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VISIR User Manual

1. 60 15C Large Cap de ih TS i Small Cap det_sub T 5 1 K 25 o o ESCH o tee BG 13 a Ai Glee Sr SH 2 1000F Geg 296 ere Qe de 9 e e ES A gt et a o o 0079 D E a so a 5 s 000 eae lt Z am 2 2000 Gi Le a 500 Hoa e L Sc Es Data o Fit of the data Photon noise J 4 n NOISE y 1 0 0x10 0x107 EART A RE 5 0x10 1 0x10 5x10 0 5 0x10 Signal e7 Figure 10 Noise as a function of the incoming flux in the large left and small right capacity mode Superimposed is the theoretical photon noise BLIP performances are approached for higher fluxes and larger DIT respectively These artifacts are less important in spectroscopy due to the lower light levels but clearly visible on objects brighter than 2 of the background However a TEL CHOP THROW between 9 to 13 shoud be avoided in particular for objects bright enough to be seen in individual DITs as one of the beams will hit some particularly hot pixels in the lower left of the spectrometer detector see Fig 14 3 5 Data acquisition system Both VISIR detectors are controlled by the ESO standard IRACE acquisition system In imaging the read out rate of the detector is high Up to 200 frames per s are read for a minimum detector integration time of DIT 5ms Such a frame rate is too high to store all exposures One VISIR image is of size 256x256 each pixel is coded with 4
2. 0 AE 16 375 16 380 16 385 16 390 16 39516 40016 405 Wavelength um 59 Figure 31 Observed sensitivity as a function of wavelength for high resolution mode Offered sensitivity is typically a factor of 2 larger 2 0104 1 5x104 1 0104 5 0x108 Sensitivity mJy 100 in 1h 0 16 900 16 910 16 920 16 930 16 940 16 950 Wavelength um Figure 32 Observed sensitivity as a function of wavelength for high resolution mode Offered sensitivity is typically a factor of 2 larger VISIR User Manual VLT MAN ESO 14300 3514 60 10000 8000 6000 4000 2000 Sensitivity mJy 100 in 1h OL r ra i 17 80 17 85 17 90 17 95 18 00 Wavelength um Figure 33 Observed sensitivity as a function of wavelength for high resolution mode Offered sensitivity is typically a factor of 2 larger LOO OOS NN A 8000 6000 4000 2000 Sensitivity mJy 100 in 1h 18 210 18 220 18 230 18 240 Wavelength um Figure 34 Observed sensitivity as a function of wavelength for high res
3. 1000 SS Sensitivity mJy 100 in 1h uge 12 46 12 48 Wavelength um OE a 12 44 12 50 Figure 29 Observed sensitivity as a function of wavelength for high resolution mode Offered sensitivity is typically a factor of 2 larger VISIR User Manual VLT MAN ESO 14300 3514 58 10000 7 7 7 TT Tt a a a TT 7 model J d median ds Ai al L M 7 L J 2 gt L gt E 1000 oO F mn E i il i 1 AL ALI F lt ye ll WLU ji IL a hola J a a apg ee A A es 12 70 12 75 12 80 12 85 12 90 wavelength um 6000 5000 4000 3000 NES 2000 1000 Sensitivity mJy 100 in 1h d 12 80 12 90 3 00 13 10 13 20 13 30 Wavelength um Figure 30 Observed sensitivity as a function of wavelength for high resolution mode Top Observed sensitivities obtained on various nights compared with the theoretical model curves corresponding to BLIP Bottom Sensitivities over an extended region encompassing the observed wavelengtgh of Nell up to z 0 038 VISIR User Manual VLT MAN ESO 14300 3514 2 0x10 TT 1 5x104 1 0x104 5 0x103 Sensitivity mJy 100 in 1h
4. Filter Nell_ref1 Filter PAH2_ref2 S EZ T T T s AO T T T DE DE DEE L J p E 0 8 IL 0 8 oet 4 5 06 J 3 E N 0 4f A 0 4 1 E 0 2H E oz O L 4 O J Z 0 0 f 1 0 0 11 8 12 0 1 2 2 12 4 12 6 10 8 ix 11 8 132 2 12 8 Wavelenght m Wavelenght m g DH g H Filter SIV_ref1 Filter SIV_ref2 s 1 2 F i s 1 2 R j ZS 1 0 F ed a 1 0 i E E o 0 8F 4 o 0 8 J 5 t 5 oet 0 6 J nO bei mee I N 0 4 O E o2 J E 02 J O O 0 0 0 0 fi i 9 5 9 7 9 9 10 1 10 3 10 4 10 6 10 8 11 0 TL Wavelenght m Wavelenght m g H g H Figure 19 Transmission curves of VISIR imager filters manufactured by OCLI Overplotted 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 VISIR User Manual VLT MAN ESO 14300 3514 52 10 Appendix Observed sensitivities in various spectro scopic settings sensitivity mJy 100 1h 0 wavelength um 1000 T 800 600 F 400 F Sensitivity mJy 100 in 1h 0 L 1 1 1 1 1 1 1 1 n 1 1 1 LS 8 0 SES Wavelength um Figure 20 Sensitivity as a function of wavelength for low resolution mode Top Four offered settings of the N band low resolution are stitched together Atmospheric molecular absorption e g at 9 55 11 8 and 12 5 um is evident Note the dete
5. To be specified Parameter Range Default Label INS GRAT1 WLEN 7 60 28 08 VODEFAULT Spectrometer Wavelength microns SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW PARALLEL PERPENDIC ULAR PARALLEL 0 10 0 20 2000 NODEFAULT 0 359 0 8 30 8 Relative Chop Nod Direc tion Random Jitter Width arc sec Total integration time sec Chopping Position Angle deg Chopping Amplitude arc sec VISIR User Manual VLT MAN ESO 14300 3514 49 9 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 23 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 Ar Filter Nell_ref2 120 i 120 y i se ont __ E 100 fe KONO eat AAN FS Lp J FRY a 2 eof 2 60 Woy E ac J E 40 5 20f 7 5 20 7 0 L L L 0 if L 8 80 8 90 9 00 9 10 9 20 12 6 12 8 13 0 Ts 13 4 Wavelenght um Wavelenght um Filter Nell Filter PAH1_ARIllref1 120 i i 120 j i i S Te 100 ay yaa sot WE uf aa 80 hy An
6. Table 6 VISIR high resolution long slit HR and cross dispersed HRX modes The second column gives the minimum and maximum allowed values for the central wavelength Ae 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 Note that the range 12 210 12 760 also covers HD 0 0 R 9 while the Nell emission line can be observed up to z 0 038 3 3 Calibration units 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 adjustable 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 8 shows the unit on top of the enclosure 3 4 Detectors The VISIR imager and spectrometer are each equipped with a DRS former Boeing 256 x 256 BIB detector The quantum efficiency of the detectors is greater than 50 and reaches 65 or more at 12 um Fig 9 VISIR User Manual VLT MAN ESO 14300 3514 14 nochromator Figure 8 Schematic drawing of the warm calibration unit on top of the VISIR vessel The detector noise has to be compared with the photon noise of the background As shown in Fig 10 the measured noise in an observation co
7. The detector integration time DIT is a few milli seconds in broad band imaging and may increase to 2s in high resolution spectroscopy The DIT is determined by the instrument software according to the filter and pfov It is not a parameter to be chosen by the observer The DRS detectors contain a fair fraction of bad pixels lt 2 Fig 12 The imager detector suffers from striping and appearances of ghosts The relatively wide rectangular area in the lower right corner South West corner for PA 0 deg of the imager detector or some other rectangular areas are masked out to avoid such disturbances Fig 13 For bright objects the DRS detector shows memory effects Stabilization is ensured by introducing dead times where necessary It is advised to observe only sources fainter than 500 Jy in N and 2500 Jy in Q VISIR User Manual VLT MAN ESO 14300 3514 15 1 0 T fe ate Am N y L MN d Re k a a ESF jv N D Y Sy e o VINO a Is Ir V MN T 0 6 py KE e Ges V 4 f z l SA 5 Ole od VA 5 de ER o ff EN i 2 0 2b f 4 O A D 4 A 0 0 1 1 1 0 5 10 15 20 25 30 Wavelength um Figure 9 Detector quantum efficiency at 12K provided by DRS solid line The same curve dashed but scaled by 0 72 reflects a lower limit of the quantum efficiency The scaling was derived from laboratory measurements Note the sharp absorption feature at 8 8 um that will appear in raw spectroscopic data
8. gt 2 E 400 E G 200 a ll n oL L f L fi fi 1 li fi 1 li L f f f L f L L el 10 5 11 0 11 5 12 0 12 5 Wavelength um Figure 22 Observed sensitivity as a function of wavelength for different settings in the medium resolution mode obtained in very good weather conditions II for 10 lum lt A lt 12 5um Offered sensitivity is typically a factor of 2 larger ac EIA WI TT TT 1500 Jul Mo A O A AAA AS 178 180 182 184 186 188 19 0 Wavelength um Sensitivity mJy 100 in 1h paro lo 11 do woen Figure 23 Observed sensitivity as a function of wavelength for different settings in the medium resolution mode obtained in very good weather conditions III for 17 7um lt lt 19 1ym Offered sensitivity is typically a factor of 2 larger VISIR User Manual VLT MAN ESO 14300 3514 55 A000 E Ty 3000 2000 1000 Sensitivity mJy 100 in 1h 0 19 90 20 00 20 10 20 20 Wavelength um Figure 24 Observed sensitivity as a function of wavelength for different settings in the medium resolution mode obtained in very good weather conditions IV for 19 9um lt lt 20 3ym Offered sensitivity is typically a factor of 2 larger 8000 J T Le 4 E 6000
