the future The entire wavelength range between 7 7 and 13 3 um is accessible with the high resolution cross dispersed HRX mode using a 4 long slit The ETC offers the possibility to take into account the earth motion to predict the observed wavelength of a given line depending on the foreseen date and time of observations In par ticular this feature allows to determine the dates when the emission line under study would appear at the same wavelength as a sky line A more detailed analysis can be done using ESO s SKYCALC Sky Model Calculator 4 3 Calibration unit A warm calibration unit WCU is located on top of the VISIR vacuum enclosure The WCU is also called star simulator It simulates either a monochromatic point source with ad
24. target coordinates name and proper motion are all set in the acquisition templates The execution of the acquisition templates presets the telescope to the target coordinates given by TEL TARG ALPHA and TEL TARG DELTA Offsets with respect to the target coordi nates can be specified by TEL TARG OFFSETALPHA and TEL TARG OFFSETDELTA and allow for example to use a bright reference star for precise acquisition see Fig 17 To guarantee proper centering within the slit when using a reference star the angular separation between the reference star and the target should not be larger than 60 Acquisition with a reference star has not been tested with the narrow 0 4 slit and should be avoided Note that the coordinates of the target TEL TARG ALPHA TEL TARG DELTA and the offsets to the reference star TEL TARG OFFSETALPHA TEL TARG OFFSETDELTA must be indicated in the acquisition template The convention TEL TARG ALPHA TEL TARG OFFSETALPHA RA offsetstar TEL TARG DELTA TEL TARG OFFSETDELTA DEC offsetstar will be used and the telescope is preset to the reference star Once the reference star is properly centered TEL TARG OFFSETALPHA is subtracted back and the telescope is moved to the target 6This convention is identical to the UVES one but differs from example from the ISAAC or NACO one VISIR User Manual VLT MAN ESO 14300 3514 25 al Figure 17 Setting the correct values of the TEL
25. template an image used to measure the slit location is always taken and archived In service mode through slit images obtained using the filter set by the INS FILT2 NAME parameter are also taken and archived so that the user can assess the correct centering of her his object The slit location image and the through slit images are automatic procedures Only the exposure time of the through slit images can be modified by a service mode observer Their execution time is included in the advertised execution time of the spectroscopic acquisition template By default if TEL TARG ALPHA and TEL TARG DELTA contain the accurate coordinates of the target the target will be located at the center of the detector including if the observing templates use SEQ CHOPNOD DIR PERPENDICULAR In this case in order to avoid to lose the chopnod images it is advisable either to e change the parameters TEL TARG ALPHA and TEL TARG DELTA so that they are off set by half the TEL CHOP THROW values to south and west for TEL ROT OFFANGLE TEL CHOP POSANG 0 so that the target falls in the upper left quadrant of the detec tor or use the parameters TEL TARG OFFSETALPHA and TEL TARG OFFSETDELTA as above the convention final coordinates RA DEC of the center of the field plus offsets equal initial coordinates is used which tranlates into RA TEL TARG OFFSETALPHA TEL TARG ALPHA DEC TEL TARG OFFSETDELTA TEL TARG DELTA Therefore if TEL RO
26. these flat fields However at the moment the scientific value of the application of these corrections is not established These day calibrations are supplied to the user on an experimental basis and may be discontinued with no previous notice 5 8 Observing constaints and OB Classification The Sky Transparency constraints used at Paranal are photometric PHO clear CLR thin THN and thick THK They mostly refer to the optical band and their translation to the IR domain and specially to the MIR is not obvious Also there is some redundancy between the sky transparency and the precipitable water vapour constraints Therefore the following scheme is applied for VISIR OBs requiring PHO conditions will be executed and classified as A fully within constraints if the sensitivity in the corresponding band is equal or better then the nominal median value and if the conversion factor is constant within 10 OBs requiring CLR THN and THK conditions will be executed and classified as A when the sensitivity is respectively within 20 30 and 50 the nominal values Classification for VISIR observations conducted in service mode is also based on the Precip itable Water Vapor PWV see 1 3 constraint Typically the following constraints should be requested w r t the PWV 1 PWV lt 1 5mm Q band imaging amp N band spectroscopy in regions affected by water vapour 2 1 5mm lt PWV lt 3 5mm Q band bright sources and N ban
27. 02 06 3 6 3 10 update for P78 CfP v78 19 06 06 cover 2 2 3 2 4 3 1 P78 release v79 30 11 06 4 8 P79 release v80 28 02 07 3 1 4 2 1 5 1 6 2 8 2 P80 release burst mode included v8l 31 08 07 3 1 4 2 1 4 3 2 P81 release new filters included v87 22 09 10 4 3 2 8 2 8 3 P87 release Exclusion of K band in science imaging templates 3 2 5 P87 release update of HR allowed A First line of Table 7 modified 8 2 P87 release non availability of jitter with IMG GenericC hopN od v88 22 02 11 4 1 P88 release upper limit of 5 filters in a single service mode OB 4 4 P88 release UCAC3 substituting USNO for guide stars selections v89 31 08 11 all P89 release removing most references to the old DRS detector and reporting the first properties of AQUARIUS detector v90 26 02 12 1 Update new schedule of VISIR upgrade v90 17 08 12 all New updates after the commissioning 1 v95 1 27 08 14 all P95 release updates after 07 2014 tests v95 2 01 02 15 all Release for P95 phase 2 v96 1 28 02 15 all Release for P96 phase 1 v1 0 v1 1 v76 1 edited by R Siebenmorgen E Pantin M Sterzik v76 2 4 v77 1 3 updated by A Smette v78 80 updated by L Vanzi v87 90 updated by Y Momany v95 1 updated by Y Momany V D Ivanov et al v95 2 and on updated by K Tristram et al iv VISIR User Manual VLT MAN ESO 14300 3514 Contents 1 VISIR Upgrade Project 1 1 1 2 1
28. 17 95 18 00 Figure 24 Observed sensitivity measured for the old DRS detector as a function of wavelength for high resolution mode V Offered sensitivity is typically a factor of 2 larger and still valid for Period 95 56 VISIR User Manual VLT MAN ESO 14300 3514 IR 01016 OD a a Zn A O En 8000 6000 4000 2000 Sensitivity mJy 100 in 1h 18 210 18 220 18 230 18 240 Wavelength um 6000 F 5000 4000 E 3000 2000 Sensitivity mJy 100 in 1h 1000 ob pa a AA a pao poo ose o pepe pe llo e ao ple Te 18 65 18 70 18 75 18 80 18 85 18 90 18 95 Wavelength um Figure 25 Observed sensitivity measured for the old DRS detector as a function of wavelength for high resolution mode VI Offered sensitivity is typically a factor of 2 larger and still valid for Period 95 VISIR User Manual VLT MAN ESO 14300 3514 57 DOMO oO SET ET AAA A 1 1 5x104 1 0x104 5 0x103 Sensitivity mJy 100 in 1h 21 280 21 290 21 300 21 310 Wavelength um Figure 26 Observed sensitivity measured for the old DRS detector as a function of wavelength for high resolution mode VII Offered sensitivity is typically a factor of 2 larger and still valid for Period 9
29. 2 3 Nodding parameters The nodding technique allows to switch from one field to another by offsetting the telescope by several tens of arcseconds It allows to correct for optical path residuals that remain after chopping 3 The number of nodding cycles SEQ NODNCYCLES is a parameter that can be modified by the observer It strongly depends on the total integration time and should be chosen such that a nodding cycle takes about 90s which will ensure a good removal of the sky residuals Faster nodding will improve the removal of background residuals especially when the object is transiting However the price is higher nodding overheads and hence a lower observing efficiency see 4 5 In all the AutoChopNod templates the nodding offset is equal to TEL CHOP THROW and cannot be modified In order to reach Nodding Position B the telescope executes an offset of TEL CHOP THROW along a position angle equal to e PA 90 360 TEL CHOP POSANG 90 if SEQ CHOPNOD DIR PERPENDICULAR e PA 180 180 TEL CHOP POSANG if SEQ CHOPNOD DIR PARALLEL The resulting distribution of images on a frame is illustrated in Fig 18 In imaging more flexibility on the nodding offsets are possible with the VISIR_img_obs_GenericChopNod template 5 3 Target acquisition 5 3 1 Introduction Observing blocks OB must start with an acquisition template Pointing to a target can only be performed through an acquisition template The
30. 3 1 4 De tector Upgrade ia coria raise Ee EEE OH Low Resolution Spectroscopy lt ee sea va sou ab sea be me da Precipitable Water Vapour ec e sy a si KR a na Ba de ba A aran de AM 2 Introduction 3 Observing in the mid infrared from the ground 3 1 The Barth a Atmosphere o ead pee art nad 3 2 apatial Resolution L 4 4 ana sen ana ea en 54 MIR COCO rar ey ma deu be Bare ha gate sa aa 34 Chopping and nodding seor eeri s e Dub ER ei dre ee Me SOW ese kb Li EL Due Rae Dh a E ar ea A ee a E h 4 Instrument description and offered observing modes DL UE lead A a Se ehr 4 2 Spectrometer 2 4 di se a MUR AE 121 ONE MIN 245 4 ascos abbr ada 122 op ctral Resolutions c 4 pda de spas ba dau asias 42 3 Low resolution offered wavelength ranges 4 2 4 Medium resolution offered wavelength ranges 42 5 High resolution offered wavelength ranges 4 3 Calbralion QUE gt amp oak Hu RS da a da AAA BeOS Ad DOCE 6 veria he 644 trend area niet 44 1 Detector Architeeture su ass sa ss arte ES 442 Detentor Reado b e eo 4 eb ea armee wa dat we Brenner 443 Detector Dark Curro ee s eee nn eme db 4 4 4 Excess Low frequency Noise dda Detector Linearity c ce au Gow baw eee sue ea eee 4 4 6 Detector Cosmetics 2 4464 Br a a re are da 45 Data acquisition Se s sc Se a see Be es 5 Observ
