Very Large Telescope Paranal Science Operations MIDI User Manual
Contents
1. Figure 3 Principle of MIDI optics For simplicity some of the mirrors are illustrated as lenses 5 Second off axis paraboloids M2 6 Folding flat mirror M3 7 Photometric beam splitter plates 8 Folding flat mirrors for one of the photometric channels M4 and M5 9 Beam combiner beam combiner plate M6 and M7 10 Filters 11 Dispersive elements prism grism 12 Cameras 13 Detector The core of the cold optics is formed by the beam combiner It consists of a half transparent plate on which the telescope beams are superimposed Nominally 50 of beam A being transmitted and nominally 50 of beam B being reflected into a common path The remaining light 50 reflection of beam A and 50 transmission of beam B is superimposed too and directed towards the detector with an extra mirror In addition the cold optics offers the possibility to extract a signal from the beams before combination by the mean of cold optics beamsplitters in order to get the photometric information see Fe bh The intensity extracted by the photometric beamsplitters for each beam is around 30 of the incoming intensity MIDI User Manual VLT MAN ESO 15820 3519 9 Figure 4 Lightpath in MIDI cold optics MIDI has two focusing mecha2. 25 eee eee Pee oe Se Re eee ee 25 7 4 1 Data handling dade a A dei a bee he A 25 ei Ae 25 a a aa aaa 26 List of Figures 1 Principle of beam combination in long baseline interferometry a 2 Mid Infrared atmospheric transmission 04004 5 3 Principle of MIDI optics For simplicity some of the mirrors are illustrated as ork SED Nene hee eh ee le ee ae wee oe ce 8 4 Lightpath in MIDI cold optics 4 2 42046 24244444 e eed eee ED 5 MIDI warm optics with individual elements labeled 2 10 6 mage of dispersed fringes obtained in laboratory by MIDI with its grism 11 7 Dispersed Fourier mode of MIDI The white fringe tracking is no longer used DEENEN 15 List of Tables 1 MIDI detector characteristics 000 00 0 ee ee 10 2 Characteristics of the MIDI ter 13 3 Sensitivity limits for MIDI ocres 19 MIDI User Manual VLT MAN ESO 15820 3519 vii List of Abbreviations AT CS DIT DRS ESO FOV FWHM IRIS ISO MACAO MIDI MIR OB OD OPC OPD OPL OS OT P2PP QU SM SNR STRAP TSF USD UT VCM VLT VLTI VM Auxiliary Telescope Constraint Set Detector Integration Time Data Reduction Software European Southern Observatory Field Of View Full Width at Half Maximum Infra Red Image Sensor Infrared Space Observatory Multi Application Curvature sensing Adaptive Optics MID infrared Interferometric instrument Mid InfraRed Observation Block Observation De
3. Highest achievable precision for calibrated visibilities require observations taken in SCILPHOT mode with both target and calibrator brigther than 15 Jy with the UTs and 200 Jy with the ATs We recommend to observe SCI CAL SCI sequences see next Sect for high precision purposes In general it is not advisable to have calibrator fluxes too close to the limit of the respective mode For the sake of a robust calibration in particular photometrically we recommend calibrator fluxes at least 1 5 times higher than the limit for photometry 6 2 Time of observation In P90 25 minutes are the estimated execution time per OB regardless of the correlated mag nitude in N band of the target and both for science and calibrator If the user requests a single calibrator per science target SCI CAL slots of 50 minutes per calibrated visibility vs wavelength curve at a given u v point will be allocated If the user requests two calibrator observations per science target CAL SCI CAL sequence slots of 75 minutes will be allocated For certain visitor mode programs in particular for targets very close to the limit of MIDI or too faint for IRIS and otherwise challenging acquisitions a value of 80 minutes per calibrated visibility point SCI CAL has turned out to be more realistic and the user is requested to state such circumstances in the proposal in order to ensure a proper time allocation Since the DIT detector integration time of MIDI is us