9. 1 E c d d 2 eof a 5 60 ly 1 E 40F J E 40 n i Y E E 1 KE fi 2 or 2 20 i g fi o La di y 12 4 12 6 128 13 0 132 TD 8 0 8 5 9 0 9 5 Wavelenght um Wavelenght um Filter Pah2 Filter QO 120 S 120 KS Be E oes J Be E A EG aa Du ra i on T eof pH ga z t C D O L al O 3 g o A E 4of J E 40 ui H E L C d 2 20 2 20 Wy yd 0 an f oL A ro 10 3 10 9 11 6 12 2 12 8 15 8 16 2 16 8 17 2 17 8 Wavelenght um Wavelenght um Figure 23 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 1370 16 5 se 18 5 Wavelenght um Filter Q3 120 100F ef 40 F 20F y 0 Transmissions in BOF an 18 5 120 1005 Eis 605 40 Transmissions in 205 0 19 0 20 0 Wavelenght um Filter SIV 10 1 10 3 10 5 10 7 Wavelenght um 10 9 Transmissions in Transmissions in Filter Q2 l 16 5 17 5 18 5 19 5 20 5 Wavelenght wm Filter SIC 120 i i i 100 PY YA A Wi Th 80 il fy y 60 y 40 l 20 O y 8 10 12 14 16 Figure 18 continued Wavelenght um 50 VISIR User Manual VLT MAN ESO 14300 3514 51
10. 30 8 Relative Chop Nod Direc tion Random Jitter Width arc sec Total integration time sec Chopping Position Angle deg Chopping Amplitude arc sec VISIR_spec_obs_MRAutoChopNod tsf To be specified Parameter Range Default Label INS GRAT1 WLEN 7 5 28 08 NODEFAULT Spectrometer Wavelength microns SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW PARALLEL PERPENDIC ULAR PARALLEL 0 10 0 180 3600 NODEFAULT 0 359 0 8 30 8 Relative Chop Nod Direc tion Random Jitter Width arc sec Total integration time sec Chopping Position Angle deg Chopping Amplitude arc sec VISIR_spec_obs_HRAutoChopNod tsf To be specified Parameter Range Default Label INS FILT2 NAME INS GRAT1 WLEN SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW NEIL2 H251 NEIL2 7 80 19 18 12 810 H25_4 PARALLEL PERPENDIC ULAR PARALLEL 0 10 0 180 3600 NODEFAULT 0 359 0 8 30 8 Spectrometer Filter Spectrometer Wavelength microns Relative Chop Nod Direc tion Random Jitter Width arc sec Total integration time sec Chopping Position Angle deg Chopping Amplitude arc sec VISIR User Manual VLT MAN ESO 14300 3514 46 VISIR_spec_obs_HRXAutoChopNod tsf To be specified Parameter Range Default La
11. CATALOGUE SETUP FILE NONE CATA LOGUE 0 359 0 8 30 8 ra dec 0 359 0 0 TA 0 TEL TARG ADDVELDELTAO 0 TEL TARG ALPHA TEL TARG DELTA TEL TARG EQUINOX TEL TARG OFFSETALPH ra dec 2000 0 A 0 0 TEL TARG OFFSETDELTA 0 0 Imager Filter Imager pixel scale Relative Chop Nod Direc tion 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 RA additional tracking ve locity DEC additional tracking ve locity RA blind offset DEC blind offset VISIR img acq Preset tsf To be specified Parameter Range Default Label TEL AG GUIDESTAR TEL GS1 ALPHA TEL GS1 DELTA TEL ROT OFFANGLE TEL TARG ADDVELALPE CATALOGUE FILE NONE LOGUE ra dec 0 359 0 0 1 0 SETUP CATA TEL TARG ADDVELDELTAO 0 TEL TARG ALPHA TEL TARG DELTA ra dec TEL TARG EQUINOX 2000 0 Get Guide Star from Guide star RA Guide star DEC Rotator on Sky PA on Sky RA additional tracking ve locity DEC additional tracking ve locity VISIR User Manual VLT MAN ESO 14300 3514 42 VISIR_spec_acq MoveToSlit tsf To be specified Parameter Range Default Label INS FILT2 NAME N SW N_LW ARIII NEIT1 Acquisition Filter NEIL NODEFAULT INS SLIT1 TYPE LONG SHORT LONG
12. 2 5 Brightness IMItationS s e soe ig a bk AA COON AI 4 7 Calibration observations lt lt lt 2 bao oe be Pew a e aS OBS Eeer cos A we a a ee aos ee Se Gee ee eRe See eo A9 Koon problems sc Boe bas beh ae Pe ARERR E DHE e EES eS 10 1 Decreased Image quay lt o sess so sussa 60 he we A ee 4 9 2 Low level stripes AA EN EEN raise a ES 290 Bad residuals 2445 dada Se SOE DREDGE EA EE Eo AGA Residuals of sky emission lines 24 42 264 44 254 44 eRe RS O G N H r A 10 11 11 11 12 12 13 13 15 VISIR User Manual VLT MAN ESO 14300 3514 A A oe A a ae he eRe SERS Rw eS bE OS eee Sx 5 VISIR data SC AE TOR escran eh Re Se Rs ee ee et BO ee iA ees or mej e eae ee ee ee ee oe ee eRe BES 5 3 VISIR spectrometer data o e o se ce CSS E DARA RED ka RE OR 6 VISIR templates description DL emgeet sr meei me aa wy EE AE Oo REE HH BS Observing with the Imager gt gt eca 114445 242 Bee Ee ARE Ee RES 6 3 Observing with the spectrometer e GA CalbraGon opuso he be eege SS oe eg ee SS eee SS eee cd 7 Checklist TLE Eresch ons 3k ea eR Boe Se Be eee SE ee A fee PUBS oes a e e a ew ae ew bere we wn ee we ES 8 Appendix VISIR template parameters S1 O iia Ae aw Ee ERw ESSERE ESSE ARA 8 2 Observado 52 4 65 364 a a eee a ew a 8 3 Calibration e 9 Appendix Filter transmission curves 10 Appendix Observed sensitivities in various spectroscopic settings 31 31 31 32 34 3
13. CHOP THROW 8 11 13 and 14 from left to right Note the presence of significant striping when the left beam hits some hot pixels at the lower left of the detector For the location of the object along the slit pixel X 123 at row Y 128 this occured for TEL CHOP THROW between 10 and 13 approximatively The horizontal lines at the middle of the images are caused by the lack of detector response at 8 8um VISIR User Manual VLT MAN ESO 14300 3514 18 Of fchop 0 25 Hz every Tenop 48 one VISIR image is stored as a plane in a data cube of a FITS file The number of chopping cycles within one nodding position is defined by the time spent integrating in that nodding position Thoa This nodding period is typically Thoa 90s for science observations The chopper frequency DIT and also Ti are predefined by the system The number of saved A B frames in one FITS file is Nes Thoa KE 1 The number of nodding cycles is computed from the total integration time as given by the observer The total number of stacked images for each secondary position respectively chopper half cycle is NDIT This parameter is computed according to NDIT 2 DIT fonop NDITSKIP 2 and is given by the system It depends on DIT chopping frequency and NDITSKIP some read outs at the beginning of each chopper half cycle are rejected during stabilization of the secondary Typical stabilization times of the secondary are 25ms The num
14. LONG 0 40 0 75 1 00 NODE FAULT PARALLEL PERPENDIC ULAR PARALLEL 30 3600 NODEFAULT CATALOGUE SETUP FILE NONE CATA LOGUE 0 359 0 8 30 8 ra dec 0 359 0 0 140 0 TEL TARG ADDVELDELTAO 0 TEL TARG ALPHA TEL TARG DELTA TEL TARG EQUINOX TEL TARG OFFSETALPH ra dec 2000 0 A 0 0 TEL TARG OFFSETDELTA 0 0 Acquisition Filter for the imager detector Acquisition Filter for the spectroscopy detector Spectrometer Slit Type long or short Spectrometer Slit Width arcsec Relative Chop Nod Direc tion 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 RA additional tracking ve locity DEC additional tracking ve locity RA blind offset DEC blind offset VISIR User Manual VLT MAN ESO 14300 3514 8 2 Observation 44 VISIR_img_obs_AutoChopNod tsf To be specified Parameter Range Default Label INS FILT1 NAME INS PFOV SEQ CATG SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW K BAND SIC PAHI ARI SIV1 SIV SIV2 PAH PAH22 NEI NEIL NEIL2 Q1 Q2 Q3 NODEFAULT 0 075 0 127 0 127 PRE IMAGE SCIENCE SCIENCE PARALLEL PERPENDIC ULAR PARALLEL 0 10 0 180 3600 NODEFAULT 0 359 0 8 30 8 Imager Filter Imager pixel scale O
15. Spectrometer Slit Type long or short INS SLIT1 WIDTH 0 40 0 75 1 00 NODE Spectrometer Slit Width FAULT arcsec SEQ CHOPNOD DIR SEQ TIME TEL AG GUIDESTAR TEL CHOP POSANG TEL CHOP THROW TEL GS1 ALPHA TEL GS1 DELTA TEL ROT OFFANGLE TEL TARG ADDVELALPEI PARALLEL PERPENDIC ULAR PARALLEL 30 3600 NODEFAULT CATALOGUE SETUP FILE NONE CATA LOGUE 0 359 0 8 30 8 ra dec 0 359 0 0 1 0 TEL TARG ADDVELDELTAO 0 TEL TARG ALPHA TEL TARG DELTA TEL TARG EQUINOX TEL TARG OFFSETALPH ra dec 2000 0 A 0 0 TEL TARG OFFSETDELTA 0 0 Relative Chop Nod Direc tion 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 RA additional tracking ve locity DEC additional tracking ve locity RA blind offset DEC blind offset VISIR User Manual VLT MAN ESO 14300 3514 43 VISIR_spec_acq_ImgMoveToSlit tsf To be specified Parameter Range Default Label INS FILT1 NAME INS FILT2 NAME INS SLIT1 TYPE INS SLIT1 WIDTH SEQ CHOPNOD DIR SEQ TIME TEL AG GUIDESTAR TEL CHOP POSANG TEL CHOP THROW TEL GS1 ALPHA TEL GS1 DELTA TEL ROT OFFANGLE TEL TARG ADDVELALPEI K BAND SIC PAHI ARI SIV1 SIV SIV2 PAH PAH22 NEIL1 NEIL NEIL2 Q1 Q2 Q3 NODEFAULT N SW N LW ARIII NEI NEII2 NODEFAULT LONG SHORT
16. 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 Guide Star and the USNO A 2 catalogues 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 In these cases examination of other catalogues such as the USNO B1 0 may provide suitable guide stars 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 VISIR User Manual VLT MAN ESO 14300 3514 26 e observations of objects within optically dark molecular clouds where few suitable guide stars are expected 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 astrometric accuracy is important In all these cases the use of the guidecam tool see http www eso org instruments visir doc for VISIR is strongly recommended and the coordinates of a suitable guide st
17. instrumental and atmospherical conditions The values for each filter given in Table 2 refer to the median of more than 600 different observations during September and December 2004 A graphical compilation is presented in Fig 5 for the N band and Q band imaging filters 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 4 to 6 list typical sensitivities measured in low medium and high resolution modes away from strong sky emission lines for the wavelength ranges offered in P76 Figures 20 to 36 in the Appendix 10 shows the dependence of sensitivity on wavelength The median sensitivities are the reference for classification of VISIR service mode observations 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 4 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 org observing etc is recommended t