31. EUROPEAN SOUTHERN OBSERVATORY Organisation Europ ene pour des Recherches Astronomiques dans l Hemisphere Austral Europ ische Organisation f r astronomische Forschung in der s dlichen Hemisph re ESO European Southern Observatory Karl Schwarzschild Str 2 D 85748 Garching bei M nchen Very Large Telescope Paranal Science Operations VISIR User Manual Doc No VLT MAN ESO 14300 3514 Issue 96 1 Date 28 02 2015 K Tristram amp the VISIR IOT A D eke eee RS ER ns Date Signature C Dumas Approved ve ow dees een ware at do Date Signature A Kaufer Released Date Signature 11 VISIR User Manual VLT MAN ESO 14300 3514 This page was intentionally left blank VISIR User Manual VLT MAN ESO 14300 3514 Change Record 111 Issue Date Section Parag affected Reason Initiation Documents Remarks Rev v1 0 04 09 04 creation First release for science verification in P74 and OT proposals in P75 v1 1 10 12 04 2 4 3 2 6 2 6 3 7 8 update for P75 Phase2 v76 1 01 02 05 all update for P76 CfP v76 2 06 07 05 all update for P76 Phase 2 v76 3 14 07 05 4 8 1 Corrected Legend Fig 17 v76 4 14 07 05 Cover pages Corrected typos v77 1 04 09 05 3 5 7 4 1 4 3 4 8 1 7 8 1 10 update for P77 CfP v77 2 05 09 05 4 6 match imager overhead of CfP v77 3 20 12 05 1 2 3 4 3 6 4 2 4 4 4 7 7 8 update for P77 Phase2 v78 1 27
32. In practice this is achieved by moving the secondary mirror of the telescope As the background fluctuations have typical time scales of several seconds chopping frequencies of a fraction of a Hz are sufficient to reduce the background Additionally the excess low frequency noise ELFN of the AQUARIUS detectors can be reduced effectively by chopping see 4 4 4 For this chopping rates of more than 1 Hz are required Thus chopping frequencies between 2 and 4Hz are used for imaging observations such that the signal to noise within a certain observing time including chopping overheads is maximized see 4 5 Spectroscopic observations on the other hand are performed with lower chopper frequencies at 0 1 Hz or less Note that these values are at the moment only indicative and subject to revision during the ongoing re commissioning of the instrument The chopping technique cancels most of the background However the optical path is not exactly the same in both chopper positions Therefore a residual background remains It is varying at a time scale which is long compared to that of the sky This residual is suppressed by nodding where the telescope itself is moved off source and the same chopping observations as in the on source position is repeated An illustration of the chopping and nodding technique is shown on Fig 5 Depending on the choice of chopping and nodding amplitudes and directions up to 4 images of the source can be seen on t
33. SIR instrument at VLT for next generation VLTI instruments and for the future mid IR candidate instrument METIS at the E ELT For a detailed presentation of the AQUARIUS detector we refer the reader to Ives et al Proc SPIE 8453 38 in the following we will highlight on the major properties of the detector To properly operate in the mid IR window the AQUARIUS detector is designed to deliver low thermal background high quantum efficiency and high sensitivity With respect to con ventional ones these goals have been achieved by the introduction of a new class of photo conductors called the Impurity Band Conduction IBC Raytheon designation Their Si As IBC design achieved higher sensitivities by decreasing the thickness of the photo conductor and increasing the doping of the Si As diodes 4 4 1 Detector Architecture Figure 11 displays the architecture of the AQUARIUS detector It is split into two perpen dicular areas each made of 512 rows and 1024 columns Each area has 32 outputs such that each output is configured to read out 32 x 512 pixels all 64 outputs from the two areas being 16 VISIR User Manual VLT MAN ESO 14300 3514 MOR BEE 32 outputs vAOut 1 32 con ampare Row 512A 512 rows Row 1A Row 1B 512 rows Row 5128 32 outputs vBOut 1 32 MHz nos i 8x V Y Y Y saiside a O 2 sides Figure 11 Left panel shows the AQUARIUS multiplexing readout scheme Right panel sho
34. SO 14300 3514 cycle which are rejected during the stabilization of the secondary mirror i e during the first 25 ms of each chopping half cycle Both DIT and fenop are predifined for each filter spectral setup expect in the high resoution cross dispersed mode by the instrument software in order to achieve the highest sensitivity per unit of time They are hence not changeable by the observer The observing efficiency echop due to chopping is hence NDIT 2 DIT fenop ns 7 1 2 NDITSKIP DIT Ja The total integration time on source tre is specified by the user through the parameter SEQ TIME i e tsc SEQ TIME With this the integration time including the ovserheads due to chopping but without nodding is traw tsre chop The number of nodding cycles during the entire integration period is set by the user defined parameter NODNCYCLES so that the time spend observing in one nodding half cycle is 1 tias Ina tno se q 2 NODNCYCLES nodset tsre ns 2 NDITSKIP DIT fa as 2 NODNCYCLES where tnodset 6sec is the nodding settling time required to move the telescope Typically NCYCLENOD should be chosen such that the entire nodding cycle takes 90s that is NCYCLENOD amp 90s to the first order The total integration time then is NODNCYCLES 2 taoa t z sre 2 NODNCYCLES trodset 1 2 NDITSKIP DIT faros i ttot accounting for all overheads due to chopping and nodding T
35. T OFFANGLE TEL CHOP POSANG 0 TEL TARG OFFSETALPHA and TEL TARG OFFSETDELTA should be both positive in order to reproduce the scheme shown in Fig 18 A typical value for these parameters is TEL CHOP THROW 2 where TEL CHOP THROW is the chop throw used in the subsequent templates If both the target and guide star coordinates are within the same astrometric systems the pointing accuracy is limited by the relative accuracy between the coordinates of the two objects In particular the pointing accuracy maybe affected by significant usually unknown proper motion of the guide star Note that the observatory does not guarantee the accuracy of the world coordinate systems WCS keywords in the FITS headers For a successful completion of an OB the observer has to ensure that correct target coordinates are provided for the equinox J2000 0 ideally at the epoch of the observations The following cases require special care In particular note that P2PP only accepts coordinates for J2000 0 VISIR User Manual VLT MAN ESO 14300 3514 27 e imaging in some conditions an error of less than 15 in the coordinates can bring the target outside of the field e spectroscopic acquisition in some conditions an error of less than 7 5 in the coordinates can bring the target outside of the wide slit used Errors of such scale are common in the following situations e high proper motion stars in particular if the epoch of the VISIR
36. W TEL TARG OFFSETDEI TEL TARG PMA TEL TARG PMD UTAO 10 10 0 0 10 10 0 0 Imager Filter Acquisition Filter Spectrometer Slit Type long or short Spectrometer Slit Width arcsec Relative Chop Nod Direc tion Number of nodding cycles Total integration time sec Get Guide Star from Chopping Position Angle deg Chopping Amplitude arc sec Guide star RA Guide star DEC Rotator on Sky PA on Sky Alpha coordinate for the target Delta coordinate for the tar get Epoch Equinox RA blind offset DEC blind offset Proper Motion Alpha Proper Motion Delta VISIR User Manual VLT MAN ESO 14300 3514 VISIR_img_obs_AutoChopNod tsf To be specified Parameter Range Default Label INS FILT1 NAME SEQ CATG SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ NODNCYCLES SEQ RETURN SEQ TIME TEL CHOP POSANG TEL CHOP THROW PAHI ARII SIV1 SIV SIV2 PAH PAH22 NEIL_1 NEII NEIL2 Q1 Q2 Q3 B10 7 B11 7 J7 9 J8 9 J9 8 J12 2 NODEFAULT PRE IMAGE SCIENCE CALIB TEST SCIENCE PARALLEL PERPENDIC ULAR PERPENDICU LAR 0 10 0 1 100 NODEFAULT FT T 60 3600 NODEFAULT 0 360 0 8 30 8 Imager Filter Observation Category Relative Chop Nod Direc tion Random Jitter Width arc sec Number of nodding cycles Return to Origin Total integration time sec Chopping Position Angle deg Chopping Amplitude arc sec
37. a 3 an E 40f J E 40F H 5 F 5 F d 1 3 20 S 20F 4 VU OL x gos OL f f 10 3 10 9 11 6 12 2 12 8 15 8 16 2 16 8 1 452 17 8 Wavelenght um Wavelenght um Figure 20 Transmission curves of VISIR imager filters manufactured by READING Over plotted dashed is the atmospheric transmission at low resolution The absolute transmission values are given expressed in percent VISIR User Manual VLT MAN ESO 14300 3514 Filter Q1 Transmissions in D obd 1555 16 5 17 5 18 5 Wavelenght um Filter Q3 Transmissions in D 120 100F 80t 60F 40F Transmissions in 20F 19 0 20 0 Wavelenght um Filter SIV 10 1 10 3 10 5 10 7 10 9 Wavelenght um Transmissions in Transmissions in 0 F Mh A Vi My AR Filter Q2 17 5 18 5 19 5 Wavelenght um Filter SiC 8 10 12 14 Wavelenght um Figure 18 continued 49 90 VISIR User Manual VLT MAN ESO 14300 3514 Filter Nell_ref1 1 2 To 0 8 E 0 6 E 0 4 0 25 o o Normalized transmission YT NAT D 1 2 1 0 0 8 0 6 0 4 0 2 0 0 1 Normalized transmission 11 8 12 0 12 2 12 4 Wavelenght um Filter SIV_ref1 12 6 10 8 E2 0 6 0 4 0 2 0 0 Normalized transmission 1 2 1 0 0 8 0 6 0 4 0 2 0 0 1 Normalized transmission
38. a A m a Fe E EA mg ERA ER A AER of prints EA A 11 745 11 750 11 755 11 760 11 765 11 770 11 779 Wavelength um Figure 21 Observed sensitivity measured for the old DRS detector as a function of wavelength for high resolution mode II Offered sensitivity is typically a factor of 2 larger and still valid for Period 95 VISIR User Manual VLT MAN ESO 14300 3514 53 AOOOE TT 3000 2000 1000 Sensitivity mJy 100 in 1h Wl 0 L L L 1 L L 1 1 Ll 1 12 44 12 46 12 48 12 50 Wavelength um 10000 7222 7 L model 4 L median J T o gt E gt B 1000 7 R li u 1 A AO AO mi 1 f poy l ir loo 12 70 12 75 12 80 12 85 12 90 wavelength um Figure 22 Observed sensitivity measured for the old DRS detector as a function of wavelength for high resolution mode II Offered sensitivity is typically a factor of 2 larger and still valid for Period 95 Bottom Observed sensitivities obtained on various nights compared with the theoretical model curves corresponding to BLIP 54 VISIR User Manual VLT MAN ESO 14300 3514 6000 Soo 5000 4000 3000 TITI 2000
