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Very Large Telescope Paranal Science Operations VISIR User Manual

1. 2000 TITO TIT oOo 1000 IT T 7 I T L 1 800 1500 3 600 x 1000 4 5 E i J 400 J gt d E 500 J 2 200 M s I Ws S ALL II H HERE E ee ES 0 E A A A O PR TAS A puro E CR E A E deo o xb 7 50 7 60 7 70 7 80 7 90 8 00 80 82 84 86 88 90 92 Wavelength um Wavelength um Sensitivity mJy 100 in 1h ra D A O Oo o C e e e o o o o o zs e X al ze DeL lt om O Ed D D 0a Dm ss e A Bw E Q t o Re O Figure 22 Measured on the old DRS detector Observed sensitivity as a function of wavelength for different settings in the medium resolution mode obtained in very good weather conditions ID for 10 lum lt 4 lt 12 5um Offered sensitivity is typically a factor of 2 larger Sensitivity mJy 100 in 1h Hs Hs 0006 o a C2 o o o a m o um u1sSue oAe M 8 81 98I TOLD gt OST GAI 0 6I Figure 23 Measured on the old DRS detector 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 A lt 19 1um Offered sensitivity is typically a factor of 2 larger Sensitivity mJy 100 in 1h D C3 e e ce O E o je C o O e o o e co LES ag PR EN ER EE o UV E o CL e lt O Sc o
2. TRE Une TR TE 4 3 Calibration dumis uoo obo ko o py RO ae E y EORR WR FE S 24 DEBO uu sso heck ORR XR Re Rue eue ORA 9 e OR e ow dE qe ao oe dep dE A RSA a A od 5 Observing with VISIR at the VLT 5 1 Proposal prepartallol o o sosa ARA a SAO Ge de GS bd BG dA he GOR A A A as ough He eH each 5 2 1 Instrument orientation on the sky o AAA III 5 2 3 Nodding parameters eros os is 4464 e 9 er Se aos e E dg AA Ons z sse ae x9 Wo AE RED E Inde ge E pde Ge OOo Se 2 2 43 ESS 48 3 2 03 54 Guide st rs 2423 3 9 x 49 ERR X a ed ERE EE 55 Brightness limitations 3 oad Rok 9 BU Rok A Uk bee eb oR SE RE S PES 25 0 Overb auds cre co oo E dede dins 5 7 Calibration observations e as arto ee a o Rd q a EC A EC AB e a e as e o daa Za gg A er RSA E RA ERRADA CP UE ER A SS n2 PIpelHe iu 2 624442 4 690 2908 45 39616 DAE GDE dd es Seded x 6 3 VISIR spectrometer data 7 VISIR templates description AI ACQUSION e 2s eet heehee be ee ede ado HES SEE 72 Observing with the imager 2 cosas m eee eee RE UR RES Ab 7 3 Observing with thespectrometer TA CALDO ass send A Soe Bh Ge Bh gE e 8 Checklist Sd Phase H WEEN 2 Pe do a ay Se He ay NRO eea oe eee a ee A D 9 Appendix VISIR template parameters 9 1 A ae a gt ye ee he Be dur ER ke nh as Rhe d RC 9 2 Cabo Kaw Oe Mee ORO ee 08 OX eee 9 UR ee DOLAR CUR e ae 10 Appendix Filter transmission curves 11 Ap
3. O L J A J is o Fc 4 O OL J EEN A IP ete Figure 27 Measured on the old DRS detector 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 D C3 gt O o e C e e e e e e e e e T urmm u1sue oAe M SAL TT OLL TT GOL TI 092 TI GGA TT OSA TT SPLATT Figure 28 Measured on the old DRS detector 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 Re D CA A O O O O O o o O o e o o o TTTT Se EE ER ram I eter Te a are Te E A QI J 4 gt SSS E Drs Sine e 7 S o O 5 m a J pa Ei la J 3 e a O ER QI J O o Lo AAA AAA AAA AAA EE Figure 29 Measured on the old DRS detector Observed sensitivity as a function of wavelength for high resolution mode Offered sensitivity is typically a factor of 2 larger 10000 TT model J L median J 1000 sensitivity mJy 100 1h V UU UU Lb Ca to 12 75 12 80 wavelength um or tg E 12 85 12 90 Sensitivity mJy 100 in 1h 0 C2 o o O e e e o e e a e je o 0007 0009 uin yySusjonem OTST O0OET O68 ST OBST Oc SI Ot T Figure 30 Measured
4. or be specifically mentioned to the night time astronomer in visitor mode As stated above the observatory does not guarantee the accuracy of the world coordinate systems WCS keywords in the FITS headers 5 5 Brightness limitations There are currently no brightness 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 4 4 5 6 Overheads The VLT overhead for one OB which includes active optics setting selection of guide star field stabilization is 6 min VISIR instrument configurations can be changed in a short time For example a complete change of instrument settings takes less than 2 minutes The total time for an image acquisition of a bright sources gt 1 Jy takes 5 min for one fine acquisition iteration or in blind preset 2 min Spectro scopic 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 5 7 Calibration observations MIR observations depend strongly on the ambient conditions such as humidity temperature or air mass In service mode science observati
5. 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 m2 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 14 then TEL CHOP POSANG must be equal to TEL ROT OFFANGLE In particular this is the case in spectroscopy if the observer wishes to have the 3 beams along the slit As stated in 3 5 the chopping frequency is not a parameter accessible to the observer it is fixed internally to ensure the best data quality Pointing position East Figure 12 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 O 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 i
6. A checklist to help the preparation of OBs is available in sB Acquisition observing and calibration templates are explained in 8 7 We strongly recommend to consult for additional information and updates For sup port during proposal preparation and OB submission please contact ESO s User Support Department usd help eso org 3 Observing in the MIR from the ground 3 1 The Earth s atmosphere Our atmosphere absorbs the majority of the MIR radiation from astronomical sources The main absorbing molecules are H20 CH4 CO CO O2 Os However the atmosphere is quite transparent in two atmospheric windows the N and Q bands They are centered around 10 and 20 um respec tively The transmission in the N band is fairly good at a dry site and becomes particular transparent in the wavelength range 10 5 12 um 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 spec tral 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 3 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 ring increases with wavelength as 1 22 A
7. Any VLT programme should make sure that a guide star UCAC3 with a R 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 brightness 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 1f 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 in particular for faint objects In addition objects within optically dark molecular clouds may have few or no suitable guide stars at least in the catalogues currently used by the Telescope Control System the UCAC3 Alternatively some bright nebulae may saturate the region of the digital sky surveys used by the telescope operator to select guide stars Considerable amount of telescope time will be saved if such cases are identified before an OB is started Providing the coordinates of a guide star in the acquisition template of an OB is therefore strongly recommended in a number of situations e observations of faint objects hardly or not visible even after a long exposure in
8. Transmissions in Transmissions in Filter Q1 120 100r 80r eor 40r 20 E a Ly 15 5 16 5 TAS 18 5 Wavelenght um Filter Q3 120 100 80 60 F 40 20F y 0 y 18 5 120 100 80r eor 40F 20H 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 16 5 17 5 18 5 19 5 Wavelenght um Filter SIC Figure 18 continued 10 12 14 Wavelenght um Normalized transmission Normalized transmission Filter Nell ref 1 2 1 0 E 0 8 F D r 0 4 F DE 0 0 SA TN LU 11 8 12 0 12 2 12 4 Wavelenght um Filter SIV ref 1 12 6 1 2 1 0r D r 0 4 E 0 2 0 0 9 5 9 7 9 9 10 1 Wavelenght um 10 5 Normalized transmission Normalized transmission 1 2 1 0 0 8 0 6 0 4 0 2 ont 10 8 1 2 1 0 0 8 0 6 0 4 0 2 0 0 10 4 Filter PAH2 ref2 v Ex SS v e Nn 4 11 2 11 8 12 2 Wavelenght um Filter SIV_ref2 12 8 am 10 6 10 8 11 0 Wavelenght um 12 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 valu
9. column Examples of dependence of sensitivity with wavelength are shown in Figures 21 to Offered slits have widths of 0 40 0 75 and 1 00 4 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 A wider range of wavelengths is accessible with the high resolution cross dispersed mode with a 4 1 long slit Offered modes and sensitivities are given in Table l6 The ETC offers the possibility to take into account the earth motion to predict the observed wavelength of a given line depending on the fore seen 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 4 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 4 4 Detectors The VISIR imager and spectrometer will be equipped with the new AQUARIUS 1k x 1k detector The quantum efficiency will be provided following the co