18. observer it is fixed internally to ensure the best data quality Pointing position East Figure 17 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 A 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 21 4 2 3 Nodding parameters The nodding technique allows to switch from one field to another by offsetting the telescope by several tens of arc seconds It allows to correct for optical path residuals that remain after chopping 2 The nodding period is a parameter that can only be modified by the instrument operator For exposures shorter than 180s SEQ TIME lt 180s as possible in acquisition images the VISIR User Manual VLT MAN ESO 14300 3514 22 nodding time is set to half the requested exposure time For exposures longer than 180s the nodding time is set to 90s In particula
19. sky lines Atmosphere absorption correction The atmosphere does not uniformly absorb the MIR radiation 2 1 At some wavelengths it is completely transparent at others 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 Photometry Spectro photometric calibration of low and medium resolution spectra can be achieved with the MIR standard star list provided by the Observatory see 4 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 VISIR User Manual VLT MAN ESO 14300 3514 34 6 VISIR templates description 6 1 Acquisition Each OB needs to start with an acquisition template they are described in 3 4 3 6 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 filter pixel scale chopper throw which is in the range of 8 to 30 The keyword SEQ CHOPNOD DIR is set to PARALLEL or PERPENDICULAR which results in images as shown in Fig 21 PARALLEL considers an equal nodding and chopping amplitude which are both in parallel direct
20. top and Q band bottom Small and intermediate field observations are displaced for clarity Background noise limits are indicated for the individual filter bandpasses VISIR User Manual VLT MAN ESO 14300 3514 8 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 The 0 075 small field SF and 0 127 intermediated field IF pixel scale are offered Ta ble 1 These offered pixel fields of view pfov ensure a proper sampling of the images in the N and Q band pfov fov diffraction diffraction um pixels 0 127 32 5 x 32 5 94 1 88 0 075 19 2 x 19 2 159 3 18 Radius of first Airy ring at A 7 7um Table 1 VISIR imager pixel scales offered The pixel size of the DRS 256x256 detector is 50 um The first airy ring at A 7 7um corresponds to a radius of 0 24 on the sky The filter wheel is located just behind the cold stop pupil mask The list of filters offered is given in Table 2 The transmission curve
21. 10 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 VISIR User Manual VLT MAN ESO 14300 3514 3 10 0 Angular resolution arcsec 0 1 L L 1 1 1 L 1 1 1 i 1 f 5 10 5 20 25 30 Wavelength um Figure 2 VLT diffraction limit full line versus seeing The Spitzer Space Telescope diffraction limits dashed are shown for comparison The Roddier dependence is shown for two optical seeings dashed dot 2 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 In practice it is achieved by moving the secondary mirror of the telescope For VISIR at Paranal a chopping frequency of 0 25 Hz has been found to be adequate for N band imaging observations while 0 5 Hz are adopted for Q band imaging Spectroscopic observations are performed with lower chopper frequencies at 0 1 Hz or less The chopping technique cancels most of the background However the optical pat
22. 4 34 36 37 38 38 38 41 41 44 47 49 52 VISIR User Manual VLT MAN ESO 14300 3514 vi List of acronyms BIB BLIP BOB DIT ETC FWHM ICS IR TRACE MIR OB P2PP PAE pfov PSF S N UT VISIR TCS TMA WCU 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 Signal to noise ratio Unit telescope VLT imager and spectrometer for the mid infrared Telescope control system Three mirrors anastigmatic Warm calibration unit VISIR User Manual VLT MAN ESO 14300 3514 1 1 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 infrared MIR atmospheric windows the N band between 8 to 13 um and the Q band between 16 5 and 24 5 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 a
23. 4 5 Staring images Staring images A chopper position A chopper position Chopped images Nodding A beam Nodding B beam EE D B chopper position Sketch of mid infrared chopping and nodding technique of observation Chopped nodded image Final Image zoomed He2 10 blue compact galaxy Figure 4 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 VISIR User Manual VLT MAN ESO 14300 3514 6 2 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 in the small field 0 075 and intermediate field 0 127 by perpendicular chopping and nodding patterns with amplitudes of 10 Calibrators are frequently observed during the night 4 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 The growing calibration database allows a statistical analysis of the sensitivity with respect to
24. 60 Acquisition with an offset star has not been tested with the narrow 0 4 slit and should be avoided Note that the coor dinates of the target TEL TARG ALPHA TEL TARG DELTA and the offsets to the ref erence star TEL TARG OFFSETALPHA TEL TARG OFFSETADELTA must be indicated with the following convention TEL TARG ALPHA TEL TARG OFFSETALPHA RA offsetstar TEL TARG DELTA TEL TARG OFFSETADELTA DEC offsetstar 4 3 2 Description 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 in intermediate field only and therefore offers the possibility to acquire fainter objects in a larger variety of filters The observing parameters are described in 38 1 The effect of all acquisition templates is first to point the telescope so that the coordinates at the center of the detector match This 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 23 _ a SS E E Figure 18 Setting the correct values of the TEL 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
25. 7 S o L S 4000F 7 E L Es L S St 2000 7 LO itt W I 2 0 fi fi L 1 li 1 ji L fi li fi fi L ji i L i 9 00 9 05 9 10 9 15 Wavelength um Figure 25 Observed sensitivity as a function of wavelength for high resolution mode Offered sensitivity is typically a factor of 2 larger VISIR User Manual VLT MAN ESO 14300 3514 Sensitivity mJy 100 in 1h 8000 J 6000 4000 2000 10 48 10 50 10 52 Wavelength um 10 54 56 Figure 26 Observed sensitivity as a function of wavelength for high resolution mode Offered sensitivity is typically a factor of 2 larger Sensitivity mJy 100 in 1h 4000 3000 2000 1000 0 Wavelength um 11 540 11 545 11 550 11 555 11 560 11 565 Figure 27 Observed sensitivity as a function of wavelength for high resolution mode Offered sensitivity is typically a factor of 2 larger VISIR User Manual VLT MAN ESO 14300 3514 57 4000 3000 2000 1000 Sensitivity mJy 100 in 1h 0 E PC AC EN E RS PR E E E RC RECO E A E ine EA E PRA a E E TP E 11 745 11 750 11 755 11 760 11 765 11 770 11 775 Wavelength um Figure 28 Observed sensitivity as a function of wavelength for high resolution mode Offered sensitivity is typically a factor of 2 larger 4000F 4 3000 2000
26. A wider range of wavelengths is accessible with the high resolution cross dispersed mode with a 4 1 long slit VISIR User Manual VLT MAN ESO 14300 3514 13 Offered modes and sensitivities are given in Table 6 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 particular this feature allows to determine the dates when the emission line under study would appear at the same wavelength as a sky line mode Ae AA line order R dispersion sensitivity ym um pixels um Jy 100 1h HR 7 970 8 270 0 02420 H2_S4 17B 32000 10544 WK HR 12 738 12 882 0 03571 Ne II 11A 17000 7145 0 9 HR 16 800 17 200 0 05156 H2 S1 8B 14000 4950 lt 10 HRX 8 970 9 140 0 02270 ArII 16A 27100 11194 4 HRX 9 360 9 690 0 02325 H2 83 15A 25000 10974 5 HRX 10 480 10 540 0 03160 STV Coll 12B 24000 8044 4 HRX 11 540 11 570 0 03210 HD 0 0 R 10 12B 23400 8000 2 HRX 11 762 0 03260 CV 12A 19100 7840 3 HRX 12 210 12 760 0 03864 H2_S2 11B 20000 6604 1 5 HRX 12 814 13 364 0 03550 Nell 11A 17500 7150 2 HRX 16 390 0 03551 Coll 9A 17300 7260 12 HRX 16 925 0 05240 Col 8B 14100 4840 8 HRX 17 790 17 980 0 04707 PII Fell 8A 13140 5400 md HRX 18 246 0 04182 NI 8A 14600 6080 8 HRX 18 680 18 960 0 06569 SI 7B 11150 3870 md HRX 21 295 0 04196 NalV 7A 14300 6058 m9