39. a chopping and nodding throw which is 1 5 times larger than the estimated diameter of the object in the mid infrared In the case of point sources the throw is usually set around 13 to ensure proper separation of the different beams The maximum chopping throw at the VLT is 30 and the minimum is 8 Note that during commissioning of the higher chopping rates required with the AQUARIUS detectors a degradation of the image quality for a chopping throw of 25 was observed For this reason the maximum chopping throw is limited to 20 for the time being i e 8 lt TEL CHOP THROW lt 20 The chopper position angle PA is the angle of chopping counted East of North see Fig 16 and specifies the direction towards which the image is offset This causes the object to move to the opposite direction in the image obtained during the second chopping half cycle see also 7 2 This parameter can be set by the observer In order to keep the same distribution of beams on the detector for a different rotator angle TEL ROT OFFANGLE as in the default rotator position see Fig 18 then TEL CHOP POSANG must be equal to TEL ROT OFFANGLE In particular this is the case in spectroscopy if the observer wishes to have the 3 beams along the slit As stated in 3 5 the chopping frequency is not a parameter accessible to the observer 24 VISIR User Manual VLT MAN ESO 14300 3514 it is fixed internally to ensure the best data quality 5
40. actual spectrometer subsystems is a reflective re imager consisting of two off axis paraboloids and three folding flats The re imager provides a 16 mm diameter cold stop pupil in parallel light and transforms the incoming VLT Cassegrain beam of F 13 4 to an F 10 beam at the spectrometer entrance The spectrometer slit wheel is also equipped with a very wide slit 14 It gives the possibility to make imaging with the spectrometer detector and is used for object acquisition and centering on the detector There are two filters available for spectroscopic acquisition ArII and Nell_2 Their measured bandpasses and approximate sensitivities for image acquisition are listed in Table 3 During the re comissioning in January 2015 it was discovered that the focussing mechanism of the spectrometer is not functional probably due to a mechanical failure Therefore the spectrograph is currently not optimally focussed To remedy the problem the instrument will have to be opened which is not forseen for the near future Despite this issue the image quality is not impaired significantly and the spectrometer is nearly diffraction limited 4 2 1 Slit widths Three different slit widths 0 4 0 75 and 1 are offered for all settings For over sized widths e g for the 1 slit with respect to the diffraction limit around 10um the spectral resolution of a point source spectrum is better than the one of the sky spectrum in addition the zero point of the w
41. an be found at https www eso org observing etc bin gen form INS MODE swspectr INS NAME SKYCALC 4 VISIR User Manual VLT MAN ESO 14300 3514 N band Q band 6 2 2 i S 2 ai E 7 3 li PRA LA id 10 15 20 25 30 wavelength um Figure 2 MIR atmospheric transmission at Paranal computed with ATRAN Lord 1992 NASA Tech Mem 103957 for an altitude of 2600 m in green for 1 mm of precipitable water vapor PWV at zenith airmass 1 in blue for 3mm of PWV and airmass 1 5 The largest changes are in the Q band and at the edges of the N band 3 2 Spatial Resolution The spatial resolution of an instrument is ultimately limited either by the diffraction of the telescope or the atmospheric seeing The diffraction limit as measured by the diameter of the first Airy ring increases with wavelength as 1 22 A D where is the observing wavelength and D the diameter of the telescope mirror see solid line in Fig 3 The wavelength dependence of the seeing can be derived by studying the spatial coherence radius of the atmosphere in the telescope beam and is to first order approximated by the Roddier formula where the seeing is oc A
42. arget Such a cali bration measurement will be performed at least once per night per instrument configuration More precisely the following settings of the VISIR spec cal LRAutoChopNod template 7 will be used SEQ TIME 180 sec TEL CHOP POSANG 0 TEL CHOP THROW 10 SEQ CHOPNOD DIR PARALLEL The slit width INS SLIT1 WIDTH will be adjusted to the science observation Important note The observatory does not provide standard calibrations for VISIR medium and high resolution spectroscopy Thus for medium and high resolution mode the observer has to supply his own calibration by supplying a calibration OB for each science OB The observing time needed to execute this calibration is charged to the observer Ideally early type stars should be chosen In particular for high resolution spectroscopy asteroids provide mostly featureless spectra on the VISIR spectral range For service mode observations all Calibrator Observations should be concatenated to their science OB For both imaging and spectroscopy day calibrations of VISIR are performed with an extended source that mimics a black body with adjustable flux by regulating its temper ature For most instrument modes a corresponding flat field is recorded which consists of a series of images with different background levels Exceptions are the spectroscopy detector for spectroscopy acquisition Bad pixels gain maps and fringing patterns can in principle be derived from
43. avelength calibration will be affected by an incorrect centering of the object within the slit VISIR User Manual VLT MAN ESO 14300 3514 13 Filter Ac half band sensitivity um width um 100 1h mJy Arlll 8 94 0 11 200 Nell 2 12 81 0 10 50 Table 3 VISIR spectrometer filter characteristics The filter transmissions have been de termined with a monochromator and the WCU The last column list the measured median sensitivities which were obtained using the curve of growth method on data obtained in par allel chopping nodding directions 3 beams Note that the sensitivities listed here are still for the ones for the old DRS detector New sensitivities will be made public on the VISIR webpage as soon as they become available 4 2 2 Spectral Resolutions The spectrometer offers three spectral resolution modes the low medium and high resolution modes which provide spectral resolving powers of R 300 R 3000 and R 15000 respec tively In high resolution long slit mode narrow wavelength ranges around the 8 02 H2_S4 12 813 Ne II and 17 03 um H2_S1 line are offered Table 2 A minimum flux in an emission line below 10716 W m arcsec can be achieved with the 1 slit This value corresponds to an approximate sensitivity limit around 1 Jy in the continuum A high resolution cross dispersed mode with a 4 short slit is also available Note that the medium resolution mode is currently not offered
44. bservation templates will be supported by the pipeline reductions Raw images of imaging and spectroscopic observations are recombined by the pipeline Spectra are extracted and cal ibrated in wavelength 6 3 for all spectroscopic modes in low medium and high resolution 32 VISIR User Manual VLT MAN ESO 14300 3514 Sensitivity estimates based on standard star observations are provided both in imaging and spectroscopy 5 7 At the time of writing the manual the pipeline was still under develop ment and only securely worked for the standard imaging template VISIR_img_obs_AutoChopNod It is planned that in the course of Period 95 the pipeline will support all modes offered dur ing that period with the excepton of VISIR_img_obs_GenericChopNod Also the coronography mode newly offered in Period 96 will most likely not be supported by the pipeline The public release of the VISIR pipeline is accessible at http www eso org sci software pipelines The pipeline will support the following templates e VISIR img obs AutoChopNod VISIR_spec_obs_LRAutoChopNod VISIR_spec_obs_MRAutoChopNod VISIR_spec_obs HRAutoChopNod VISIR_spec_obs HRXAutoChopNod In mosaic or raster mode VISIR img obs GenericChopNod only raw frames are delivered e g mapping reconstruction algorithms are not supported 6 3 Spectrometer data Optical distortion correction Spectra are deformed by optical distortion and slit curvatures The VISIR spectrometer use
45. carried out Since the overall astrometric accuracy of an image is actually limited by the accuracy on the coordinates of the guide star it is strongly recommended that all OBs of a same field use the same guide star in particular for faint objects In addition objects within optically dark molecular clouds may have few or no suitable guide stars at least in the catalogues currently used by the Telescope Control System the UCAC3 Alternatively some bright nebulae may saturate the region of the digital sky surveys used by the telescope operator to select guide stars Considerable amount of telescope time will be saved if such cases are identified before an OB is started Providing the coordinates of a guide star in the acquisition template of an OB is therefore strongly recommended in a number of situations e observations of faint objects hardly or not visible even after a long exposure in partic ular if this exposure has to be combined with other ones e observations of objects within optically dark molecular clouds where few suitable guide stars are expected 28 VISIR User Manual VLT MAN ESO 14300 3514 e observations of objects within bright nebulae larger than the field of view accessible by the guide probe that appeared saturated in the digital sky surveys example Orion e observations for which astrometric accuracy is important In all these cases the use of the Guidecam tool see http www eso org sci observing p