10. in order to have the 3 beams along the slit 9 Appendix VISIR template parameters VISIR_img_acq_MoveToPixel tsf To be specified Parameter Range Default Label INS FILTI NAME K BAND SIC PAHI ARII Imager Filter SIV 1 SIV SIV_2 PAH2 PAH2 2 NEII NEII NEII 2 Q1 Q2 Q3 NODE FAULT INS PFOV 0 045 0 076 0 076 Imager pixel scale SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ TIME 30 3600 NODEFAULT Total integration time sec TEL AG GUIDESTAR CATALOGUE SETUPFILE Get Guide Star from NONE CATALOGUE TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 30 8 Seen Amplitude arcsec TEL GS1 ALPHA ra Guide star RA TEL GS1 DELTA dec Guide star DEC TEL ROT OFFANGLE 0 359 0 0 Rotator on Sky PA on Sky TEL TARG ADDVELALPHA 0 0 RA additional tracking veloc ity sec TEL TARG ADDVELDELTA 0 0 DEC additional tracking ve locity sec TEL TARG ALPHA ra TEL TARG DELTA dec TEL TARG EQUINOX 2000 0 TEL TARG OFFSETALPHA 0 0 RA blind offset TEL TARG OFFSETDELTA 0 0 DEC blind offset VISIR img acq Preset tsf To be specified Parameter Range Default Label TEL AG GUIDESTAR CATALOGUE SETUPFILE Get Guide Star from NONE CATALOGUE TEL GS1 ALPHA ra Guide star RA TEL GS1 DELTA dec Guide star DEC TEL ROT OFFANGLE 0 359 0 0 Rotator on Sky PA on Sky TEL TARG ADDVELALPHA 0 0 RA additional trac
11. observations In spectroscopy the observatory will provide spectro photometric observations of a telluric K or M type standard star in the Low Resolution mode based on the same catalog as for imaging with an airmass difference no larger than 0 2 AM respect to the science target Such a calibration mea surement will be performed at least once per night per instrument configuration More precisely the following settings of the VISIR spec cal LRAutoChopNod template 7 will be used SEQ TIME 180 sec TEL CHOP POSANG 0 TEL CHOP THROW 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 spec troscopy 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 cali bration 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 imaging 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 flat field is recorded which consists o
12. on the old DRS detector Observed sensitivity as a function of wavelength for high resolution mode 7op 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 Sensitivity mJy 100 in 1h ol Re Re D Oo o a e x x x x ma Ha RE o o o o Zo S A 4 urmm u1sue oAe M GOV 97007 9166691066 91 6686 9108 9TGZE OT Figure 31 Measured on the old DRS detector 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 a Re Re D o o al O x x x x ma E E E O O O O o O co ES P S Ro o O E e co Re o Su oO co SG D ran e Es 20 E d g e Ser a ES e co D O E 2 eo CH o Figure 32 Measured on the old DRS detector 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 0003 0007 0009 0008 100001 i 08 AT 06 4 urmm u1Sue oAe M 6 Al LE 00 8 Figure 33 Measured on the old DRS detector Observed sensitivity as a function of wavelength for high resolution mode Offered sensitivity is typically a factor of 2 larger Sens
13. only sources fainter than 500 Jy in N and 2500 Jy in Q Due to the lowe flux levels eventual detector artifacts are less important in spectroscopy As in previous periods however it is advised that a TEL CHOP THROW between 9 to 13 shoud be avoided until a full characterization of the AQUARIUS detector is done Figure 9 An example of the bad cosmetics on the old DRS detector Sequence of chop nod reduced spectra obtained in the Medium Resolution mode with a central wavelength 8 8um The TEL 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 4 5 Data acquisition system As part of the upgrade project the VISIR new AQUARIUS detectors will be controlled by an NGC 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 5 ms Such a frame rate is too high to store all exposures One VISIR image is of size 1024 x 1024 each pixel is coded with 4 bytes long integer Thus one read out has a size of 262 kB During each chopping cycle the elementary exposures are added i
14. particular if this exposure has to be combined with other ones e observations of objects within optically dark molecular clouds where few suitable guide stars are expected e observations of objects within bright nebulae larger than the field of view accessible by the guide probe that appeared saturated in the digital sky surveys example Orion e observations for which astrometric accuracy is important In all these cases the use of the guidecam tool see http www eso org instruments visir for VISIR is strongly recommended and the coordinates of a suitable guide star should be inserted in the acquisition templates If TEL AG GUIDESTAR is CATALOGUE a guide star from the guide star catalog will be automat ically 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 con ditions 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
15. the given setting The wavelength range per setting in given in the 3rd column A4 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 The dispersion is given in the 7th column and has been estimated for the new AQUARIUS detector pixel size A ee EU EE SE sapport gl PR ETATS Y Liatrubtare AR ES Lv Figure 8 Schematic drawing of the warm calibration unit on top of the VISIR vessel The detectors have a switchable pixel well capacity The large capacity is used for broad band imaging and the small capacity for narrow band imaging and spectroscopy Detector saturation due to the enormous MIR background is avoided by a storage capacity of 0 6 10 e in small and 6 0 10 e in large capacity modes respectively 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 new AQUARIUS detector cosmetic testing shows that it does not show the fair fraction of bad pixels seen in the old DRS detector Moreover the AQUARIUS detector can be considered free from stripping effects and ghosts For period 90 it is still advised to observe
16. 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 Mi cron sized particles such as silicates silicon carbide carbon coals aluminum oxides or polycyclic aromatic hydrocarbon PAH molecules are major contributors to the thermal MIR emission The gaseous phase emits through a large number of ionic and atomic lines Examples are Nell 12 8 um and the pure rotation lines of molecular hydrogen at 8 02 9 66 12 27 and 17 03um Because of the very high background from the ambient atmosphere and telescope the sensitivity 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 84 provides a technical description of VISIR and its offered ob serving modes offered An overview on how to observe with VISIR at the VLT can be found in 5 A description of the structure of the imaging and spectroscopic data files is given in 8 6
17. 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 15 Illustration of generating raster maps with VISIR img obs GenericChopNod An illustration of generating an raster map can be found in Fig The following parameters correspond to this setting SEQ NOFF 3 SEQ OFFSET1 LIST 30 10 10 SEQ OFFSET2 LIST 30 10 10 SEO OFFSET COORDS SKY Note that depending on choice of the integration time SEQ TIME several nodding cycles might re sult e g pattern like ABIBIAAB1B1A 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 VISIR img obs BurstAutoChopNod Th
18. 0 NODEFAULT Total integration time sec TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 30 8 SE Amplitude arcsec 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 PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 180 3600 NODEFAULT Total integration time sec TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 30 8 Gate Amplitude arcsec VISIR_spec_obs_HRAutoChopNod tsf To be specified Parameter Range Default Label INS FILT2 NAME NEI 2 H28 1 H2S 4 Spectrometer Filter NEII 2 INS GRAT1 WLEN 7 80 19 18 12 810 Spectrometer Wavelength microns SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 180 3600 NODEFAULT Total integration time sec TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 30 8 SE Amplitude arcsec VISIR spec obs HRXAutoChopNod tsf To be specified Parameter Range Default Label INS GRAT1 WLEN 7 60 28 08 NODEFAULT Spectrometer Wavelength microns SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TI