27. AR PARALLEL 0 10 0 20 2000 NODEFAULT 0 359 0 8 30 8 Relative Chop Nod Direc tion Random Jitter Width arc sec Total integration time sec Chopping Position Angle deg Chopping Amplitude arc sec VISIR User Manual VLT MAN ESO 14300 3514 48 VISIR_spec_cal MRAutoChopNod tsf To be specified Parameter Range Default Label INS GRAT1 WLEN 7 5 28 08 NODEFAULT Spectrometer Wavelength microns SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW PARALLEL PERPENDIC ULAR PARALLEL 0 10 0 20 2000 NODEFAULT 0 359 0 8 30 8 Relative Chop Nod Direc tion Random Jitter Width arc sec Total 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 CHOPNOD DIR SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW NEIL2 H2S_1 NEIL2 7 80 19 18 12 810 H25_4 PARALLEL PERPENDIC ULAR PARALLEL 0 10 0 20 2000 NODEFAULT 0 359 0 8 30 8 Spectrometer Filter Spectrometer Wavelength microns Relative Chop Nod Direc tion Random Jitter Width arc sec Total integration time sec Chopping Position Angle deg Chopping Amplitude arc sec VISIR_spec_cal HRX A utoChopNod tsf
28. EUROPEAN SOUTHERN OBSERVATORY ES Organisation Europ ene pour des Recherches Astronomiques dans H misph re Austral Q 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 79 Date 30 11 2006 A Smette L Vanzi Prepared ges ob EEN Date Signature A Kaufer Approved a trace k th dE aes Date Signature 0 Hainaut Released REA Date Signature VISIR User Manual VLT MAN ESO 14300 3514 This page was intentionally left blank VISIR User Manual VLT MAN ESO 14300 3514 111 Change Record Issue Rev Date Section Parag affected Reason Initiation Documents Remarks 1 0 04 09 04 creation First release for science verification in P74 and OT proposals in P75 1 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 3 6 3 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 02 06 3 6 3 10 update for P78 CfP v78 19 06 06 cover 2 2 3 2 4 3 1 P78 releas
29. RALLEL Note that nodding on the slit requires to set the telescope rotator offset angle and the M2 chopping position angle to the same value which 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 resolution 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 central wavelength INS GRAT1 WLEN the slit width INS SLIT1 WIDTH and SEQ CHOPNOD DIR 6 2 High resolution long slit mode Template for high resolution spectroscopy is VISIR_spc_obs_HRAutoChopNod Three order sorting filter at 8 02 12 81 and 17 03um INS FILT2 NAME H2_S4 Ne II H2 S1 are available See Table 6 for the corresponding list of offered central wavelengths Other observ ing parameters are total integration time SEQ TIME central wavelength INS GRAT1 WLEN the slit width INS SLIT1 WIDTH and SEQ CHOPNOD DIR 6 2 High resolution cross dispersed mode VISIR_spc_obs_HRXAutoChopNod is functionally similar to VISIR_spc_obs_HRAutoChopNod VISIR User Manual VLT MAN ESO 14300 3514 37 but uses a grism for cross dispersi
30. User Manual VLT MAN ESO 14300 3514 35 of nodding cycles Neyci_noa is computed according to the total observation time 8 3 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 SEQ TIME specified refers to only one nodding pair The total observing time is given by the product of SEQ NOFF x SEQ TIME 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 Preset Reference Position A Figure 22 Illustratio
31. _cal_LRAutoChopNod template 6 will be used SEQ TIME 180 sec TEL CHOP POSANG 0 TEL CHOP THROW 8 SEQ CHOPNOD DIR PARALLEL The wavelength setting INS GRAT1 WLEN and 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 to 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 both imaing and spectroscopy day calibrations of VISIR are performed with an extended source that mimics a black body with adjustable flux by regulating its temperature For most instrument modes a corresponding Hat Deld is recorded which consists of a series of images with different background levels Exceptions are SiC in the Intermetiate Field and all imaging obtained with the spectroscopy detector for spectroscopy acquisition Bad pixels gain maps and fringing patterns can in principle be derived from 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 experimen
32. ally in the spatial and vertically in the dispersion direction cf Fig 20 For the LR and MR modes the short wavelength appear at the top of the frames 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 5 2 Pipeline The VISIR pipeline has been developed by ESO DMD and uses the ESO CPL library The main observation templates are supported by the pipeline reductions Raw images of imag ing and spectroscopic observations are recombined Spectra are extracted and calibrated in wavelength 85 3 for all spectroscopic modes in low medium and high resolution Sensi tivity estimates based on standard star observations are provided both in imaging and spec troscopy 4 7 Public release of the VISIR pipeline is accessible at http www eso org instruments visir VISIR User Manual VLT MAN ESO 14300 3514 32 The pipeline currently supports the following templates e VISIR img obs_AutoChopNod e VISIR_spec_obs_LRAutoChopNod e VISIR spec_obs_MRAutoChopNod e VISIR_spec_obs HRAutoChopNod e 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 5 3 VISIR spectrometer data Optical distortion correction Spectra are deformed by optical distortion and slit curvatures The VISIR spectrograph
33. ar should be inserted in the acquisition templates 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 4 5 Brightness limitations There are currently no brightness limitations 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 3 4 4 6 Overheads The VLT overhead for one OB which includes active optics setting selection of guide star field stabilization is 6 min VISIR instrument configur
34. ations 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 acquisi tion of a bright sources gt 1 Jy 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 50 of the observing time The total observing time requested by the observer must include telescope and instrument overheads VISIR User Manual VLT MAN ESO 14300 3514 27 4 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 on a timescale of 3h Observations of photometric standards will be provided by the observatory within 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
35. beams VISIR User Manual VLT MAN ESO 14300 3514 10 3 2 Spectrometer VISIR offers slit spectroscopy at three spectral resolutions with a pixel scale of 0 127 This is obtained by means of two arms one with low order gratings for the low and medium spectral resolution the other with large echelle gratings providing high spectral resolution LT FOCAL DIAPHRAGM PLANE WHEEL O A B C TE IMAGER COLDSTOP Z WHEEL V WHEEL LMR GRATING UN G UNIT SS pa ee HR 4 GRATING SCANNERS IIT EEL DUO ECHELLE 1 RETURN FLAT GRATING UNIT NY ESOLUTION SELECTION MECHANISM UVR COLLIMATOR N CAMERA R COLLIMATOR CAMERA Figure 7 Schematic layout of the design of the VISIR spectrometer The long slits have a length of 32 5 and therefore cover the whole width of the detector The short slits only used in high resolution cross dispersed mode have a length of 4 1 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 VISIR spectrometer design is shown in Fig 7 The 3mirror 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 125mm Both subsystems image the spectrum onto the same detector selection between the two spectrometer arm
36. bel INS GRAT1 WLEN 7 60 28 08 VODEFAULT Spectrometer Wavelength microns SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW PARALLEL PERPENDIC ULAR PARALLEL 0 10 0 180 3600 NODEFAULT 0 359 0 8 30 8 Relative Chop Nod Direc tion Random Jitter Width arc sec Total integration time sec Chopping Position Angle deg Chopping Amplitude arc sec VISIR User Manual VLT MAN ESO 14300 3514 8 3 Calibration AT VISIR_img_cal_AutoChopNod tsf To be specified Parameter Range Default Label INS FILT1 NAME INS PFOV SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW K BAND SIC PAH1 ARII SIV_1 SIV SIV_2 PAH2 PAH22 NEII NEIL NEIL2 Q1 Q2 Q3 NODEFAULT 0 075 0 127 0 127 PARALLEL PERPENDIC ULAR PERPENDICU LAR 0 10 0 20 2000 NODEFAULT 0 359 0 8 30 8 Imager Filter Imager pixel scale Relative Chop Nod Direc tion Random Jitter Width arc sec Total integration time sec Chopping Position Angle deg Chopping Amplitude arc sec VISIR_spec_cal_LRAutoChopNod tsf To be specified Parameter Range Default Label INS GRAT1 WLEN 8 1 8 5 8 8 9 8 11 4 12 2 12 4 Spectrometer Wavelength NODEFAULT microns SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW PARALLEL PERPENDIC UL
37. ber of rejected exposures is given by NDITSKIP Similar during stabilization after each telescope movement respectively nodding position a number NCYSKIP of chopping cycles is ignored The timing organization of data is shown in Fig 15 The total on source integration time is tsource 4 Neycl nod g Neycl chop NDIT DIT 3 The total rejected time is tsxip 4 Nach aen DIT NDITSKIP Neycl noa NDIT NCYSKIP 4 and the total observing time is ttot tsource tskip 5 Typical duty cycles tsource ttot are about 70 4 Observing with VISIR at the VLT 4 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 This advice is particularly relevant for spectroscopic settings with wavelengths centered at 8 8um as they will be strongly affected by the detector feature at this wavelength VISIR User Manual VLT MAN ESO 14300 3514 19 neysk p Noe Shop nevskip Neyel_chop Aq gt An Bn Bn An al 7 i S T no