46. d imaging amp spectroscopy 3 3 5mm lt PWV N band imaging of bright sources VISIR User Manual VLT MAN ESO 14300 3514 31 OBs executed with the requested PWV constraint will be classified A those executed within 10 of the requested PWV value will be classified B Almost within constraints and OBs executed under PWV conditions greater than 10 of the requested PWV value will be classified C out of constraints Observations qualified as C will be repeated The time required to do so will not be charged to the observer s program For VISIR the lunar illumination and distance constraints are not user selectable param eters They are fixed to 1 and 30 respectively meaning that observations can be carried out irrespective of the lunar phase and with a minimum distance of 30 from the moon Similarly the twilight constraint should remain fixed at 30 meaning that the observation can be carried out up to half an hour into the twilight The very high background from the ambient atmosphere and telescope in the mid infrared does not change significantly from day to night or due to the presence of the moon All background emission is removed using the chopping and nodding see 3 4 Therefore observations in the mid infrared are not sensitive to the lunar phase or to the twilight the only limitation being the telescope being able to giude using the guide star in the optical In fact VISIR observations are carri
47. e of fast tracking speeds that do not allow proper background cancelation through nodding A final recommendation concerning service mode observations is that no more than 5 filters are grouped together in a single Observing Blocks This will ensure a proper calibration of each single filter Moreover it is also recommended that N and Q band filters are not grouped together as the Q band sensitivities can be significantly different from that in the N band depending on the atmospheric conditions especially the water vapour Questions related to the VISIR Phasel and Phase 2 observing preparation should be directed to the User Support Department usd help eso org 5 2 Observing Parameters 5 2 1 Instrument orientation on the sky By default the imager orientation is such that North is at the top and East is to the left For the spectrometer the default orientation is mirrored along the North South axis y axis respective to the imager so that East is to the right with the slit orientation along the North South direction Figure 15 summarizes the situation Since VISIR is mounted on a rotator at the Cassegrain focus of Melipal it is possible to change the default orientation of VISIR on the sky for example to obtain the spectra of two objects A and B at once The parameter TEL ROT OFFANGLE defaulted to 0 is used for this purpose If PA represents the required position angle of object B relative to A measured on the sky east of north i
48. ease to 2s in high resolution spectroscopy The actual DIT used for each filter or setup is predefined in the instrument software in order to ensure that the detector is operated at a reasonable 30 approximately 20000 counts of the potential well and that the combination of DIT and chopping frequency lead to the highest observing efficency It is hence not a parameter to be chosen by the observer 4 4 6 Detector Cosmetics The new AQUARIUS detector cosmetic testing shows that it does not show the high fraction of bad pixels seen in the old DRS detector Moreover the AQUARIUS detector can be considered free from striping effects see Fig 13 For bright sources falling on the edge between two detector outputs there is however a memory effect which can lead to the appearance of bright stripes in horizontal direction x direction in the two outputs affected This bleeding effect is currenty still under investigation but it seems that it is not caused by a redistribution of flux For accurate photometry the flux in the feature should therefore not be added to the 18 VISIR User Manual VLT MAN ESO 14300 3514 60000 T T T 7 05 f ith Gai 04 G 50000 Low Gain 0 3 y 32881 4x 5763 2 40000 ye 3 30 1 30000 So 4 5 y F 40815x 4146 8 5 10000 gt 30000 400 50000 20000 i S i Signal Level One 0 2 i Ei gt 10000 0 3 2 amp 0 4 t 0 bd 0 0 5 2 25 05 Ekposure tite s
49. ed out preferentially during bright time i e close to full moon as observations with other instruments operating at shorter wavelengths are less efficient during this time The other constraints seeing and airmass are the same as for other instruments Note that the seeing is specified at the wavelength of observation which for the mid infrared is typically much better than the seeing in the optical as measured by e g the seeing monitor DIMM see also 3 2 6 VISIR data 6 1 Data format One FITS file is saved for each telescope nodding position This file is a data multi extension fits file and contains for each chopping cycle 1 a general header 2 the averaged half cycle frames for the on A and off source B positions of the chopper 3 the average of the current and all previous chopped frames that is the difference of the two averaged half cycle frames For the default value of the rotator angle 0 the images are oriented north up and east left Spectroscopic data are aligned vertical in the spatial and horizontal in the dispersion direction cf Fig 13 For the LR and MR modes the wavelength of the spectrum increases towards the left For the HR and HRX modes the short wavelength is at the top of the frame if the side B of the dual grating is used and at the bottom of the frame of the side A is used 6 2 Pipeline A pipeline for the reduction of the new VISIR data is been developed by ESO The main o
50. eing The Spitzer Space Telescope diffraction limits dashed are shown for comparison The Roddier dependence is shown for two optical seeings dashed dot T m T y 0 6 2 L jon CA On E a gt ae fen Y vol pe E the a 2047 wi Fu A F 4 Ea Be a Y ai Pal ae s 4 gt Y 4 0 6 PE z ye a a 7 Z a J ue LL A 0 4 A poe CCE El A E F D yes un q 3 2 o Z A ma DAA a Br ao RTS do a 0 2 yl Pr Books o a 7 a A AM L l 1 ot 19 1 1 1 L L L 1 y 0 5 1 15 FWHM VIS Figure 4 Measures of the VISIR image quality versus optical seeing obtained during 2005 The dashed lines indicates the prediction of Roddier s formula 6 VISIR User Manual VLT MAN ESO 14300 3514 photons s Therefore the exposure time of an individual integration the Detector Integration Time DIT is short of the order of a few tens of milli seconds in imaging mode 3 4 Chopping and nodding The basic idea to suppress the MIR background is to perform differential observations using the chopping nodding technique In the chopping technique two observations are performed One set of exposures on source include the background and the astronomical source A second set of off source exposures measures the pure background The on and off source observations have to be alternated at a rate faster than the rate of the background fluctuations
51. ere will be two bright sources in the VISIR field of view The science target is the southernmost of these two For high resolution cross dispersed spectroscopy the finding chart should inlcude a copy of the output spectrum of the ETC at the requested wavelength including and indication of the exact position of the emission line This is required so that the DIT can be adjusted manually depending on the atmospheric conditions in order to ensure the optimal S N 22 VISIR User Manual VLT MAN ESO 14300 3514 0 076 pixel o m 38 14 imager spectrometer Figure 15 Field orientation and pixel scale for the imaging and spectroscopic modes of VISIR for TEL ROT OFFANGLE 0 While the imaging detector follows the normal astronomical convention the spectrosopic detector is reversed in x direction so that East is to the right The gray area marks the region of the detector which is typically read out the spectroscopy detector being windowed in y direction The mangenta areas mark the areas illuminated by the sky in case of the spectrometer for the very wide slit OPEN The sizes of these usable areas on sky are given in blue see also 4 4 e It is mandatory to check that a guide star brighter than V 13 within a field of 7 5 arcmin radius around the science target is available This can be done using the Guidecam tool see 5 4 Note that observations close to zenith during meridian crossing should be avoided becaus
52. g and spectroscopic data files is given in 6 A checklist to help the preparation of OBs is available in 8 Acquisition observing and calibration templates are explained in 7 We strongly recommend to consult http www eso org instruments visir for additional information and updates For support during proposal preparation and OB submission please contact ESO s User Support Department at usd help eso org 3 Observing in the mid infrared from the ground 3 1 The Earth s Atmosphere Our atmosphere absorbs the majority of the MIR radiation from astronomical sources The main absorbing molecules are H20 CH4 CO2 CO O Oz However the atmosphere is quite transparent in two atmospheric windows the N and Q bands They are centered around 10 and 20 um respectively The transmission in the N band is fairly good at a dry site and becomes particular transparent in the wavelength range between 10 5 and 12 0 ym However the transmission of the Q band is rapidly decreasing with wavelength and can be viewed as the superposition of many sub bands having a typical spectral coverage of AA lum at an average transmission of 60 Observations in this band require low water vapor content in the atmosphere The atmospheric transmission in the N and Q bands is displayed in Fig 2 A detailed modelling of both transmission and emission of the atmosphere can be done using ESO s SKYCALC Sky Model Calculator 2The SKYCALC Sky Model Calculator c