19. 3 1 4 2 1 4 3 2 P81 release new filters included v87 22 09 10 4 3 2 8 2 8 3 P87 release Exclusion of K band in science imaging templates 3 2 5 P87 release update of HR allowed A First line of Table 7 modified 8 2 P87 release non availability of jitter with IMG GenericChopNod v88 22 02 2011 4 1 P88 release upper limit of 5 filters in a single service mode OB 4 4 P88 release UCAC3 substituting USNO for guide stars selections v89 31 08 2011 all P89 release removing most references to the old DRS detector and reporting the first properties of the AQUARIUS detector v90 26 02 2012 1 Update the new schedule of the VISIR upgrade v1 0 v1 1 v76 1 edited by R Siebenmorgen E Pantin M Sterzik v76 2 4 v77 1 3 updated by A Smette v78 80 updated by L Vanzi v87 90 updated by Y Momany Contents 1 VISIR Upgrade Project 1 1 Detector Uperade a 252 9 50 3 46 3 9 See SETA GAS 246648 404 1 2 Low Resolution Spectroscopy 42k ox 9 624 54 AS A se x rwr rrr 3 Observing in the MIR from the ground 3 1 The Earth s atmosphere 473 9a 3 R3 9c s 33 bom EE 4 rcp 3 3 MIR background ix dues y Roda Ep A RR UU do x ERS 3 4 Chopping and nodding ooa Bo OCUSHIVELY 2 2 oe wokos III 4 Instrument description and offered observing modes A AA a A A A A AA Re de Ai e a y AA se DE EASE RO HAS ORE EUR e 4 Wee es T vr 4 2 3 Low resolution offered central wavelengths
20. D where 2 is the observing wavelength and D the diameter MIR atmosphere transmission at Paranal 1 2 1 0 0 8 0 6 Transmission 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 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 179 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 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 15 in N The VISIR instrument is cooled to avoid internal background contaminatio
21. E K BAND SIC PAHI ARII Acquisition Filter for the im SIN 1 SIV SIV 2 PAH2 ager detector PAH2 2 NEI NEI NEI 2 Q1 Q2 Q3 NODE FAULT INS FILT2 NAME N_SW N_LW ARII NEIL 1 Acquisition Filter for the NEII 2 NODEFAULT spectroscopy detector INS SLIT1 TYPE LONG SHORT LONG Spectrometer Slit Type long or short INS SLIT1 WIDTH 0 40 0 75 1 00 NODE LAN Slit Width arc FAULT sec SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ TIME 30 3600 NODEFAULT Total integration time sec TEL AG GUIDESTAR CATALOGUE SETUPFILE Get Guide Star from NONE CATALOGUE TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 30 8 nas Amplitude arcsec TEL GS1 ALPHA ra Guide star RA TEL GS1 DELTA dec Guide star DEC TEL ROT OFFANGLE 0 359 0 0 Rotator on Sky PA on Sky TEL TARG ADDVELALPHA 0 0 RA additional tracking veloc ity sec TEL TARG ADDVELDELTA 0 0 DEC additional tracking ve locity sec TEL TARG ALPHA ra TEL TARG DELTA dec TEL TARG EQUINOX 2000 0 TEL TARG OFFSETALPHA 0 0 RA blind offset TEL TARG OFFSETDELTA 0 0 DEC blind offset 9 1 Observation VISIR img obs AutoChopNod tsf To be specified Parameter Range Default Label INS FILTI NAME SIC PAHI ARII SIN 1 Imager Filter SIV SIV 2 PAH2 PAH2 2 NEII 1 NEI NEII 2 QI Q2 Q3 NODEFAULT INS PFOV 0 045 0 076 0 076 Imager pixel scale SE
22. ES O EUROPEAN SOUTHERN OBSERVATORY Organisation Europ ene pour des Recherches Astronomiques dans H misph re Austral Europ ische Organisation f r astronomische Forschung in der s dlichen Hemisph re ESO European Southern Observatory Karl Schwarzschild Str 2 D 85748 Garching bei M nchen Very Large Telescope Paranal Science Operations VISIR User Manual Doc No VLT MAN ESO 14300 3514 Issue 90 Date 26 02 2012 Y Momany o EE Date Signature C Dumas Approved A TTE Date Signature A Kaufer Released eset else brit tato ege eds Date Signature This page was intentionally left blank 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 release v79 30 11 06 4 8 P79 release v80 28 02 07 3 1 4 2 1 5 1 6 2 8 2 P80 release burst mode included v8l 31 08 07
23. ME 180 3600 VODEFAULT Total integration time sec TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 30 8 o Amplitude arcsec 9 2 Calibration VISIR img cal AutoChopNod tsf To be specified Parameter Range Default Label INS FILTI NAME SIC PAHI ARII SIN 1 Imager Filter SIV SIV 2 PAH2 PAH2 2 NEIL 1 NEI NEU 2 Q1 Q2 Q3 NODEFAULT INS PFOV 0 045 0 076 0 076 Imager pixel scale SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PERPENDICULAR SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 30 3600 NODEFAULT Total integration time sec TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 30 8 i m Amplitude arcsec VISIR spec cal LRAutoChopNod tsf To be specified Parameter Range Default Label INS GRATI WLEN 8 1 8 5 8 8 9 8 11 4 12 2 12 4 Spectrometer Wavelength NODEFAULT microns SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 30 3600 NODEFAULT Total integration time sec TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 30 8 ME Amplitude arcsec VISIR spec cal MRAutoChopNod tsf To be specified Parameter Range Default Label INS GRAT1 WLEN 7 5 28 08 NODEFAULT Spectrometer Wavele
24. NDITSKIP Neyer chop NDIT NCYSKIP 4 and the total observing time is tiot source skip 5 Typical duty cycles source ftot are about 70 wee N yelchop Ncvsxip Neeyel chop q p An Bn Bn An di 7 gt T_nod NDITSKIP NDIT NDITSKIP q DIT Ac Bc A E i T_chop Figure 10 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 5 Observing with VISIR at the VLT 5 1 Proposal preparation Tools are available to prepare the observations either during phase 1 call for proposals or during phase 2 creation of observing blocks by the observer e The exposure time calculator ETC available at http www eso org observing etc may be used to estimate the integration time needed to obtain the required S N for a given instrument setting because of the numerous sky absorption lines see Fig 20 and following it is recommended to display the S N as a function of wavelength when using the spectrograph ETC 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 e As for all VLT instruments astronomers with granted VISIR telescope time prepare their ob servations using the phase 2 proposal preparation tool P2PP d
25. 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 14 A typical value for these parameters is TEL CHOP THROW 2 where TEL CHOP THROW 1s the chop throw used in the subsequent templates If both the target and guide star coordinates are within the same astrometric systems the point ing accuracy is limited by the relative accuracy between the coordinates of the two objects In particular the pointing accuracy maybe affected by significant usually unknown proper mo tion 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 12 in 60s on source integration for a S N 10 Please note that the K Band filter is not offered for science 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 o
26. Q CATG PRE IMAGE SCIENCE Observation Category SCIENCE SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 180 3600 NODEFAULT Total integration time sec TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 30 8 Ges Amplitude arcsec VISIR img obs GenericChopNod tsf To be specified Parameter Range Default Label INS FILTI NAME SIC PAHI ARII SIN 1 Imager Filter SIV SIV 2 PAH2 PAH2 2 NEII 1 NEI NEII 2 QI Q2 Q3 NODEFAULT INS PFOV 0 045 0 076 0 076 Imager pixel scale SEQ CATG PRE IMAGE SCIENCE Observation Category SCIENCE SEQ NOFF 1 100 NODEFAULT Number of offset positions SEQ OFFSET COORDS SKY DETECTOR NODE Offset coordinates FAULT SEQ OFFSETI LIST NODEFAULT List of offsets in RA or X SEQ OFFSET2 LIST NODEFAULT List of offsets in DEC or Y SEQ TIME 180 3600 NODEFAULT Total integration time sec TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 30 10 Gia Amplitude arcsec 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 PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 180 360
27. Qr OE a 5 O rt os E A a e OL D Er E A ES Figure 24 Measured on the old DRS detector 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 A lt 20 3um Offered sensitivity is typically a factor of 2 larger Sensitivity mJy 100 in 1h D gt O O o Oo o o e o e e e e Cx E e il Or e EM CO gt lt L rel Or 4 e ge T Ge M oo E Es co e a F E ape ys Sap ae Sja mrt mo E O1 Figure 25 Measured on the old DRS detector 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 D DE O o o o Oo o C C o o o Oo o Er 5 T RR al a J dx pu o MES o Ol ES oO 5 ga gud Ent E J jo NIE OA E SE CH Figure 26 Measured on the old DRS detector 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 gt IW Co AB o o e o m Ce e e e ru o o o o OQ TTTTTTTTERTTTTTTTTTTTTTTTTTTTTTTTTTTTTT E L ol J d pel J cn b 4 WL or J px ok J mo J Q tF te Sol lt 0 J e peur al PE Oa OV rF 7 on LU sol oo Era eb gi d Qr A O