38. bservation Category Relative Chop Nod Direc tion Random Jitter Width arc sec Total integration time sec Chopping Position Angle deg Chopping Amplitude arc sec VISIR img obs GenericChopNod tsf To be specified Parameter Range Default Label INS FILT1 NAME INS PFOV SEQ CATG SEQ JITTER WIDTH SEQ NOFF SEQ OFFSET COORDS SEQ OFFSET1 LIST SEQ OFFSET2 LIST SEQ TIME TEL CHOP POSANG TEL CHOP THROW K BAND SIC PAHI ARI SIV1 SIV SIV2 PAH2 PAH22 NEI NEIL NEIL2 Q1 Q2 Q3 NODEFAULT 0 075 0 127 0 127 PRE IMAGE SCIENCE SCIENCE 0 10 0 1 100 NODEFAULT SKY DETECTOR NODE FAULT NODEFAULT NODEFAULT 180 3600 NODEFAULT 0 359 0 8 30 10 Imager Filter Imager pixel scale Observation Category Random Jitter Width arc sec Number of offset positions Offset coordinates List of offsets in RA or X List of offsets in DEC or Y Total integration time sec Chopping Position Angle deg Chopping Amplitude arc sec VISIR User Manual VLT MAN ESO 14300 3514 45 VISIR_spec_obs_LRAutoChopNod tsf To be specified Parameter Range Default Label INS GRAT1 WLEN 8 1 8 5 8 8 9 8 11 4 12 2 12 4 Spectrometer Wavelength NODEFAULT microns SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW PARALLEL PERPENDIC ULAR PARALLEL 0 10 0 180 3600 NODEFAULT 0 359 0 8
39. bytes long integer Thus one read out has a size of 262kB During each chopping cycle the elementary exposures are added in real time and only the result is stored on disk At a chopping frequency VISIR User Manual VLT MAN ESO 14300 3514 16 2 0x1 a E Ge Ser 1 o _ _ _ mg sd 1 8x1 Se g eg D An s SKIO ant 4 E F J FR a ae 1 3x1 Sa Es 3 Es vi fa 10x10 F F 4 H a 4 a a a E af e a n6 af E 5 0x10 Be 7 F e L e L a e E e o Oe f A Ed n 0 2 4 6 8 0 0 0 2 0 4 ae integration time s integration time s Figure 11 Linearity curve of the detector in the large left and small right capacity modes The break in the response at 2 3 at 1 8 107 e7 of the large and at 1 9 10 e7 of the small capacity are indicated by full lines The top lines indicate the well capacities SPECTROMETER Figure 12 Bad pixel maps of the imager left and spectrometer right detectors The large grey rectangular areas correspond to pixels masked electronically in order to decrease detector striping VISIR User Manual VLT MAN ESO 14300 3514 17 Figure 13 The DRS detector shows stripes and repeating ghosts for very bright sources left The ghosts are distributed every 16 columns For other sources striping is not apparent right Figure 14 Sequence of chop nod reduced spectra obtained in the Medium Resolution mode with a central wavelength 8 8m The TEL
40. 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 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 MoveToSlit the first acquisition images are obtained with the OPEN 15 3 slit For VISIR_spec_acq_ImgMoveToSlit the first acquisition images are obtained with the imager detector Intermediate Field Then e The VISIR_img_acq MoveToPixel VISIR_spec_acq MoveToSlit and VISIR_spec_acq_ImgMoveToSlit requires 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 the center 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 det
41. ctor feature at 8 8 um Dots indicate individual observations full lines represent median and the dashed line the best sensitivities Theoretical model curves correspond to BLIP Bottom Sensitivity measured in the bluer setting centered at 8 1um VISIR User Manual VLT MAN ESO 14300 3514 93 2000 TT 1500 1000 900 Sensitivity mJy 100 in 1h 7 50 7 60 7 70 7 80 7 90 8 00 Wavelength um 1000 800 600 400 du Sensitivity mJy 100 in 1h 0 pe eh IE MES VO EE 8 0 8 2 8 4 8 6 8 8 9 0 9 2 Wavelength um Figure 21 Observed sensitivity as a function of wavelength for different settings in the medium resolution mode obtained in very good weather conditions I for 7 5um lt lt 9 4ym Offered sensitivity is typically a factor of 2 larger VISIR User Manual VLT MAN ESO 14300 3514 54 1000 gt AAA 7 8005 F S S E 6007
42. curacy 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 Acquisition with the VISIR_img_acq MoveToPixel or VISIR_spec_acq ImgMoveToSlit tem plates can make use of the K BAND filter for which a preliminary conservative limiting magnitude is 19 in 3600s on source integration for a S N 5 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 25 e imaging in the small field in some conditions an error of less than 10 on the coordi nates can bring the target outside of the field e spectroscopic acquisition in some conditions an error of less than 7 5 on the coordi nates 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 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 g
43. d NDITSKIP NDIT NDITSKIP ae rat DIT Ac Bc i Ac Bc T_chop Figure 15 Data timing in VISIR Ac and Bc refer to the two chopper positions An and Bn refer to the two nodding telescope positions Note the AnBnBnAn cycle sequence for the nodding to save observing time 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 http www eso org observing p2pp P2PP tool 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 available VISIR templates and their parameters is given in 6 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 observing p2pp ServiceMode html the following VISIR re quirements 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 may be preferable 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
44. dary mirror of the telescope It allows to alternatively observe a field then another field VISIR User Manual VLT MAN ESO 14300 3514 21 offset from the first by a chopping distance or throw called TEL CHOP THROW see Fig 17 This parameter can be set by the user To avoid chopping inside the object it is recommended to use a chopping and nodding throw which is 1 5 times larger than the estimated MIR diameter of the object In the case of point sources the throw is usually set around 10 to ensure proper separation of the different beams The maximum chopping throw at the VLT is 30 and the minimum is 8 For good image quality and good background cancelation chopping and nodding throws below 15 are recommended see 4 9 1 Note that for chopping throws larger than the field of view the negative beams will not be seen on the detector and the integration times have to be adjusted accordingly The chopper position angle PA is the angle of chopping counted East of North see Fig 17 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 21 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
45. der 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 6 with the following settings 3Cohen et al 1999 AJ 117 1864 VISIR User Manual VLT MAN ESO 14300 3514 28 SEQ TIME 180 sec for N and 360sec for Q band TEL CHOP POSANG 0 TEL CHOP THROW 10 SEQ CHOPNOD DIR PERPENDICULAR Filter INS FILT1 NAME and pixel scale INS PFOV will be set according to the science observa tions 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 with an airmass difference no larger than 0 2 AM respect to the science target Such a calibration measurement will be performed at least once per night per instrument configu ration More precisely the following settings of the VISIR_spec
46. e v79 30 11 06 4 8 P79 release v1 0 v1 1 v76 1 edited by R Siebenmorgen E Pantin M Sterzik v76 2 4 v77 1 3 updated by A Smette VISIR User Manual VLT MAN ESO 14300 3514 Contents 1 Introduction 2 Observing in the MIR from the ground St The Parra atmosphere dai ur OSS POR Sw Er ee RES 22 ARG spatial resoluti n ira AAA A AR La ER Dackoround ea rodear ed rra aert die 24 Acheter and nodding s s sos See wa ROE HERE EN PERE HER Do BENIN A soe ace as a ee ee he e e ge Instrument description and offered observing modes eck AUS aaa A A A A A E we ES o EEE hr AMES cis a RR OR RA A RE A 3 22 Resolution e cae ara AAA A 3 2 3 Low resolution offered central wavelengths 3 2 4 Medium resolution offered central wavelengths 3 2 5 High resolution offered central wavelengths 39 Calibrafion WIS o aa ee RA ee A Pe Bae Ea Oe Detects ete Sea rs ia aora a 20 Data CCW Sere and a ees REA A OR A Observing with VISIR at the VLT 2 1 Proposal preparation o seco soosoo ee ee ee De ee Dee ee 4 2 Telescope observing parameters 4 2 1 Instrument orientation on thesky 42 665 2255 4846446 42 2 Chopping parameters 4 a s tuad HE ER Ee E ES e A h 42 3 Nodding parameters oscense Re eS da Target acquisition oe s orce oe Se AR AE RAE ee ES Aad EE a ae Se e ee ee eh RSS e Set E RE Ae e E 432 TCS socs e en a E a MA Eege SETS csm is sri naur edhe bed eras
47. e blue wing of the ozone band off the wavelength range covered by the detector at the expense of having a larger part of the detector covering the red wing of the water vapour band The 73 Zum setting avoids the CO2 band Additionally a bluer setting centered at 8 1 um covers the spectral range down to the trans mission cut off at 7 lum It aims at covering the 7 6 um methane band and 7 7 wm PAH feature VISIR User Manual VLT MAN ESO 14300 3514 12 In all cases classification of OBs in service mode will be based on the clean parts of the spectra Note that the exposure time calculator ETC cannot currently provide sensitivity estimates for A lt 7 6u range Ac grating spect resol dispersion um um order measured 1 slit pixels um 7 1 8 7 8 1 2 300 390 160 02 7 7 9 3 8 5 2 300 390 160 02 8 0 9 6 8 8 2 300 390 160 02 9 0 10 6 9 8 2 305 360 160 05 10 34 12 46 11 4 1 185 220 119 94 11 14 13 26 12 2 1 215 250 119 96 11 34 13 46 12 4 1 215 250 119 96 Table 4 VISIR low resolution offered settings The first column gives the wavelength range of a spectrum for the central wavelength Ae listed in the 2nd column The measured sensitivities are 50mJy at 100 1h in the clean regions of the spectrum cf Fig 20 for a slit width of 1 Offered slits have widths of 0 40 0 75 and 1 00 The spectral resolution of the 8 1m 8 5um and 12 2um settings has not been independently