53. gration time sec Chopping Position Angle deg Chopping Amplitude arc sec VISIR img_cal AutoChopNod tsf To be specified Parameter Range Default Label INS FILT1 NAME SEQ CATG SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ NODNCYCLES SEQ RETURN SEQ TIME TEL CHOP POSANG TEL CHOP THROW PAHI ARII SIV1 SIV SIV2 PAH PAH22 NEIL1 NEII NEIL2 Q1 Q2 Q3 B10 7 B11 7 J7 9 J8 9 J9 8 J12 2 NODEFAULT PRE IMAGE SCIENCE CALIB TEST CALIB PARALLEL PERPENDIC ULAR PERPENDICU LAR 0 10 0 1 100 NODEFAULT FT N 60 3600 NODEFAULT 0 360 0 8 30 8 Imager Filter Observation Category Relative Chop Nod Direc tion Random Jitter Width arc sec Number of nodding cycles Return to Origin Total integration time sec Chopping Position Angle deg Chopping Amplitude arc sec 45 46 VISIR User Manual VLT MAN ESO 14300 3514 VISIR spec_cal LRAutoChopNod tsf To be specified Parameter Range Default Label SEQ CATG SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ NODNCYCLES SEQ RETURN SEQ TIME TEL CHOP POSANG TEL CHOP THROW SCIENCE CALIB TEST CALIB PARALLEL PERPENDIC ULAR PARALLEL 0 10 0 1 100 NODEFAULT FT N 30 3600 NODEFAULT 0 360 0 8 30 8 Observation Category Relative Chop Nod Direc tion Random Jitter Width arc sec Number of nodding cycles Return to Origin To
54. gurations The left panel of Fig 12 displays the measured signal level as a function of exposure time Typically these data are taken in a non destructive read mode such that many hundreds of frames are taken between the signal detector starvation level up to its saturation level A linear fit was applied to the data points between 15 000 40 000 DN and the differences between the fit and the data points is plotted for the high gain setup right panel of Fig 12 Over this signal range the detector shows an excellent linearity of the order of 0 5 For this particular detector the gain and therefore the detector saturation level and read noise can be changed by a factor of approximately eight At the operating temperature of the detector 9K the dark current which is the signal obtained when the detector receives no photons is negligible compared to the background generated by the photons emitted by the telescope and the atmosphere The dark current is removed by the observation technique chopping or nodding The detectors have a switchable pixel well capacity The large capacity is used for broad band imaging and the small capacity for narrow band imaging and spectroscopy Detector saturation due to the enormous MIR background is avoided by a storage capacity of 0 6 x 10 e in small and 6 0 x 10 e in large capacity modes respectively The detector integration time DIT is a few milli seconds in broad band imaging and may incr
55. hase2 SMGuidelines GuidecamVISIR html for VISIR is strongly recommended and the coordinates of a suitable guide star should be inserted in the acquisition templates Note that a new version of the Guidecam tool taking into account the post upgrade features of VISIR will be released for phase II of Period 96 The currently available version is only provided for reference If TEL AG GUIDESTAR is CATALOGUE a guide star from the guide star catalog will be automatically selected by the TCS If TEL AG GUIDESTAR is SETUPFILE the observer has to provide the coordinates of the GS The coordinates of the guide star also fix the reference point for the World Coordinate System coordinates that appear in the FITS header of the files In both cases the telescope operator acknowledges the guide star Depending on the weather conditions or if the star appears double in the guide probe the telescope operator may have to select another guide star Therefore if the observer has selected a guide star for astrometric purposes for example to insure the repeatability of the pointings between different OBs a clear note should be given in the README file for service mode observations or be specifically mentioned to the night time astronomer in visitor mode As stated above the observatory does not guarantee the accuracy of the world coordinate systems WCS keywords in the FITS headers 5 5 Brightness Limitations There are currently no brightness l
56. he AnBnBnAn cycle sequence for the nodding to save observing time http www eso org sci observing phase2 P2PP3 html Acquisitions observations and calibrations are coded via observing templates One or more templates build up an observing block OB They contain all the information necessary for the execution of a complete observing sequence An overview of the curently available and commissioned VISIR templates and their parameters is given in 7 of this manual e For each science template the user has to provide a finding chart so that the target can be acquired In addition to the general instruction on how to create these finding charts see http www eso org sci observing phase2 SMGuidelines html the following VISIR requirements apply All finding charts have to be made using existing infrared K band or longer wave length images Typically 2MASS or DENIS K band images are acceptable al though higher spatial resolution is prefered especially if the object has a complex structure If the wavelength at which the finding chart has been taken is different from that of the science observation e g a K band finding chart for a 104m spectroscopic template the user has to describe clearly how to identify the target at the observ ing wavelength in the README section of the programme description Adequate examples of such comments are The target will be the brightest source in the field of view at 10um x At 10um th
57. he algorithm described above ensures that the integration time on source requested by the observer using SEQ TIME will actually also be the true integration time on source Typical duty cycles tsrc ttot are between 50 and 80 5 Observing with VISIR at the VLT 5 1 Proposal Preparation Tools are available to prepare the observations either during phase 1 call for proposals or during phase 2 creation of observing blocks by the observer e The exposure time calculator ETC available at http www eso org observing etc may be used to estimate the integration time needed to obtain the required S N for a given instrument setting because of the numerous sky absorption lines see Fig 20 and following it is recommended to display the S N as a function of wavelength when using the spectrograph ETC in order to ensure that the correct S N is reached for the targeted line emission or absorption e As for all VLT instruments astronomers with granted VISIR telescope time prepare their observations using the phase 2 proposal preparation tool P2PP described at VISIR User Manual VLT MAN ESO 14300 3514 21 noysk p N Cycl_chop nevskp N Cycl_chop 4 bb An Bn Bn An 7 gt T_nod NDITSKIP NDIT NDITSKIP lt lt Mi DIT Ac Bc rn Ac Bc T_chop Figure 14 Data timing in VISIR Ac and Be refer to the two chopper positions An and Bn refer to the two nodding telescope positions Note t
58. he frame and used for scientific analysis Of course the free field of view on the chop nod images can be severely reduced depending on the particular chopping and nodding parameters chosen 3 5 Sensitivity Measurements of VISIR sensitivities are based on observations of mid infrared calibration standard stars Cohen et al 1999 AJ 117 1864 In imaging mode the stars are recorded using perpendicular chopping and nodding patterns with amplitudes of 13 Calibrators are frequently observed during the night 5 7 Flux and noise levels are extracted by multi aperture photometry using the curve of growth method the aperture used for all 4 beams in a given frame is the one for which the flux to noise ratio is the largest By combining all 4 beams the sensitivity in a given set up filter field of view is defined as the limiting flux of a point source detected with a S N of 10 in one hour of on source integration For details see the VISIR Pipeline User Manual 3With the old DRS detector standards were also recorded using the intermediate field 0 076 and 8 chopping The VISIR Pipeline User Manual is downloadable at http www eso org sci software pipelines VISIR User Manual VLT MAN ESO 14300 3514 7 Staring images Staring images chopper position chopper position Prep rre bi Tae Chopped images l Nodding beam Nodding B beam B chopper position Final Image zoomed He2 10 blue compact galaxy
59. hich is in general different from 0 This is useful to acquire two targets simultaneously in the slit The keyword SEQ JITTER WIDTH allows to apply random offsets along the slit More complex source geometries might require larger amplitudes and or SEQ CHOPNOD DIR PERPENDICULAR in order to avoid self cancellation Low and medium resolutionThe templates for low and medium resolution spectroscopy are VISIR spec obs LRAutoChopNod and VISIR spec obs MRAutoChopNod respectively Observ ing parameters are total integration time SEQ TIME the slit width INS SLIT1 WIDTH SEQ CHOPNOD DIR and for medium resolution the central wavelength INS GRAT1 WLEN see Se High resolution long slit mode Template for high resolution spectroscopy is VISIR_spc_obs_HRAutoChopNod Three order sort ing filter at 8 02 12 81 and 17 03um INS FILT2 NAME H2_S4 Nell H2_S1 are avail able See Table 2 for the corresponding list of offered central wavelengths Other observing parameters are total integration time SEQ TIME central wavelength INS GRAT1 WLEN the slit width INS SLIT1 WIDTH and SEQ CHOPNOD DIR 87 2 High resolution cross dispersed mode VISIR_spc_obs_HRXAutoChopNod is functionally similar to VISIR_spc_obs_HRAutoChopNod but uses a grism for cross dispersion and order separation The effective length of the spectrograph slit in this mode is limited to 4 The entire wavelength range between 7 7 and 13 3 um is accessible using the