28. 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 detector 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 ser vice 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 requested in the README file photometric standard stars are not necessarily observed in the same
29. ates or atmospheric lines In the VISIR FITS file chopper half cycle frames which are dominated by sky emission lines are stored s 6 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 chopping process results in a near perfect cancelation of sky lines Atmosphere absorption correction The atmosphere does not uniformly absorb the MIR radiation B 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 5 7 For high resolution spectroscopy only calibrators known with high precision such as A stars or asteroids should be considered However even early A stars are known to have some hydrogen absorption lines in the N and Q band 7 VISIR templates descr
30. ci facilities paranal instruments visir upgradeproject html 1 1 Detector Upgrade The major part of the upgrade project concerns the replacement of the current detector DRS 256 x 256 pixel array with The Raytheon AQUARIUS 1024 x 1024 pixel array This hardware upgrade is expected to significantly improve the VISIR performance in terms of field coverage and sensitivity Currently two pixels scales are offered 07075 and 07127 so called Small Field and Intermediate Field SF and IF providing a field of view of 19 x 19 and 32 x 32 arcseconds respectively The new AQUARIUS detector will be offered in two pixel scales 07045 and 07076 SF and IF respectiely The projected field of view will therefore be 46 x 46 and 78 x 78 arcseconds As for Exposure time calculation the astronomical community is invited to use the well established DRS detector sensitivities both for spectroscopy and imaging as provided in Tables 2 3 6 and 7 Indeed the new AQUARIUS detector has proven to show better performance however a final characterization of the achieved sensitivities 1s to await the commissioning and science verification The cosmetic quality of the AQUARIUS detector is proven to be excellent The regions of masked pixels Fig 12 and stripes Fig 13 are not present anymore 1 2 Low Resolution Spectroscopy The second major improvement is that concerning the N band 8 13um Low Resolution Spec troscopy Currently this is achieved by means of a
31. d and VISIR_spec_obs_MRAutoChopNod respectively Ob serving parameters are total integration time SEQ TIME central wavelength INS GRAT1 WLEN the slit width INS SLIT1 WIDTH and SEQ CHOPNOD DIR s 7 2 High resolution long slit mode Template for high resolution spectroscopy is VISTR_spc_obs_HRAutoChopNod Three order sort ing filter at 8 02 12 81 and 17 03um INS FILT2 NAME H2_S4 Ne 11 H2_S1 are avail able See Table 6 for the corresponding list of offered central wavelengths Other observing param eters are total integration time SEQ TIME central wavelength INS GRAT1 WLEN the slit width INS SLIT1 WIDTH and SEO CHOPNOD DIR s 7 2 High resolution cross dispersed mode VISIR spc obs HRXAutoChopNod is functionally similar to VISIR spc obs HRAutoChopNod but uses a grism for cross dispersion and order separation See Table 6 for a list of offered wave lengths 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 72 7 4 Calibration Specific templates exist for the observations of photometric and spectro photometric standard stars They offer the same functionality as the corresponding science templates but allow to monitor the sensitivity and image quality by observing calibration standard stars Their use is recommended to be properly recognized by the VISIR
32. e observations in burst mode are analogous to the observations with the template VISIR img obs AutoChopNod but single elementary frames are saved These can be single DIT frames or the average of a number NDIT of DIT frames The minimum total integration time is 2 minutes and the total integration time must be a multiple of this value NDIT can assume values from 1 to 10 7 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 oth erwise LONG is the default setting A preferred observing strategy is called nodding on the slit where the chopping and nodding amplitudes are small SEQ CHOPNOD DIR PARALLEL Note that nodding on the slit requires to set the telescope rotator offset angle and the M2 chopping position angle to the same value 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_LRAutoChopNo
33. er BLIP and excellent weather conditions and the measured median 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 beams filter Ac half band width sensitivity um um mJy 100 1h SF IF B 8 7 8 92 0 97 B 9 7 9 82 0 84 9 20 B10 7 10 65 1 37 5 8 B11 7 11 52 0 85 5 6 B 124 12 47 0 99 8 12 J 7 9 7 76 0 55 14 25 J 8 9 8 70 0 73 3 5 J 9 8 9 59 0 94 T 10 J12 2 11 96 0 52 8 10 Table 3 Sensitivity values are still valid for Period 89 proposal preparation VISIR imager filter characteristics determined with a monochromator and the WCU The last 2 columns give the mea sured median sensitivities for the Small and Intermediate Field obtained in good weather conditions The measured sensitivities were obtained using the curve of growth method on data obtained in perpendicular chopping nodding directions 4 beams The burst read out is offered for the imager in visitor mode only The burst read out allows the user to save every single DIT frame of the exposure In this way it is possible to follow rapidly evolving events or to improve the spatial resolution by taking short enough exposures to freeze the atmospheric turbulence This mode can be used only for objects bright enough to provide a S N high enough in a single elementary exposure Typical flux l
34. es are normalized so that their peak transmission is equal to 1 11 Appendix Observed sensitivities in various spectroscopic set tings median best ever 2 4 theoretical limit 100 sensitivity mJy 106 1h wavelength um Sensitivity mJy 100 in 1h v A o o o o o o o o T T 008 10001 GL um u18uoe oAe M 0 8 Figure 20 Measured on the old DRS detector Sensitivity as a function of wavelength for low resolution mode 7op Four offered settings of the N band low resolution are stitched together At mospheric molecular absorption e g at 9 55 11 8 and 12 5 um is evident Note the detector 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 Sensitivity mJy 100 in 1h
35. escribed at Acquisitions observa tions 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 com plete observing sequence An overview of the available VISIR templates and their parameters is given in Silo this manual e For each science template the user has to provide a finding chart so that the target can be acquired In addition to the general instruction on how to create these finding charts see http www eso org sci observing phase2 SMGuidelines html the following VISIR requirements apply All finding charts have to be made using existing infrared K band or longer wavelength images Typically 2MASS or DENIS K band images are acceptable although 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 for a 10um spectroscopic template the user has to describe clearly how to identify the target at the observing wavelength in the README section of the programme description Adequate examples of such comments are The target will be the brightest source in the field of view at 10m 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 fie
36. f 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 experimental basis and may be discontinued with no previous notice 5 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 factor is constant within 10 Refer to the web page http www eso org instruments visir inst 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 5 9 Known problems The cosmetic quality of the AQUARIUS detector is very good However this and eventual other problems e g decreased image quality bad resid