48. ector in both X and Y for VISIR_spec_acq MoveToSlit at the center of the chosen slit for VISIR_spec_acq ImgMoveToSlit at the center of the imager detector Spectroscopic acquisition using the imager detector with the VISIR_spec_acq_ ImgMoveToSlit template is limited to airmass smaller than 1 4 and slit witdh of 0 75 and 1 00 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 VISIR User Manual VLT MAN ESO 14300 3514 24 As part of the execution of the VISIR_spec_acq_MoveToSlit and VISIR_spec_acq_ImgMoveToSlit templates 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 e If the target coordinates are well known VISIR imaging modes allow to perform b
49. ength with time has also been observed Post processing involving wavelength calibration of individual chopping cycles may be necessary for optimal results 4 9 5 Fringes The DRS detector shows fringes which are generated in the detector substrate One example of such fringes is shown in Fig 20 for the medium resolution mode The fringes are stable and are not apparent in chopped images but the spectra are modulated Division of the extracted spectra by standard star spectra simultaneously removes most of the fringes and corrects for telluric features VISIR User Manual VLT MAN ESO 14300 3514 31 Figure 20 VISIR spectrum in staring medium resolution mode showing the detector fringing white The detector absorption feature at 8 8 um is visible as black horizontal bar cf Fig 9 Dark vertical stripes are caused by the non uniform gain of the different electronic amplifiers These features are largely removed by chopping 5 VISIR data 5 1 Data format One FITS file is saved for each telescope nodding position This file is a data cube and contains for each chopping cycle 1 each half cycle frame of the on source position A of the chopper 2 the average of the current and all previous A B chopped frames In addition the last plane of the cube contains the average of all chopped frames For the default value of the rotator angle 0 images are oriented North up and East left Spectroscopic data are aligned horizont
50. for a 10um 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 x The target will be the brightest source in the field of view at 10um x At 10um there will be two bright sources in our field of view The science target is the southernmost of these two 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 http www eso org instruments visir doc See 4 4 VISIR User Manual VLT MAN ESO 14300 3514 20 Note that observations close to zenith during meridian crossing should be avoided because of fast tracking speeds that do not allow proper background cancelation after nodding Questions related to the VISIR Phasel and Phase 2 observing preparation should be directed to the User Support Department usd help eso org 4 2 Telescope observing parameters 4 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 rotated by 90 respective to the imager so that the North is to the left and the East to the bottom with the slit orientation along the North South direction Since VISIR is mounted on a rotator at the Cassegrain focus of Meli
51. g increases with wavelength as 1 22 A D where A is the observing wavelength and D the diameter of the telescope mirror see solid line in Fig 2 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 x A7 2 see dot dashed lines in Fig 2 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 3 2 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 emits 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 5 6K and the interior of the cryostat at 33K The background radiation at 10m is typically my 5mag arcsec 3700 Jy arcsec and at 20m mo 7 3mag arcsec 8300 Jy arcsec Consequently the number of photons reaching the detector is huge often more than
52. gain maps will appear as low level stripes Such stripes tend to smooth out on long integrations 4 9 3 Bad residuals The chopping and nodding technique does not always lead to a satisfactory removal of all the structures seen in individual images Bad residuals have been found to occur in the following situations e in observations carried out close to zenith and to a lesser extent close to the meridian in general the likely cause is the fast rotation of the field relative to the telescope structure e in variable atmospheric conditions In addition it seems that imaging of extended objects are also more likely to be affected by low level bad residuals similar to fringes in some aspects whose orientation on the images changes at the same angular velocity as the rotator The origin of these structures is not understood 4 9 4 Residuals of sky emission lines In spectroscopy the scanners of the grating units may still show a small residual motion at the beginning of an exposure or mainly for the HR or HRX modes show some jitter after a nodding offset The first few frames at a given wavelength setting may therefore show stronger than expected residuals at the wavelength of the sky emission lines more exactly of the wings of sky emission lines For the HR and HRX modes the residuals of the scanner jitter tend to cancel out on long integrations and lead to a very slight decrease of the spectral resolution An overall drift of wavel
53. ght nebula that saturates the digitized sky survey DSS used by the telescope and instrument operator VISIR User Manual VLT MAN ESO 14300 3514 39 The guidecam tool see http www eso org instruments visir doc can help in selecting appropriate guide stars 4 Calibrations For calibration OBs use the appropriate VISIR_img_cal_AutoChopNod or VISIR_spc_cal_LR MR HR HRXAutoChopNod templates 5 Position angle If the observations must be carried out at a position angle different from 0 check 4 2 1 and 4 2 2 In particular it is useful to clearly indicates 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 40 VISIR User Manual VLT MAN ESO 14300 3514 41 8 Appendix VISIR template parameters 8 1 Acquisition VISIR img acq_MoveToPixel tsf To be specified Parameter Range Default Label INS FILT1 NAME INS PFOV SEQ CHOPNOD DIR SEQ TIME TEL AG GUIDESTAR TEL CHOP POSANG TEL CHOP THROW TEL GS1 ALPHA TEL GS1 DELTA TEL ROT OFFANGLE TEL TARG ADDVELALPEI K BAND SIC PAHI ARIII SIV 1 SIV SIV2 PAH2 PAH22 NEI NEIL NEIL2 Q1 Q2 Q3 NODEFAULT 0 075 0 127 0 127 PARALLEL PERPENDIC ULAR PARALLEL 30 3600 NODEFAULT
54. h 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 4 Depending on the choice of chopping and nodding amplitudes and directions up to 4 images of the source can be seen on the 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 VISIR User Manual VLT MAN ESO 14300 3514 4 a E ac e 0 6 De D SE i on DOE i 3 7 e BW F we 7 i as m D Se D Zo F a aD an KR o4 L ane ae NA Dec aa at H y 0g z a an 2 7 Al 7 A o P J es 02 7 e KR 4 t ae L oe x B Pa LI JE A A E A fe Z F NW Ka ac S a z 0 4 A a Ki O w P 4 r D bh D a 4 LE ba g A po gs a D D e e a a a m Li ma A DAA F 4 TA a DTA we Ee 45 O 7 rs 0 2 a E O e el d Ax C 1 fl d 0 5 1 1 5 2 FWHM VIS Figure 3 Measures of the VISIR image quality versus optical seeing obtained during 2005 The dashed lines indicates the prediction of Roddier s formula VISIR User Manual VLT MAN ESO 14300 351
55. hrow does the S N take into account the fact that some beams would fall outside the 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 the target and mode in the GTO protected list see http www eso org observing proposals gto is there a guide star brighter than V 13 5 mag within a radius of 7 5 arcmin around the object 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 4 3 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 bri
56. ical seeing conditions have been carried out The images shown in Fig 19 were obtained with optical seeing at zenith of 1 2 and 1 0 respectively They were obtained with TEL CHOP POSANG 270 deg and TEL CHOP THROW 29 The filter was PAH2 in the Small Field In chop position A the image appears symmetric with a FWHM close to the expected limited for diffraction limited imaging However in chop position B the image is affected by a coma caused by the motion of the M2 which decreases the image quality mainly in the chopping direction and adds significant structure Figure 19 Image of a star obtained in the PAH2 filter in the Small Field 0 0757 pixel and with TEL CHOP THROW 25 SEQ CHOPNOD DIR PARALLEL and TEL CHOP POSANG 2700 Left Chopping Position A the image is symmetric with a FWHM close to the one expected given the optical seeing conditions Right Chopping Position B the wings of the image shows a coma caused by the motion of M2 VISIR User Manual VLT MAN ESO 14300 3514 30 4 9 2 Low level stripes The background level of individual DIT images fluctuates not only with the varying sky background but also with the detector temperature The latter follows the 1Hz period of the closed cycle cryo coolers The mean background level in two consecutive half cycle frames corresponding to the two chopper positions may therefore not be equal If this difference is larger than a few tens of ADUs structures in the