60. imager for the small field provides a usable field of view of 38 0 x 38 0 pfov Because the detector is slightly larger than the area of sky imaged VISIR User Manual VLT MAN ESO 14300 3514 11 THT focal plane detector II ES Figure 7 The optical path of the imager with a pixel scale of 0 045 is shown from the focal plane down to the detector by the VISIR optics a part of the detector has been masked and only the pixels between x 20 880 and y 75 945 receive light from the sky The filter wheel is located just behind the cold stop pupil mask The list of filters offered is given in Table 1 The transmission curves of the filters measured at 35K are plotted in App 10 The VISIR upgrade includes an Annular Groove Phase Mask AGPM coronograph The AGPM will be offered for the first time starting in P96 pending its successful commissioning during P95 The AGPM consists of a vector vortex induced by a rotationally symmetric subwavelength grating and allows for coronagraphic imaging of the close environment of stars especially enabling the search for exoplanets and circumstellar disks This mode is currently only offered in visitor mode Note that for the time being burst mode imaging is not offered due to hardware limita tions Further and more up to date information can be found at http
61. imitations with VISIR However it is advised to observe only sources fainter than 500 Jy in N and 2500 Jy in Q to avoid detector artifacts 4 4 5 6 Overheads The VLT overhead for one OB which includes active optics setting selection of guide star field stabilization is 6 min VISIR instrument configurations can be changed in a short time For example a complete change of instrument settings takes less than 2 minutes The total time for an image acquisition of a bright sources gt 1Jy takes 5min for one fine acquisition iteration or in blind preset 2min Spectroscopic acquisitions take longer and are strongly dependent on the source brightness an overhead of 15 min is accounted for sources gt 1 Jy while 30 min are required for sources between 0 2 and 1 Jy respectively Instrument overheads due to chopping and nodding duty cycle losses have been measured to be between 25 and 50 of the observing time leading to an observing efficiency between 50 and 80 see also 4 5 The total observing time requested by the observer must include telescope and instrument overheads 5 7 Calibration Observations MIR observations depend strongly on the ambient conditions such as humidity temperature or airmass In service mode science observations are interlaced by calibration observations VISIR User Manual VLT MAN ESO 14300 3514 29 on a timescale of 3h That is observations of photometric standards will be provided by the observatory w
62. ing with VISIR at the VLT 5 1 Proposal Preparati n coxis ehe ha Date Das we ed we 5 2 Observing Parameters 5 2 1 Instrument orientation on the sky 52 2 Chopping Parameter und de dd db td d a 5 2 3 Nodding parameters lt 2 424 iris rra Dada ke 5o BI OUI bac eh be bee ISSN era rien es w OFF ph Me DD gt ww 10 10 11 12 13 13 13 14 14 15 15 16 16 17 17 17 18 VISIR User Manual VLT MAN ESO 14300 3514 Poot OIG os ua e as Dom Acquisition Templates ssie co reis eR a ea a od A EII ES 5 5 Brightness Limitations lt a s soo sa 244 hate a amp dor pn aio Overheads o ea ii er aaar Lande p ku aea e EAE EGER RAD 5 Galibration Opestvations 2 44 lt 54 6664 484 dame da ERE 5 8 Observing constaints and OB Classification 6 VISIR data CL Daa A lt A Ga PODIAS ee ees aaa da dus 7 Description of VISIR templates T L Acguisition MAI 7 2 Observing with the imager 7 3 Observing with the spectrometer Ta TA Se pleca pa ns ds Du De de d amp Erd E Bere HS 8 Checklist S TEN a de at ca re ee D Be ee a e ee he ee er eee ri ae 9 Appendix VISIR template parameters 10 Appendix Filter transmission curves 11 Appendix Sensitivities in various spectroscopic settings 31 31 31 32 34 34 34 36 36 37 37 37 39 48 51 vi Li
63. ion is not so crucial PERPENDICULAR con siders an equal nodding and chopping amplitude however in perpendicular direction Note that while the telescope offset is in positive East direction the resulting image on the de tector will move to the West This technique is recommended for point or relatively small extended lt 10 sources Fig 5 goen Nodding Position A Nodding Position B Nodding Position A Nodding Position B Figure 18 Schematic drawing of the content of a frame obtained with TEL ROT OFFANGLE TEL CHOP POSANG 0 and SEQ CHOPNOD DIR PARALLEL top and SEQ CHOPNOD DIR PERPENDICULAR bottom In the individual nodding positions the positive beams correspond to the chopper position A and the negative beams to the chopper position B Note that the default pointing position of the telescope corresponds to the center of the detector Within the accuracy of the telescope pointing this location matches the nodding position A chopper position A if SEQ CHOPNOD DIR PARALLEL The keywords SEQ JITTER WIDTH allows chopping and nodding with random offsets so that a jitter pattern is performed This technique allows to reconstruct bad pixels For SEQ JITTER WIDTH 0 no jitter is performed and the resulting image depends on the setting of SEQ CHOPNOD DIR VISIR User Manual VLT MAN ESO 14300 3514 35 The chopping period is set by the system and the number of nodding cycles during the obser vations period is set by the
64. ion spectroscopy are not yet available For the time being the maximum chop throw is limited to 20 arcsec The astronomical community is encouraged to monitor the latest VISIR news reported on http www eso org sci facilities paranal instruments visir news html http www eso org sci facilities paranal instruments visir upgradeproject html 1 1 Detector Upgrade The major part of the upgrade project concerns the replacement of the old detector DRS 256 x 256 pixel array with a new Raytheon AQUARIUS 1024 x 1024 pixel array This hardware upgrade improves the VISIR performance in terms of field coverage and sensitivity The new AQUARIUS detector is offered with a pixel scale of 0 045 SF providing a usable field of view of 38 0x38 0 The cosmetic quality of the AQUARIUS detector is proven to be excellent The regions of masked pixels and stripes which were characteristic for the old DRS detector are not present anymore 1 2 Low Resolution Spectroscopy The second major improvement concerns the N band 8 13m low resolution spectroscopy Formerly this was achieved by means of a grism which had the disadvantge of requiring 4 independent exposures in order to cover the 8 13 5um range The introduction of a low resolution prism R 300 for a 074 slit now allows to achieve the same wavelength coverage in a single exposure and reach sensitivities of 30 50 mJy 10o h 1 3 Precipitable Water Vapour The amount of Precipitable Water Vapo
65. it the first acquisition images are obtained with the wide slit 14 width Then e If the target coordinates are well known VISIR imaging modes allow to perform blind preset observations with the VISIR_img_acq Preset template In this case no acquisition images are taken e The VISIR_img_acq_MoveToPixel and VISIR_spec_acq Move ToSlit require interaction with the instrument operator or night support astronomer in order to center the target at the appropriate location on the detector Without further indication given by the observer the default locations are for VISIR_img_acq MoveToPixel and SEQ CHOPNOD DIR PARALLEL 3 North from the center of the detector to avoid the central outward readout of the detector for VISIR_img_acq_MoveToPixel and SEQ CHOPNOD DIR PERPENDICULAR in the top left quadrant of the detector at a distance equal to TEL CHOP THROW 2 from the center of the detector in both X and Y 26 VISIR User Manual VLT MAN ESO 14300 3514 for VISIR_spec_acq MoveToSlit at 3 South of the center of the slit In service mode acquisition with the VISIR_spec_acq_MoveToSlit template is limited to objects brighter than 0 2 Jy All acquisition images are recorded and archived Note that except if specifically re quested in the README file photometric standard stars are not necessarily observed in the same filter as the acquisition filters As part of the execution of the VISIR_spec_acq_MoveToSlit
66. ithin a time interval of three hours w r t the science observations Calibrators unless provided by the observer are selected from the MIR spectro photometric standard star catalog of the VLT http www eso org instruments visir This catalog is a sub set of the radiometric all sky network of absolutely calibrated stellar spectra by Cohen et al This list is supplemented by MIR standards formerly used by TIMMI2 see http wuw ls eso org lasilla sciops 3p6 timmi html stand html At present the standard star catalog contains 425 sources Continuous observations over 3 hours of the same standard star indicates that photometric stability better than 3 can be achieved with VISIR at the VLT In order to test if a photo metric precision of the same order can be obtained a reduced set of standard stars has been built consisting of the Cohen et al stars which obey the following criteria e visibility from Paranal e no variability detected by Hipparcos non variables Var 0 in the Hipparcos catalogue e absolute flux calibration errors as reported by Cohen et al lt 20 e all spectral types reported in SIMBAD no more than 1 sub class different from that used by Cohen et al e not visual binaries as reported by SIMBAD This catalogue of 81 stars is also made available at http www eso org instruments visir From this catalogue a further selection to provide a reduced list of 12 stars has been carried out see also http www
67. justable wavelength or an extended black body source with adjustable temperature A selection mirror allows to switch from the telescope to the simulator beam It can be used for calibration and tests also during daytime Fig 10 shows the unit on top of the enclosure The SKYCALC Sky Model Calculator can be found at https www eso org observing etc bin gen form INS MODE swspectr INs NAME SKYCALC VISIR User Manual VLT MAN ESO 14300 3514 15 IE Monochromator Figure 10 Schematic drawing of the warm calibration unit on top of the VISIR vessel 4 4 Detectors Following the upgrade of VISIR the imager and spectrometer are equipped with two AQUAR IUS 1k x 1k detectors with a pixel size of 30 um In principle this provides a field of view that is four times larger in area than that offered by the old DRS 256 x 256 detector However the unvignetted and usable part of the AQUARIUS detector of the imager is smaller than the entire detector Only a region of approximately 860 x 860 pixels is illuminated by the sky providing a field of view of 38 x38 with a pixel scale of 0045 Similarly onla a region of 190 x 450 pixels is illuminated by the sky using the imaging capability of the spectrometer i e the wide slit providing a field of view of 14 x34 with a pixel scale of 0 076 The AQUARIUS array was developed at Raytheon Vision Systems at Santa Barbara USA The development was funded by ESO to upgrade the VI