37. f the observationg The following cases require special care e imaging in the small field in some conditions an error of less than 10 on the coordinates can bring the target outside of the field e spectroscopic acquisition in some conditions an error of less than 7 5 on the coordinates can bring the target outside of the wide slit used Errors of such scale are common in the following situations e high proper motion stars in particular if the epoch of the VISIR observations is significantly different from the epoch for which the coordinates were determined e point like sources within extended objects such as an AGN a number of catalogues do not provide accurate coordinates of the nucleus Coordinates given by 2MASS are 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 nuta tion 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 6 and 6 must be given in s 3In particular note that P2PP only accepts coordinates for J2000 0 5 4 Guide stars Guide stars are mandatory for active optics and field stabilization
38. factured by the company READING Their absolute transmission curves are displayed in Fig 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 Filter Arlll Filter Nell ref 120 ER 120 i 8 100f e 100r sob j uoBsOR o M SY j S cof 9 eof Ke E 40 J E 40 J S 20 J 5 20 E J O Bae Set desc eet OE 1 8 80 8 90 9 00 9 10 9 20 12 6 12 8 TO 13 2 13 4 Wavelenght um Wavelenght um Filter Nell Filter PAH1 ARIIIref 1 120 E i 1206 T ta a 100r yaaa iE Vf f U A gol J po 8 e WE gol ul n I c E al 5 cof 9 n d E 40 4 E 40 p i V bo iW 2 20F 8 20f 12 4 12 6 12 8 13 0 13 2 7 5 8 0 8 5 9 0 9 5 Wavelenght um Wavelenght um Filter Pah2 Filter QO 120 j T 120 TOO peso t mmm env VY WA rd E 100r 80 n 2 BL 1 c 9 sok J 9 ek y E E arm E sot E 40b vull c c x S zo J 9 20 Wo Wy ud E ES L A 0 Chat l 10 3 10 9 11 6 12 2 12 8 15 5 16 2 16 8 TZ 17 8 Wavelenght um Wavelenght um Figure 16 Transmission curves of VISIR imager filters manufactured by READING Overplotted dashed is the atmospheric transmission at low resolution The absolute transmission values are given expressed in percent Transmissions in
39. ferent 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 Cohen et al 1999 AJ 117 1864 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 uniformly as possible in Right Ascension with spectral types as similar as possible In addition their flux in the N band of the order of 10Jy is bright enough to be observable in the Q band without reaching non linearity levels in the N band even in non ideal background conditions At least one star in this reduced catalogue will be observed every night VISIR is in use Note that this list could be modified without previous notice A PSF can be derived from these photometric standard star observations However 1t is not guar anteed that its S N is sufficient for deconvolution purposes If the observer requires a specific PSF measurement s he has to provide the corresponding PSF OB Observations of photometric standards provided by the observatory are taken using the VISIR img cal AutoChopNod template 7 with the following settings SEQ TIME 180 sec for N and 360 sec for Q band TEL CHOP POSANG 0 TEL CHOP THROW 10 SEQ CHOPNOD DIR PERPENDICULAR Filter INS FILT1 NAME and pixel scale INS PFOV will be set according to the science
40. filter as the acquisition filters 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 f the target coordinates are well known VISIR imaging modes allow to perform blind preset observations with the VISIR img acq Preset template In this case no acquisition images are taken 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 off set by half the TEL CHOP THROW values to south and west for TEL ROT OFFANGLE TEL CHOP POSANG 0 oruse the parameters TEL TARG OFFSETALPHA and TEL TARG
41. grism and has the disadvantge of requiring 4 independent exposures in order to cover the 8 13um range The introduction of the low resolu tion prism R 300 for a 073 slit will allow to achieve the same wavelength coverage in a single exposure and reach improved sensitivities 1 3 Precipitable Water Vapour The amount of Precipitable Water Vapour PWV present in the Earth s atmosphere can heavily impact on Mid infrared observations Operations wise a prior knowledge of the PWV content will seriously impact on efficient service and visitor mode VISIR observations As part of the VISIR upgrade project and starting December 2011 real time PWV monitor is now available on Paranal The commissioning of the PWV monitor shows that it meets all specifications e PWV range 0 5 9 mm validated e PWV precision ca 30 um http www eso org tecarch Documents VLT 14300 mid_ir_imager_spectrometer 14330 VISIR_Upgrade SoW_for 20_RS_campaign_5504 pdf e PWV accuracy ca 0 1 mm e High time resolution sec e All sky pointing 2D capability e Autonomous operation It is expected that the PWV value will be used as user defined a constraint parameter starting Period 90 October 2012 April 2013 2 Introduction The VLT xxx 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
42. he VISIR spectrometer design is shown in Fig The 3 mirror system of the low and medium resolution arm gives a 53 mm diameter collimated beam the collimated beam diameter in the high resolution arm is 125 mm Both subsystems image the spectrum onto the same detector selection between the two spectrometer arms is done by two pairs of folding flat mirrors In front of the 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 37 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 4 together with their measured bandpasses and approximate sensitivities for image acquisition filter Ac half band width sensitivity um um mJy 10c71h NSW 8 85 1 35 40 NLW 121 1 9 40 All 8 94 0 11 200 Nell 1 12 35 0 50 80 Nell 2 12 81 0 10 50 Table 4 VISIR spectrometer filter characteristics The filters transmissions have been determined with a monochromator and the WCU The last column list the measured median sensitivities which were obtained usi
43. imates based on standard star observations are provided both in imaging and spectroscopy 5 7 The public release of the VISIR pipeline is accessible at http www eso org sci software pipelines The pipeline currently supports the following templates e VISIR img obs AutoChopNod e VISIR spec obs LR AutoChopNod e VISIR spec obs MR AutoChopNod e VISIR spec obs HR AutoChopNod e VISIR spec obs HRS AutoChopNod In mosaic or raster mode VISIR img obs GenericChopNod only raw frames are delivered e g mapping reconstruction algorithms are not supported 6 3 VISIR spectrometer data Optical distortion correction Spectra are deformed by optical distortion and slit curvatures The VISIR spectrograph 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 Y 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 increasing x and e by decreasing y respectively Measured values of the distortion parameters are in the low and med
44. imits are of the order of 5 Jy in N and 10 Jy in Q Some compromise between sensitivity and time resolution can be reached by averaging a number of elementary frames Further and more up to date information can be found at http www eso org instruments visir inst 4 2 Spectrometer VISIR offers slit spectroscopy at three spectral resolutions with a pixel scale of 07076 This is ob tained 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 VLT FOCAL DIAPHRAGM J PLANE WHEEL Y MO A BIC a b IMAGER B RE IMAGER je RSTO FILTER WHEEL c estem WHEEL IMR GUN SS wit HR 4 GRATING SCANNERS DUO ECHELLE 1 RETURN FLAT GRATING UNIT N ESOLUTION SELECTION MECHANISM LMR MLW COLLIMATOR S R d CAMERA COLLIMATOR p Sc T CAMERA RETURN FLAT gt i N s d B LENS LN V B E amp FOCUS PESOS MECHANISM 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 t
45. iption 7 1 Acquisition Each OB needs to start with an acquisition template they are described in 5 3 7 2 Observing with the imager VISIR img obs AutoChopNod This template permits observing a source in imaging configuration with various sub settings The observer must specify 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 PARALLEL considers an equal nodding and chopping amplitude which are both in parallel direction It is recommended for faint extended sources for which the spatial 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 CO CO CO Nodding Position A Nodding Position B Nodding Position A Nodding Position B Figure 14 Schematic drawing of the content of a frame obtained with TEL ROT OFFANGLE TEL CHOP POSANG 8 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 c