57. ion It is recommended for faint extended sources for which the spatial resolution is not so crucial PERPENDICULAR considers 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 detector will move to the West This technique is recommended for point or relatively small extended lt 5 sources Fig 4 ees Nodding Position A Nodding Position B Nodding Position A Nodding Position B Figure 21 Schematic drawing of the content of a frame obtained with TEL ROT OFFANGLE TEL CHOP POSANG O 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 The chopping period is set by the system and the nodding period is fixed to 90s The number VISIR
58. iven by 2MASS are 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 Is 4 4 Guide stars Guide stars are mandatory for active optics and field stabilization Any VLT programme should make sure that a guide star USNO A 2 or Guide Star Catalog 2 with a V 11 13 mag is available within a field of 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 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
59. lind preset observations with the VISIR_img_acq_ Preset template In this case no acquisi tion images are taken 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 change the parameters TEL TARG ALPHA and TEL TARG DELTA so that they are offset by half the TEL CHOP THROW values to south and west for TEL ROT OFFANGLE TEL CHOP POSANG 0 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 ROT OFFANGLE TEL CHOP POSANG 0 TEL TARG OFFSETALPHA and TEL TARG OFFSETDELTA should be positive in order to reproduce the scheme shown in Fig 21 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 ac
60. measured values for the 8 8um and 12 4um settings are reported instead 3 2 4 Medium resolution offered central wavelengths In Medium Resolution mode the central wavelength A can be freely chosen within the wave length ranges listed in Table 5 Note that the exposure time calculator ETC cannot currently provide estimates of S N for A lt 7 6um Table 5 provides offered sensitivities A range AX grating spect resol dispersion sensitivity um um order measured 1 slit pixels um mJy at 100 in 1h 75 80 0 195 2 3500 1315 1000 8 0 9 3 0 188 2 3500 1360 200 10 2 13 0 0 21 2 3500 1450 1040 200 17 1 19 0 037 1 1800 695 1200 20 12 0 36 1 1800 720 2000 Table 5 VISIR medium resolution settings The first column gives the minimum and maxi mum allowed values for the central wavelength Ae in the given setting The wavelength range per setting in given in the 2nd column AA The spectral resolution measured with a 1 slit is given in the 3rd column The dispersion is given in the 4th column Typical offered sensi tivites are given in the last column Examples of dependence of sensitivity with wavelength are shown in Figures 21 to 24 Offered slits have widths of 0 40 0 75 and 1 00 3 2 5 High resolution offered central wavelengths The VISIR spectrometer offers a high resolution long slit mode for 3 passbands centered in the wavelengths of the H2_S4 Nell and H2_S1 lines
61. n of generating raster maps with VISIR_img_obs_GenericChopNod An illustration of generating an raster map can be found in Fig 22 The following parameters correspond to this setting VISIR User Manual VLT MAN ESO 14300 3514 36 SEQ NOFF 3 SEQ OFFSET1 LIST 30 10 10 SEQ OFFSET2 LIST 30 10 10 SEQ OFFSET COORDS SKY Note that depending on choice of the integration time SEQ TIME several nodding cycles might result e g pattern like AB1BIAABIB1A AB2B2AAB2B2A AB3B3AAB3B3A Currently 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 keyword must be set to PRE IMAGE In addition the name of the OB must start with the prefix PRE 6 3 Observing with the spectrometer Conceptually the same observing techniques applies for spectroscopy as well as for imaging The default slit orientation is in 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 PA
62. nd in y direction by n 2 A 2Ra A It is corrected along the cross dispersion falo y a y sign e Re yR x 5 0 9 VISIR User Manual VLT MAN ESO 14300 3514 33 and along the dispersion fs z y a sign A Ra VRA y y A 0 10 Finally the origin of the coordinate system is moved back from the fix point to 1 1 n 1 pte 2 977 fel y x 11 Spectral extraction is similar to the TIMMI2 pipeline and described by Siebenmorgen et al 2004 AA 414 123 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 chopper half cycle frames which are dominated by sky emission lines are stored 5 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 The chopped frames cannot be used for calibration with atmospheric lines because the chop ping process results in a near perfect cancelation of
63. nsists of read out noise and fixed pattern noise which are both independent of the detector integration time DIT At the operating temperature of the detector 6K 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 It is at least 6 times lower than the photon noise for the spectrometer and negligible for the imager 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 1 9 10 e7 in small and 1 8 107 e in large capacity modes respectively For background limited noise performance BLIP the optimal operational range of the detector is half of the dynamic range for the large capacity and between 1 2 and 1 5 for the small capacity The detector is linear over 2 3 of its dynamic range Fig 11 and its working point is set in the middle of the dynamic range During commissioning it was found that for about half of the array the gain does not differ by more than 2 peak to peak By comparison with other limitations flat field corrections which are difficult to implement in the MIR are not considered important
64. o estimate the on source integration time 3 Instrument description and offered observing modes VISIR offers two spatial scales in imaging and several spectral resolution modes in slit spec troscopy The imager and spectrograph 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 Gifftord McMahon closed cycle coolers are used to maintain the required temperature 33K for most of the structure and optics and lt 15K for the parts near the detector The detectors are cooled down to 5 6K 3 1 Imager The imager is based on an all reflective design The optical design is shown in Fig 6 It consists of two parts VISIR User Manual VLT MAN ESO 14300 3514 7 AAA EE Amedan small field 1 00 E median intermed field ma e Y E El C y 7 o y L A i i Me 4 gt a a y Y Y ved 10 i 6 o 4 S E SS El de 2 e 3 ER D A o L R ARIII SIV PAH2 PAH2_2 NEII 7 PAH1 SIV_1 SIV_2 SIC NEI NEII_2 REENEN EEN Pr a ad AAA Ki 8 9 1 11 12 13 wavelength um A AAA A Amedan small field median intermed field A y 3 100 E 4 7 TZ C n Y J L A J lo S A m J D a gt Ge C v 105 C Q1 Q2 Q3 J 17 0 175 180 185 19 0 19 5 20 0 wavelength um Figure 5 Sensitivities for the VISIR imager for the N
65. olution mode Offered sensitivity is typically a factor of 2 larger VISIR User Manual VLT MAN ESO 14300 3514 61 6000 5000 4000 3000 2000 1000 Sensitivity mJy 100 in 1h O on ES Ob ae o Elle 18 65 18 70 18 75 18 80 18 85 18 90 18 95 Wavelength um Figure 35 Observed sensitivity as a function of wavelength for high resolution mode Offered sensitivity is typically a factor of 2 larger DOK Oe fe EE AAA rrr 1 5x104 1 0x10 5 0x108 Sensitivity mJy 100 in 1h 21 280 21 290 21 300 21 310 Wavelength um Figure 36 Observed sensitivity as a function of wavelength for high resolution mode Offered sensitivity is typically a factor of 2 larger
66. on and order separation See Table 6 for a list of offered wavelengths Note that the effective length of the spectrograph slit is limited to 4 Total integration time SEQ TIME the slit width INS SLIT1 WIDTH and SEQ CHOPNOD DIR are specified as usual 6 2 6 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 38 7 Checklist This section provides a number of advice regarding the preparation of the proposal 7 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 t
67. pal 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 e counted positively from north to east within the range 0 to 360 then setting TEL ROT OFFANGLE 360 PA allows one to have both A and B objects on the slit A more general case is illustrated in Figure 16 The object B is located at a Position Angle on the sky of PA 162 relative to A see left side graph on the figure One wishes to bring it at an apparent location on the detector indicated by the square F The angle F A B counted east of north is O PA 90 252 Therefore one needs to set TEL ROT OFFANGLE 360 O 108 in order to obtain the requested final orientation as shown on the right side graph of Fig 16 Figure 16 Setting the correct value of the TEL ROT OFFANGLE parameter Left side Object B located at a position angle PA 162 is to be brought at the apparent location F on the detector Here O PA 90 Therefore TEL ROT OFFANGLE 360 O 108 Right side Requested final orientation after completion of the telescope setup 4 2 2 Chopping parameters The chopping technique as described in 2 is based on beam switching using the moving secon