68. nge that is set by the Closed Cycle Coolers typically 6 9 K Lower operating temperatures allow to minimize the leakage and most importantly the dark current Laboratory experiments showed that the dark current ranged between 2200 and 0 56 e pixel s at 10 0 and 5 6 K respectively The upgrade operational goals aimed at reaching a 1 0 e pixel s and this was achieved at an operating temperature of 7 K VISIR User Manual VLT MAN ESO 14300 3514 17 4 4 4 Excess Low frequency Noise The AQUARIUS detectors by Raytheon suffer from excess low frequency noise ELFN which is the result of a design optimized for operations in a low background environment such as space applications The ELFN is a form of correlated noise caused by fluctuations in the space charge induced by ionization recombination in the blocking layer It manifests itself as a memory of photons in subsequent frames This correlation can be broken by modulating the scene seen by the detector sources and background at sufficiently high speed Chopping has been demonstrated in the lab and on sky to significantly reduce the impact of ELFN with increasing chopping frequences Therfore VISIR is now operated at much higher chopping frequencies as before up to 4 Hz in imaging The chopping frequencies are predfined and chosen such that the signal to noise is maximised 4 4 5 Detector Linearity The AQUARIUS detector linearity was derived for both the high gain and the low gain confi
69. observations is signif icantly different from the epoch for which the coordinates were determined e point like sources within extended objects such as an AGN a number of catalogues do not provide accurate coordinates of the nucleus Coordinates given by 2MASS are the more reliable e coordinates obtained with low spatial resolution instrument such as MSX etc For solar system objects the J2000 0 equinox topocentric ICRF or FK5 coordinates at the epoch of the observations are required as the Telescope Control System takes into account precession nutation annual aberration and refraction On the contrary the topocentric apparent coordinates at the observatory often used in other observatories should not be used Additional velocity parameters corresponding to a cos and u 9 must be given in fs 5 4 Guide stars Guide stars are mandatory for active optics and field stabilization Any VLT program should make sure that a guide star UCAC3 with a R 11 13 mag is available within 7 5 around the object Sensivity in the mid IR for a ground based observatory is strongly limited by the sky bright ness In addition the VISIR field is small compared to other VLT instruments Therefore images of a field can often appear empty in short to medium length exposures However objects may become visible in longer ones Combining different exposures taken on different nights may be tricky if a proper astrometric alignment is not
70. observer with SEQ NODNCYCLES 4 5 VISIR_img_obs_GenericChopNod This imaging template enhances the flexibility of nodding offsets and allows the user to specify them in a list of relative offset positions In the most simple application only one offset position is specified This allows to record nodding pairs i e cycle of on off observations using a flexible offset position Additional jitter offsets can be specified More than one entry in the offset list results in a freely programmable pattern of nodding pairs Note that the integration time specified SEQ TIME refers to the entire observation sequence As for the normal imaging template the time spent in a certain nodding position will depend on the number of nodding cycles i e on SEQ NOFF The offset positions are calculated as the cumulative sum of offsets i e are defined relative to the previous offset positions Note that the telescope always returns to the first reference position when specifying a list of offsets This mode can be exploited to perform mosaic or raster imaging The first reference position can then be considered as a sky observation while the offsets refer to object positions It is recommended to offset to positions that result in observations of overlapping fields which enhances the redundancy after image reconstruction Nodding Position B1 Nodding Position B2 Nodding Position B3 E Reference Position A Preset Figure 19 Illustration of
71. r PWV present in the Earth s atmosphere can heavily impact on mid infrared observations However the effect of PWV is strongly dependent on wavelength Whereas a PWV column of 3 mm or larger is generally acceptable for observations in the N band the sensitivity of observations in the Q band depends strongly on the PWV contents and can only be carried out for PWV columns below 3mm Operations wise a prior knowledge of the PWV content will seriously impact the efficiency of service and visitor mode observations with VISIR As part of the VISIR upgrade project and starting December 2011 real time PWV monitoring is available on Paranal The commissioning of the PWV monitor shows that it meets all specifications e PWV range 0 5 9 0mm validated http www eso org tecarch Documents VLT 14300 mid_ir_imager_spectrometer 14330 VISIR_ Upgrade SoW_for 20_RS_campaign_5504 pdf 2 VISIR User Manual VLT MAN ESO 14300 3514 Paranal PWV across the year PWV mm Figure 1 Average PWV distribution over Paranal across the year PWV precision ca 30 um PWV accuracy ca 0 1 mm High time resolution sec All sky pointing 2D capability Autonomous operation The median PWV over Paranal is 2 1 mm with strong seasonal variations see Fig 1 The fraction of time in which the PWV contents over Paranal is lower than 1 mm is about 10 The PWV value is a user defined constraint parameter 1 4 Main Remaining Issues
72. rs partly or completely opaque Differential absorption is often corrected by dividing the extracted spectrum by a reference spectrum This procedure may cause numerical instabilities at wavelengths close to strong sky lines that might amplify the noise VISIR User Manual VLT MAN ESO 14300 3514 33 Photometry Spectro photometric calibration of low and medium resolution spectra can be achieved with the MIR standard star list provided by the Observatory see 5 7 For high resolution spectroscopy only calibrators known with high precision such as A stars or asteroids should be considered However even early A stars are known to have some hydrogen absorption lines in the N and Q band 34 VISIR User Manual VLT MAN ESO 14300 3514 7 Description of VISIR templates 7 1 Acquisition Each OB needs to start with an acquisition template The acquisition process is described in 3 8 7 2 Observing with the imager VISIR_img_obs_AutoChopNod This template permits observing a source in imaging configuration with various sub settings The observer must specify the filter and the chopper throw which can be for the time be ing in the range between 8 and 20 The keyword SEQ CHOPNOD DIR is set to PARALLEL or PERPENDICULAR which results in images as shown in Fig 18 PARALLEL considers an equal nodding and chopping amplitude which are both in parallel direction It is recommended for faint extended sources for which the spatial resolut
73. s curved slits to cancel the distortion of the pre slit optics Thus the slit projected on the sky is straight There is an additional linear distortion in both dispersion and cross dispersion direction of the detector The distortions have not been estimated yet for the new AQUARIUS detector and will be reported after the full commissioning of the new detectors Wavelength calibration A first order wavelength calibration is given by the optical model of the instrument Its precision is about 10 pixels for the low and medium resolution mode and 15 pixels for the high resolution mode The wavelength calibration can be refined by using Fabry Perot Etalons plates or atmospheric lines In the VISIR FITS file the averaged half cycle frames which are dominated by sky emission lines are stored 86 1 They can be used to fine tune the wavelength calibration to sub pixel precision by comparison with a model of the atmospheric lines This method is used by the pipeline More specifically the zero point of the wavelength calibration is obtained by cross correlating the observed sky spectrum with a HITRAN model of the sky emission lines Note that the chopped frames cannot be used for calibration with atmospheric lines because the chopping process results in a near perfect cancelation of sky lines Atmosphere absorption correction The atmosphere does not uniformly absorb the MIR radiation 3 1 At some wavelengths it is completely transparent at othe
74. see dot dashed lines in Fig 3 However initial results from VISIR data indicate that this formula overestimates the measured MIR seeing at Paranal by 20 50 as the size of a UT mirror is comparable to the turbulence outer scale As a result VISIR data are already diffraction limited for optical seeing below 0 6 The results of measures obtained in 2005 are shown in Fig 4 3 3 MIR background The atmosphere does not only absorb MIR photons coming from astrophysical targets but also emits a strong background with the spectral shape of a black body at about 253 K Kirchhoff s law The telescope gives an additional MIR background The VLT telescopes emit at 283 K with a preliminary emissivity estimate of lt 15 in N The VISIR instrument is cooled to avoid internal background contamination The detectors are at 9K and the interior of the cryostat at 29K The background radiation at 10um is typically my 5 mag arcsec 3700 Jy arcsec and at 20um mg 7 3 mag arcsec 8300 Jy arcsec Consequently the number of photons reaching the detector is huge often more than 10 VISIR User Manual VLT MAN ESO 14300 3514 5 10 0 E ss AS a a me SPITZER diffraction c WS 2 r 5 05 VLT VISIR diffraction _ 2 Ba E FERNEN 5 a ee 0 5 visible seeing We lt 0 1 1 1 1 1 L 1 1 1 1 i 1 1 1 1 L 1 1 1 f L fi f fi fi 5 10 15 20 25 30 Wavelength um Figure 3 VLT diffraction limit full line versus se