46. itivity mJy 100 in 1h 0003 0007 0009 0008 00001 OTe St T tur yy Suga M 02281 a L 06281 mE ji Ove BI ETC Figure 34 Measured on the old DRS detector 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 Il C2 A q O o o O D o Q e o e o e o e o e e o e e EETTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTTH B M M gt tur y Suaga M S6 81 06 8T S887 08 8T Gol 0481 S9 BT Figure 35 Measured on the old DRS detector 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 al Re Re D oO o Ol o x x x x E E C ce ce O e CA FS ES 4 T IT FE IT d QI eo no co er svf SE w O T Do tJ J m c LE SN El dor 7 e J av ps w F A we es Figure 36 Measured on the old DRS detector Observed sensitivity as a function of wavelength for high resolution mode Offered sensitivity is typically a factor of 2 larger
47. ium resolution mode 1 6 and Y 0 7 The curvatures in the low resolution mode are 1 04 pixel A 0 08 pixel and for the medium resolution mode are e 0 26 pixel A 0 08 pixel The center of the lower left of the detector is at 1 1 Therefore the fix point which is the detector center is at 512 2 512 2 for the n 1024 AQUARIUS pixel array The fix point is moved to 1 1 by f x y gt nel 6 and the skew is corrected along the cross dispersion fax y x y tanQP y 7 and along the dispersion direction fix y x y x tan 8 The curvature is a segment of a circle with radius R in x direction given by n 2 ye 2R and in y direction by n 2 NA 2R A It is corrected along the cross dispersion falx y x y sign e Re VR 2 5 0 9 and along the dispersion fo y x sign A Ra JR 3 9 3 A 0 10 Finally the origin of the coordinate system is moved back from the fix point to 1 1 1 n 1 n fen y y 5 11 Spectral extraction is similar to the TIMMI 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 pl
48. king veloc ity sec TEL TARG ADDVELDELTA 0 0 DEC additional tracking ve locity sec TEL TARG ALPHA ra TEL TARG DELTA dec TEL TARG EQUINOX 2000 0 TEL TARG OFFSETALPHA 0 0 RA blind offset TEL TARG OFFSETDELTA 0 0 DEC blind offset VISIR_spec_acq_MoveToSlit tsf To be specified Parameter Range Default Label INS FILT2 NAME N_SW N_LW ARII NEH 1 Acquisition Filter NEII 2 NODEFAULT INS SLIT1 TYPE LONG SHORT LONG Spectrometer Slit Type long or short INS SLIT1 WIDTH 0 40 0 75 1 00 NODE Ee Slit Width arc FAULT sec SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ TIME 30 3600 NODEFAULT Total integration time sec TEL AG GUIDESTAR CATALOGUE SETUPFILE Get Guide Star from NONE CATALOGUE TEL CHOP POSANG 0 359 0 Chopping Position Angle deg TEL CHOP THROW 8 30 8 Ge Amplitude arcsec TEL GS1 ALPHA ra Guide star RA TEL GS1 DELTA dec Guide star DEC TEL ROT OFFANGLE 0 359 0 0 Rotator on Sky PA on Sky TEL TARG ADDVELALPHA 0 0 RA additional tracking veloc ity sec TEL TARG ADDVELDELTA 0 0 DEC additional tracking ve locity sec TEL TARG ALPHA ra TEL TARG DELTA dec TEL TARG EQUINOX 2000 0 TEL TARG OFFSETALPHA 0 0 RA blind offset TEL TARG OFFSETDELTA _ 0 0 DEC blind offset VISIR_spec_acq_ImgMoveToSlit tsf To be specified Parameter Range Default Label INS FILTI NAM
49. l require only one exposure to cover the 8 13um range This is achieved by replacing the current grisms with a new R 300 for a 073 slit prism The sensitivities measured on the old DRS detector still apply for Period P90 proposal preparation These are 50 mJy at 100 1h in the clean regions of the spectrum Offered slits have widths of 0 40 0 75 and 1 00 4 2 4 Medium resolution offered central wavelengths In Medium Resolution mode the central wavelength A can be freely chosen within the wavelength ranges listed in Table 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 AA grating spect resol dispersion sensitivity um um order measured l slit pixels um mJy at 100 in 1h 75 80 0 195 2 3500 2192 1000 8 0 9 3 0 188 2 3500 2267 200 10 2 13 0 0 21 2 3500 2417 1733 200 17 1 19 0 0 37 1 1800 1158 1200 20 12 0 36 1 1800 720 1200 Table 5 VISIR medium resolution settings The first column gives the minimum and maximum allowed values for the central wavelength 4 lt in the given setting The wavelength range per setting in given in the 2nd column A4 The spectral resolution measured with a 1 slit is given in the 3rd column The dispersion is given in the 4th column and has been estimated for the new AQUARIUS detector pixel size Typical offered sensitivites are given in the last
50. ld 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 5 4 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 A final recommendation concerning service mode observations is that no more than 5 filters are grouped together in a single Observing Blocks This is rather necessary for a proper calibration of each single filter Moreover it is also recommended that N and Q band filters are not grouped together as the Q band sensitivities can be quite lower from that in the N band Questions related to the VISIR Phasel and Phase 2 observing preparation should be directed to the User Support Department usd help eso org 5 2 Telescope observing parameters 5 2 1 Instrument orientation on the sky By default the imager orientation is such that North is at the top and East is to the left For the spectrometer the default orientation is 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 Figure summarizes the situation N N io o Slit E pix 0 076 r E pix 0 045 pix 0 076 78 46 E IMG 1 IMG S IMG SPC Figure 11 Field orientation and scale for the imaging and spectrosc
51. mmissioning of the detector At the operating temperature of the detector 6 K 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 mode Ac AA line order R dispersion sensitivity um um pixels um Jy 100 1h HR 7 800 8 100 0 02420 H2 S4 17B 32000 17573 3 HR 12 738 12 882 0 03571 Ne II 11A 17000 11908 0 9 HR 16 800 17 200 0 05156 H2_S1 8B 14000 8250 lt 10 HRX 8 970 9 140 0 02270 ArI 16A 27100 18757 4 HRX 9 360 9 690 0 02325 H2 S3 ISA 25000 18290 5 HRX 10 480 10 540 0 03160 SIV Col 12B 24000 13407 4 HRX 11 540 11 570 0 03210 HD 0 0 R 10 12B 23400 13333 2 HRX 11 762 0 03260 CUV 12A 19100 13067 3 HRX 12 210 12 760 0 03864 H2 S2 11B 20000 11007 1 5 HRX 12 814 13 364 0 03550 Nell 11A 17500 11917 2 HRX 16 390 0 03551 Coll 9A 17300 12100 12 HRX 16 925 0 05240 Col 8B 14100 8067 8 HRX 17 790 17 980 0 04707 PII Fell 8A 13140 9000 4 HRX 18 246 0 04182 Nill 8A 14600 10133 8 HRX 18 680 18 960 0 06569 SIII 7B 11150 6450 4 HRX 21 295 0 04196 NaIV TA 14300 10097 9 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 Ac in
52. n The detectors are at 5 6 K and the interior of the cryostat at 33 K The background radiation at 10um is typically my 5 mag arcsec 3700 Jy arcsec and at 20um mo 7 3 mag arcsec 8300 Jy arcsec Consequently the number of photons reaching the detector is huge often more than 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 3 4 Chopping and nodding The basic idea to suppress the MIR background is to perform differential observations using the chopping nodding technique In the chopping technique two observations are performed One set of exposures on source include the background and the astronomical source A second set of off source exposures measures the 10 0 E e n mdi SPITZER diffraction c NT e 2 f L VLT VISIR diffraction _ Z DUE Gir O Fr 1 visible seeing 1 d Md uL UNDE cnm EL 5 Ge o cos q gt 0 5 visible seeing lt 0 1 1 1 1 l 1 1 1 L 1 1 1 1 L 1 1 1 1 L 1 1 1 L 5 10 15 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 pure background The on and off source observations have to be alternated at a rate fa
53. n one of the nodding position images illustrated in Fig 5 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 8B The nodding period is a parameter that can only be modified by the instrument operator For expo sures shorter than 180s SEQ TIME 1805 as possible in acquisition images the nodding time is set to half the requested exposure time For exposures longer than 180s the nodding time is set to 90s In particular 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 l4 In imaging more flexibility on the nodding offsets are possible with the VISIR img obs GenericChopNod template 5 3 Target acquisition 5 3 1 Introduction Observing blocks OB must start with an acquisition template Pointing to a target can only be performed thro