68. pect to the diffraction limit around 10m the spectral resolution of a point source spectrum is better than the one of the sky spectrum in addition the zero point of the wavelength calibration will be affected by an incorrect centering of the object within the slit 3 2 2 Resolution In the N band the low resolution and medium resolution modes provide spectral resolving power of 300 Table 4 and 3000 Table 5 respectively In high resolution long slit mode narrow wavelength ranges around the 8 02 H2_S4 12 813 Ne II and 17 03 ym H2_S1 line are offered With the 1 slit the measured spectral resolution is R 15000 Table 6 and a minimum flux in an emission line below 107 W m arcsec can be achieved This value cor responds to an approximate sensitivity limit around 1 Jy in the continuum A high resolution cross dispersed mode with a 4 1 short slit is available for a number of wavelength settings Table 6 Please consult http ww eso org instruments visir for the latest update of the list of offered modes and slits 3 2 3 Low resolution offered central wavelengths The clean part of the N band can be covered in 4 settings with central wavelengths 8 8 9 8 11 4 and 12 4 ym Two additional settings at 8 5 and 12 2 um are offered as alternative to the 8 8 and 12 4 um settings respectively mostly for cosmetic reason as they avoid strong sky emission lines to fall on bad pixels Indeed the 8 5um setting moves th
69. r exposure time given in the template will be internally changed by the software to be the closest to a multiple of 90s 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 21 In imaging more flex ibility on the nodding offsets are possible with the VISIR_img_obs_GenericChopNod template 4 3 Target acquisition 4 3 1 Introduction Observing blocks must start with an acquisition template Pointing to a target can only be performed through an acquisition template Target coordinate 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 offset star for precise acquisition see Fig 18 To guarantee proper centering within the slit when using an offset star the angular separation between the offset star and the target should not be larger than
70. re 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 ym 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 0 3 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 2 83 provides a technical description of VISIR and its offered observing modes offered An overview on how to observe with VISIR at the VLT can be found in 4 A description of the structure of the imaging and spectroscopic data files is given in 5 A checklist to help the preparation of OBs is available in 87 Acquisition observing and calibration templates are explained in 6 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 usd help eso org 2 Ob
71. s is done by two pairs of folding flat mirrors In front of the actual spectrometer subsystems is a reflective re imager consisting of two off axis paraboloids and three folding flats The re imager provides a 16mm 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 15 3 named OPEN in P2PP It gives the possibility to make imaging with the spectrometer detector and is used for object acquisition and centering on the detector The list of available filters for spectroscopic acquisition offered is given in Table 3 together with their measured bandpasses and approximate sensitivities for image acquisition VISIR User Manual VLT MAN ESO 14300 3514 11 filter Ac halt band width sensitivity um um mJy 100 1h NSW 8 85 1 85 40 NLW 12 1 1 9 40 All 8 94 0 11 200 NelL_1 12 35 0 50 80 Nell 2 12 81 0 10 50 Table 3 VISIR spectrometer filter characteristics The filters 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 3 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 res
72. s of the filters measured at 35K are plotted in the Appendix p entrance window diaphragm focal plane TMA optics filter Figure 6 The optical path of the imager in the intermediate field 0 127 pixel is shown from the entrance window down to the detector VISIR User Manual VLT MAN ESO 14300 3514 9 filter Ac half band width maximum sensitivity um um transmission mJy 100 1h theory median BLIP SF IF PAH1 8 59 0 42 77 1 6 5 8 ArII 8 99 0 14 72 4 1 6 70 SIV_1 9 82 0 18 72 4 0 30 60 SIV 10 49 0 16 70 4 5 8 13 SIV_2 10 77 0 19 70 4 6 9 20 PAH2 11 25 0 59 75 2 3 6 9 SiC 11 85 2 34 75 1 2 T 18 PAH22 11 88 0 37 58 4 1 7 15 NelT_1 12 271 0 18 51 6 9 12 20 Nell 12 81 0 21 64 6 1 12 18 Nell 2 13 04 0 22 68 6 3 15 22 Q1 17 65 0 83 59 11 1 50 120 Q2 18 72 0 88 49 13 6 50 80 Q3 19 50 0 40 50 41 7 100 160 Table 2 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 and the measured me dian sensitivities for the Small and Intermediate Field obtained in various weather conditions The measured sensitivities were obtained using the curve of growth method on data obtained in perpendicular chopping nodding directions 4
73. serving in the MIR from the ground 2 1 The Earth s atmosphere Our atmosphere absorbs the majority of the MIR radiation from astronomical sources The main absorbing molecules are H20 CH4 CO CO Oz O3 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 10 5 12 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 on Fig 1 VISIR User Manual VLT MAN ESO 14300 3514 2 MIR atmosphere transmission at Paranal 1 2 1 0 0 8 0 6 Transmission i i 5 10 15 20 5 30 Wavelength um Figure 1 MIR atmospheric transmission at Paranal computed with HITRAN for an altitude of 2600 m and 1 5 mm of precipitable water vapor at zenith The US standard model atmosphere is used 2 2 The 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 rin
74. tal basis and may be discontinued with no previous notice 4 8 OBs 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 The following scheme is applied for VISIR OBs requiring PHO conditions will be executed and classified as A if the sensitivity in the corresponding band is equal or better then the nominal median value and if the conversion VISIR User Manual VLT MAN ESO 14300 3514 29 factor is constant within 10 Refer to the web page to know the values of nominal sensitivities in each mode 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 4 9 Known problems In addition to effects caused by the cosmetic quality of the detectors mentioned above 83 4 the following problems may affect the quality of the observations 4 9 1 Decreased image quality During the first months of VISIR operations the image quality has been severely degraded for observations obtained with a large gt 15 chopper throw The origin of this problem has been localized and at least for the most severe consequences appears to be solved However since the solution to the problem has been applied no tests for large chop throw gt 25 in excellent opt
75. used by TIMMI2 see http www 1s eso org lasilla sciops 3p6 timmi html stand html At present the standard star catalog contains 425 sources Zero point fluxes Jy have been calculated for the VISIR filter set by taking into account the measured transmission curves Fig 23 the detector efficiency Fig 9 and an atmosphere model Fig 1 However continuous observations over 3 hours of the same standar star indicates that photo metric stability better than 3 can be achieved with VISIR at the VLT In order to test if a photometric 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 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 or
76. uses 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 following algorithm is supported by the pipeline for low and medium resolution mode Let us define the detector pixels in dispersion direction by x and in cross dispersion direction by y respectively a The skew angle along x with and along y with W b The maximum curvature along x with A and along y with e is defined positive in clockwise direction and Y counter clockwise A is positive by increas ing x and e by decreasing y respectively Measured values of the distortion parameters are in the low and medium resolution mode 1 6 and Y 0 7 The curvatures in the low resolution mode are e 1 04 pixel A 0 08 pixel and for the medium resolution mode are 0 26 pixel A 0 08 pixel The center of the lower left of the detector is at 1 1 There fore the fix point which is the detector center is at 128 5 128 5 for the n 256 pixel array of the DRS The fix point is moved to 1 1 by n 1 n 1 filz y 2 Y a 6 and the skew is corrected along the cross dispersion falx y a y tan y 7 and along the dispersion direction f x y z y x tan 8 The curvature is a segment of a circle with radius R in x direction given by n 24 2R a

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