75. ser Manual VLT MAN ESO 14300 3514 Filter Ag half max sensitivity 100 1h Note um band trans mJy width mission theory median effective um BLIP J7 9 7 76 0 55 19 24 PAH1 8 59 0 42 77 1 6 5 6 J8 9 8 70 0 73 3 4 B8 7 8 92 0 97 Arlll 8 99 0 14 72 4 1 9 12 J9 8 9 59 0 94 7 9 SIV 1 9 82 0 18 72 4 0 17 32 B9 7 9 82 0 84 SIV 10 49 0 16 70 4 5 9 11 B10 7 10 65 1 37 5 6 SIV 2 10 77 0 19 70 4 6 8 11 PAH2 11 25 0 59 75 2 3 B11 7 11 52 0 85 SiC 11 85 2 34 75 12 PAH2_2 11 88 0 37 58 4 1 6 8 J 12 2 11 96 0 52 10 12 Nell 1 12 27 0 18 51 6 9 for spectr acquisition B12 4 12 47 0 99 Nell 12 81 0 21 64 6 1 15 25 Nell_2 13 04 0 22 68 6 3 23 25 for spectr acquisition Q1 17 65 0 83 59 11 1 36 42 Q2 18 72 0 88 49 13 6 67 78 Q3 19 50 0 40 50 41 7 65 78 Table 1 VISIR imager filter characteristics following the manufacturer specifications except for the central wavelengths noted with which were re determined with a monochromator and the WCU because they deviate from specifications The last 3 columns give respectively the theoretical expectations under BLIP and excellent weather conditions the median and effective sensitivities obtained in various weather conditions during the re commissioning of VISIR The median sensitivites only take into account the pure integration time 4 while the effecti
76. st of acronyms AGPM BIB BLIP BOB DIT ETC FWHM ICS IR IRACE MIR OB P2PP PAE pfov PSF PWV SAM S N TCS TMA UT VISIR WCU VISIR User Manual VLT MAN ESO 14300 3514 Annular Groove Phase Mask Blocked impurity band Background limited performance Broker of observation blocks Detector integration time Exposure time calculator Full width at half maximum Instrument control software Infrared Infrared array control electronics Mid infrared Observing block Phase 2 proposal preparation Preliminary acceptance in Europe pixel field of view Point spread function Precipitable Water Vapor Sparse Aperture Masking Signal to noise ratio Telescope control system Three mirrors anastigmatic Unit Telescope VLT imager and spectrometer for the mid infrared Warm calibration unit VISIR User Manual VLT MAN ESO 14300 3514 1 1 VISIR Upgrade Project VISIR has been undergoing an upgrade starting from May 2012 P89 The instrument is offered again since Period 95 However the re commissioning of the instrument will continue during 2015 and only a limited number of instrument modes are offered for P96 imaging with a pixel size of 0 045 arcsec long slit low resolution spectroscopy and long slit and cross dispersion high resolution spectroscopy Newly offered in Period 96 pending successful commissioning in early March 2015 will be the coronagraph Annular Groove Phase Mask AGPM Burst mode SAM and medium resolut
77. tal integration time sec Chopping Position Angle deg Chopping Amplitude arc sec VISIR spec cal HRAutoChopNod tsf To be specified Parameter Range Default Label INS FILT2 NAME INS GRAT1 WLEN SEQ CATG SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ NODNCYCLES SEQ RETURN SEQ TIME TEL CHOP POSANG TEL CHOP THROW NEI2 H251 NEIL2 7 5 28 0 NODEFAULT H25_4 SCIENCE CALIB TEST CALIB PARALLEL PERPENDIC ULAR PARALLEL 0 10 0 1 100 NODEFAULT FT N 30 3600 NODEFAULT 0 360 0 8 30 8 Spectrometer Filter Spectrometer microns Observation Category Wavelength Relative Chop Nod Direc tion Random Jitter Width arc sec Number of nodding cycles Return to Origin Total integration time sec Chopping Position Angle deg Chopping Amplitude arc sec VISIR User Manual VLT MAN ESO 14300 3514 VISIR spec_cal HRXAutoChopNod tsf To be specified Parameter Range Default Label INS GRAT1 WLEN 7 5 28 0 NODEFAULT Spectrometer Wavelength microns SEQ CATG SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ NODNCYCLES SEQ RETURN SEQ TIME TEL CHOP POSANG TEL CHOP THROW SCIENCE CALIB TEST CALIB PARALLEL PERPENDIC ULAR PARALLEL 0 10 0 1 100 NODEFAULT FT T 30 3600 NODEFAULT 0 360 0 8 30 8 Observation Category Relative Chop Nod Direc tion Random Jitter Width arc sec
78. tion The templates are subject to change during the continuing re commissioning of the instrument VISIR_img_acq_Preset tsf Parameter Range Default Label TEL AG GUIDESTAR TEL GS1 ALPHA TEL GS1 DELTA TEL ROT OFFANGLE TEL TARG ALPHA TEL TARG DELTA TEL TARG EPOCH TEL TARG EQUINOX TEL TARG OFFSETAL TEL TARG OFFSETDE TEL TARG PMA TEL TARG PMD SETUP CATA CATALOGUE FILE NONE LOGUE ra dec 0 360 0 0 ra dec 2000 3000 2000 0 2000 3000 2000 0 HOW TAO 10 10 0 0 10 10 0 0 Get Guide Star from Guide star RA Guide star DEC Rotator on Sky PA on Sky Alpha coordinate for the target Delta coordinate for the tar get Epoch Equinox RA blind offset DEC blind offset Proper Motion Alpha Proper Motion Delta 40 VISIR User Manual VLT MAN ESO 14300 3514 VISIR_img_acq_MoveToPixel tsf To be specified Parameter Range Default Label INS FILTI NAME SEQ CHOPNOD DIR SEQ NODNCYCLES SEQ TIME TEL AG GUIDESTAR TEL CHOP POSANG TEL CHOP THROW TEL GS1 ALPHA TEL GS1 DELTA TEL ROT OFFANGLE TEL TARG ALPHA TEL TARG DELTA TEL TARG EPOCH TEL TARG EQUINOX PAHI ARII SIV1 SIV SIV2 PAH PAH22 NEII_1 NEI NEIL2 Q1 Q2 Q3 B10 7 B11 7 J7 9 J8 9 J9 8 J12 2 NODEFAULT PARALLEL PERPENDIC ULAR PERPENDICU LAR 1 100 NODEFAULT 30 3600 NODEFAULT CATALOGUE SETUP FILE NONE CATA
79. trumental and atmospherical conditions The newly determined values of the sensitivity for each filter are given in Table 1 and refer to the median of the observations during re commissioning of the instrument in November 2014 and January 2015 A graphical compilation is presented in Fig 6 Some of the best measurements approach theoretical expectations i e they are close to background limited performance BLIP Sensitivity estimates for the VISIR spectroscopy observing modes are obtained in a similar way However in this case chopping and nodding are executed in parallel Consequently only 3 beams are obtained with the central one containing twice as much flux as the two other ones Table 2 lists typical sensitivities measured in high resolution spectrosopy away from strong sky emission lines for the wavelength ranges currently offered for this mode Figures 20 to 26 in the Appendix 11 show the dependence of sensitivity on wavelength for the high resolution mode Not all of these sensitivities have been determined with the new detectors yet However the measurements carried out so far indicate that VISIR is at least as sensitive as before the upgrade with the new detectors Therefore the old sensitivities can be used as guidelines for the preparation of OBs until they are updated with new measurements using the AQUARIUS detectors The median sensitivities are the reference for classification of VISIR service mode observations VISIR U
80. ve sensitivities include overheads due to chopping and nodding ttot see 4 5 The sensitivities were obtained using the curve of growth method on data obtained in perpendicular chopping nodding directions 4 beams 10 VISIR User Manual VLT MAN ESO 14300 3514 mode c AA line order R dispersion sensitivity um um pixels um Jy 100 1h HR 7 800 8 100 0 02420 H2 54 17B 32000 17573 3 HR 12 738 12 882 0 03571 Ne II 11A 17000 11908 0 9 HR 16 800 17 200 0 05156 H2S1 8B 14000 8250 lt 10 Table 2 VISIR high resolution long slit HR mode The second column gives the mini mum and maximum allowed values for the central wavelength Ac in the given setting The wavelength range per setting in given in the 3rd column AA R is the theoretical spectral resolution Offered slits have widths of 0 40 0 75 and 1 00 The dispersion is given in the 7th column and has been estimated for the new AQUARIUS detector pixel size The sensitivites are still the ones for the old DRS detector and are valid until further notice and the basis to assess the feasibility of an observing programme In particular classification of service mode OBs will be based on sensitivity measurements made at zenith Calibrations will be provided following the guidelines given in 85 7 For up to date information please consult http www eso org instruments visir The use the VISIR exposure time calcu lator ETC located at http www eso
81. ws the detector mounted in its socket read in parallel This readout scheme also allows for 16 outputs rather than 64 to simplify the system design for low background applications With this multiplexer configuration it is possible to read out the full detector at 150 Hz 7 milli seconds frame rates each output operational at 3 MHz pixel rates 4 4 2 Detector Readout For imaging with detector integration times of a few milliseconds the detector is read out in rolling mode In this mode a row is read then immediately reset then the next row is read and reset and so forth for a programmable row time which can overlap the read of the next row For the high resolution Echelle mode with significantly longer intergration times the detector is read out using correlated double sampling A windowed readout is also possible A number of rows can be selected to be read out from the center outwards with the remaining rows reset automatically For example a 1024 x 150 sized window centered in the middle of the device can be readout at 1 kHz 0 001 milli seconds frame rates There is no advantage to windowing in the column direction since all outputs run in parallel Note that in the Q band the detector is by default windowed to 256 pixels in y direction corresponding to 23 in order to guarantee sufficiently short exposure times and to avoid saturation 4 4 3 Detector Dark Current Mid IR detectors operate at a temperature ra

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