54. n real time and only the result is stored on disk At a chopping frequency of Senop 0 25 Hz every Tchop 4s 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 Taoa This nodding period is typically Taoa 90 s for science observations The chopper frequency DIT and also T 4 are predefined by the system The number of saved A B frames in one FITS file is Neycl chop Thoa T chop 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 Lee 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 Typ ical stabilization times of the secondary are 25 ms The number of rejected exposures is given by NDITSKIP Similar during stabilization after each telescope movement respectively nodding posi tion a number NCYSKIP of chopping cycles is ignored The timing organization of data is shown in Fig The total on source integration time is tsource 4 g Neyet_nod z Noycl_chop NDIT DIT 3 The total rejected time is Lin 4 Noel soa DIT
55. nd bottom Small and intermediate field observations are displaced for clarity Background noise limits are indicated for the individual filter bandpasses Pixel size Field of view 07076 78 x 78 07045 46 x 46 Table 1 VISIR imager pixel scales offered The pixel size of the AQUARIUS 1kx1k detector is 30 um p entrance window diaphragm focal plane TMA optics Figure 6 The optical path of the imager in the intermediate field 07076 is shown from the entrance window down to the detector filter Ac half band width maximum sensitivity um um transmission mJy 100 1h 96 theory median BLIP SF IF PAHI 8 59 0 42 77 1 6 5 8 All 8 99 0 14 72 4 1 6 70 SIV_1 9 82 0 18 12 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 75 229 6 9 SiC 11 85 2 34 75 1 2 7 18 PAH2 2 11 88 0 37 58 4 1 7 15 Nell 1 1227 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 Ql 17 65 0 83 59 11 1 50 120 Q2 18 72 0 88 49 136 50 80 Q3 19 50 0 40 50 41 7 100 160 Table 2 Sensitivity values are still valid for Period 90 proposal preparation 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 und
56. ng the curve of growth method on data obtained in parallel chopping nodding directions 3 beams 4 2 1 Slit widths Three different slit widths 0 4 0 75 and 1 are offered for all settings For over sized widths e g for the 1 slit with respect to the diffraction limit around 10um the spectral resolution of a point source spectrum is better than the one of the sky spectrum in addition the zero point of the wavelength calibration will be affected by an incorrect centering of the object within the slit 4 2 2 Resolution In the N band the low resolution and medium resolution modes provide spectral resolving power of 300 and 3000 Table D respectively In high resolution long slit mode narrow wavelength ranges around the 8 02 H2 S4 12 813 Ne II and 17 03 um H2 S1 line are offered With the l slit the measured spectral resolution is R 15000 Table 6 and a minimum flux in an emission line below 1076 W m arcsec can be achieved This value corresponds to an approximate sensitiv ity 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 www eso org instruments visir for the latest update of the list of of fered modes and slits 4 2 3 Low resolution offered central wavelengths Following the VISIR upgrade project and throughtout Period 90 the N band 8 13um Low Resolution Spectroscopy wil
57. ngth microns SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 30 3600 NODEFAULT Total integration time sec TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 30 8 c Amplitude arcsec VISIR spec cal HRAutoChopNod tsf To be specified Parameter Range Default Label INS FILT2 NAME NEIL2 H28 1 H2S 4 Spectrometer Filter NEII 2 INS GRATI WLEN 7 80 19 18 12 810 Spectrometer Wavelength microns SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 30 3600 NODEFAULT Total integration time sec TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 30 8 es Amplitude arcsec VISIR spec cal HRXAutoChopNod tsf To be specified Parameter Range Default Label INS GRAT1 WLEN 7 60 28 08 NODEFAULT Spectrometer Wavelength microns SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 30 3600 NODEFAULT Total integration time sec TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 30 8 Sane Amplitude arcsec 10 Appendix Filter transmission curves The filter transmission has been measured using a Fourier Transform Spectrometer at a temperature of 35 K for filters manu
58. ons 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 stan dard 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 alf This list is supplemented by MIR standards used by TIMMI2 see Tasilla sciops 3p6 timmi html stand html At present the standard star catalog contains 425 sources Zero point fluxes Jy have been calcu lated for the VISIR filter set by taking into account the measured transmission curves Fig 16 the detector efficiency and an atmosphere model Fig D Continuous observations over 3 hours of the same standar star indicates that photometric 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 O 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 dif
59. opic modes of VISIR Since VISIR is mounted on a rotator at the Cassegrain focus of Melipal it is possible to change the default orientation of VISIR on the sky for example to obtain the spectra of two objects A and B at once The parameter TEL ROT OFFANGLE defaulted to 0 is used for this purpose If PA represents the required position angle of object B relative to A measured on the sky east of north i e counted positively from north to east within the range O to 360 then setting TEL ROT OFFANGLE 360 PA allows one to have both A and B objects on the slit 5 2 2 Chopping parameters The chopping technique as described in sB is based on beam switching using the moving secondary mirror of the telescope It allows to alternatively observe a field then another field offset from the first by a chopping distance or throw called TEL CHOP THROW see Fig 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 Note that for chopping throws larger than the field of view
60. orresponds 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 8 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 of nodding cycles N yci_noa is computed according to the total observation time 4 5 VISIR img obs GenericChopNod This imaging template enhances the flexibility of nodding offsets and allows the user to specify them in a list of relative offset positions In the most simple application only one offset posi tion 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 off set 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 prod uct 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
61. ped 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 commission ing team June 2004 The growing calibration database allows a statistical analysis of the sensitivity with respect to in strumental 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 mea surements approach theoretical expectations 1 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 5 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 B6 in the Appendix 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 obse
62. pendix Observed sensitivities in various spectroscopic settings 30 30 30 32 33 34 34 34 36 39 4 43 46 List of acronyms BIB BLIP BOB DIT ETC FWHM ICS IR IRACE 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 1 VISIR Upgrade Project During the period May August 2012 P89 VISIR will go through the implementaion of its upgrade project It is expected that VISIR will be offered back to regular service and visitor mode observa tions the last 2 weeks of September 2012 Period 89 and for the entire Period 90 The post upgrade VISIR performance is expected to be significantly better However no details can be provided at the moment For the time being we here list the major modifications that VISIR P90 science users will need to plan their proposals The astronomical community is encouraged to monitor the latest news on the upgrade reported on http www eso org s
63. pipeline 8 Checklist This section provides a number of advice regarding the preparation of the proposal 8 1 Phase 1 It is very important that the time justification Box 9 of the proposal contains enough information so that its feasibility can be correctly assessed The following points must be respected 1 the expected S N for each object and modes must be given 2 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 3 in spectroscopy does the S N refer to one pixel in the dispersion direction as given by the ETC or to one resolution element 4 in case of large throw does the S N take into account the fact that some beams would fall outside the detector 5 does the overhead calculation include the time required for each preset given that OBs should in general not be longer than 1 hour 6 is there a guide star brighter than V 13 5 mag within a radius of 7 5 arcmin around the object 8 2 Phase 2 1 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 2 Acquisition If the VISIR img ac
64. q 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 co ordinates of TEL TARG ALPHA and TEL TARG DELTA or use TEL TARG OFFSETALPHA and TEL TARG OFFSETDELTA See 5 3 3 Acquisition Itis 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 1s required e the object is faint or diffuse and unlikely to be visible on short exposures e the object appears in the field of a bright nebula that saturates the digitized sky survey DSS used by the telescope and instrument operator The guidecam tool see http www eso org instruments visir doc can help in se lecting 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 5 2 1 and 5 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 instrument operator about the non standard configuration In particular in spectroscopy TEL CHOP POSANG must be equal to TEL ROT OFFANGLE
65. right star used to center the target the faint object B at the center of the field The telescope will first point at the object A The instrument operator centers it properly Once done the telescope is offset so that object B is now properly centered and the observation templates can be executed Following the convention described in the text and since the target object B is at the east of the offset star TEL TARG OFFSETALPHA is negative on the other hand the target is at the south of the offset star so TEL TARG OFFSETDELTA is positive 5 3 2 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 9 The effect of all acquisition templates is first to point the telescope so that the coordinates at the center of the detector match e the target coordinates if no blind offset is used 2This convention is identical to the UVES one but differs from example from the ISAAC or NACO one e the offset star coordinates otherwise within the accuracy of the VLT pointing see below For VISIR spec
66. rns with amplitudes of 10 Calibrators are frequently observed during the night 95 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 ofview is defined as the limiting flux of a point source detected with a S N of 10 in one hour of on source integration T T vd L Z H a 0 6 F Pd E E ut E 4 D m La a Ld 4 o s m D m P Ro Nu nu x al TES mat E aH a a 0 4 gt LL n e E le di ma D 3 Bg a E PLI n E d 7 o L gt 4 0 2 F pa J Ed E P T T T T T T E T T T T L 7 B Ee 0 6 e 4 H Y a L 4 A A H y 4 E L E ua E o J vd Li E 0 4 m A P d d O w 5 Gel a na D B mna E P y x x d W D B Caia tt o L e AB r e AR A a 4 E E a ard o0 A ma BAS n D E L D D al 02 7 qid A IR x Es A 1 ri l 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 Staring images Staring images A chopper position A chopper position Chopped images Nodding A beam Nodding B beam subtr chopping and nodding technique of observation Chop
67. rving 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 s 5 7 For up to date information please consult http www eso org instruments visir The use the VISIR exposure time calculator ETC located at http www eso org observing etc is recommended to estimate the on source integration time 4 Instrument description and offered observing modes VISIR offers two spatial scales in imaging and several spectral resolution modes in slit spectroscopy 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 Gifford McMahon closed cycle coolers are used to maintain the required temperature 33 K for most of the structure and optics and lt 15 K for the parts near the detector The detectors are cooled down to 5 6K 4 1 Imager The imager is based on an all reflective design The optical design is shown in Fig l6 It consists of two parts e A collimator which provides an 18 mm 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 strayliht and excessive background emission The collimator mirror M1 is a concave aspherical mirror I
68. ster 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 path is not exactly the same in both chopper positions Therefore a residual background remains It is varying at a time scale which is long compared to that of the sky This residual is suppressed by nodding where the telescope itself is moved off source and the same chopping observations as in the on source position is repeated An illustration of the chopping and nodding technique is shown on Fig 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 3 5 Sensitivity Measurements of VISIR sensitivities are based on observations of mid infrared calibration standard stars Cohen et al 1999 AJ 117 1864 In imaging mode the stars are recorded in the small field 0 045 and intermediate field 0 076 by perpendicular chopping and nodding patte
69. t 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 0 045 small field SF and 0 076 intermediated field IF pixel scale are offered Table 1 These offered pixel fields of view pfov ensure a proper sampling of the images in the N and Q band The filter wheel is located just behind the cold stop pupil mask The list of filters offered is given in Table 2 The transmission curves of the filters measured at 35 K are plotted in the Appendix Starting Period 81 another set of filters is offered their characteristics are summarized in Table B EE AAA A median small field 1 00 E WV median intermed field d e Y 4 y 7 o j i A CH L n 2 4 gt E A ZS 10 pog AM E T E T e A A zm D A o e E 4 lh ARIII SIV PAH2 PAH2 2 NEII 1 PAH1 SIV_1 SIV_2 SIC NEII 1 NEII 2 AAA itis ata ARR Cee Oni zh EEE anol EE Ki 8 9 1 11 12 13 wavelength um DR E A median small field WV median inteimod field E S 100 E 4 E T C Sr We L A E E L E A y gt L gt O Cc o 105 E L Q1 Q2 Q3 J 170 175 18 0 185 190 195 20 0 wavelength um Figure 5 Measured on the old DRS detector but also valid for Period 90 Sensitivities for the VISIR imager for the N top and Q ba
70. uals stripes etc still need to be investigated 6 VISIR data 6 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 In Burst mode a number of FITS files is saved Each file is a data cube containing up to 1500 ele mentary frames The total number of files is such that it sums up the total integration time requested For the default value of the rotator angle 0 images are oriented North up and East left Spec troscopic data are aligned horizontally in the spatial and vertically in the dispersion direction cf Fig 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 6 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 imaging and spec troscopic observations are recombined Spectra are extracted and calibrated in wavelength 86 3 for all spectroscopic modes in low medium and high resolution Sensitivity est
71. ugh an acquisition template The target coordinates name and proper motion are all set in the acquisition templates The execution of the acquisition templates presets the telescope to the target coordinates given by TEL TARG ALPHA and TEL TARG DELTA Offsets with respect to the target coordinates can be spec ified by TEL TARG OFFSETALPHA and TEL TARG OFFSETDELTA and allow for example to use a bright reference star for precise acquisition see Fig 13 To guarantee proper centering within the slit when using a reference star the angular separation between the reference star and the target should not be larger than 60 Acquisition with a reference star has not been tested with the nar row 0 4 slit and should be avoided Note that the coordinates of the target TEL TARG ALPHA TEL TARG DELTA and the offsets to the reference star TEL TARG OFFSETALPHA TEL TARG OFFSETADEL I must be indicated in the acquisition template Thus the following convention TEL TARG ALPHA TEL TARG OFFSETALPHA RA offsetstar TEL TARG DELTA TEL TARG OFFSETADELTA DEC offsetstar will be used and the telescope is preseted to the reference star Once the reference star is properly centered TEL TARG OFFSETALPHA is subtracted back and the telescope is moved to the target A Is e B Figure 13 Setting the correct values of the TEL TARG OFFSETALPHA and TEL TARG OFFSETDELTA for a blind offset Here the object A is a b

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