4. are in phase opposition Subtracting in each detector frame the area illuminated by one interferometric channel from the area illuminated by the other will therefore cancel out the thermal background in the frame 2 5 Atmospheric turbulence Atmospheric turbulence is a major contributor to the difficulty of optical and infrared interfer ometry from the ground Rapid atmospheric variations of the OPD between the two arms of an optical or infrared interferometer if uncorrected will cause a smearing of the fringe pattern and a strong decorrelation of the observed signals The correlation time of the atmosphere is milliseconds in the visible and increases to hundreds of milliseconds in the mid IR Seeing is the historical term for the fact that the image produced by a telescope which is larger than approximately ten centimeters in optical is not as sharp as one would ex pect from its size and optical quality but is fuzzy The fuzziness changes with time and weather conditions A short exposure milliseconds of such an image reveals a large number of speckles These speckles move around disappear and reappear on short time scales The speckle image is in fact a result of interference between wavefronts of randomly appearing moving and disappearing sub apertures These sub apertures are defined by the atmospheric turbulence bubbles in the first few tens to hundreds of meters above the telescope The sensitivity of interferome
5. target that is embedded in a complex structure in IR etc In this case 30 minutes of overheads for the scientific target observation are granted and the allocated time to get a calibrated visibility is 80 minutes 7 3 The VLT software environment for phase 2 Observations are described by Observing Blocks OBs A standard OB consists of e A target coordinate set e A target acquisition template e A set of observation templates for data collection e A constraint set e A list of intervals of the local sidereal times at which the observations shall be executed The templates are the atoms of an OB sequence They represent the simplest units of an observation and are described extensively in the Template Manual The P2PP software enables users to create lists of targets For each target one or several OBs can be created with P2PP by selecting templates and by filling request keyword values free parameters of the templates and intervals of the local sidereal times at which the observations shall be executed The P2PP user manual is available at the ESO website http www eso org sci observing phase2 P2PP P2PPDocumentation html For a detailed description of the MIDI templates please refer to the P90 MIDI Template Manual This document will be available online with the announcement of observing time web letters 7 4 Post observation process 7 4 1 Data handling Data from the MIDI instrument will be stored as FITS binary
6. 2 29 arcsec on sky with the 1 8 m auxiliary telescopes It is important to notice that the prism and the grism setups use different cold cameras providing different magnification factors on the detector e Prism mode Field camera as for image acquisition magnification 3 pixels per A D 1 A D 0 26 arcsec on sky with UTs 1 14 arcsec with ATs e Grism mode Spectral anamorphic camera magnification 2 pixels per A D along the y axis and 1 pixel per A D along the x axis dispersion direction 4 4 Fringe exposure In P90 only the classical dispersed Fourier Mode mode is offered 4 4 1 Dispersed Fourier mode In dispersed Fourier mode Fig 7 fringes are scanned over an OPD that is several A long to get interferograms fringe packets showing the main lobe of the coherence envelope As the background is strongly correlated between the two interferometric channels subtraction will cancel the background and enhance the interferometric signal fringes are shifted by 7 between the two recombined beams The signal obtained for one scan if the OPD has been compensated to allow fringe detection is the actual interferogram Since self fringe tracking is used the zero OPD point is computed in real time by MIDI for each scan and then converted into an offset sent to the tracking delay line In dispersed Fourier mode we now systematically use group delay tracking the OPD is measured from the position of a fringe peak in
7. PRISM 4 1 UT HIGH_SENS GRISM 2 8 3 UT SCIPHOT PRISM 3 2 2 UT SCI PHOT GRISM 2 6 AT HIGH SENS PRISM 0 74 20 AT HIGH SENS GRISM 0 31 30 AT SCLPHOT PRISM 0 0 40 AT SCLPHOT GRISM 0 44 60 The correlated flux CF is defined by the uncorrelated flux PF for photometric flux mul tiplied by the estimated visibility i e the product brightness x visibility Extended objects might be well above these limits for uncorrelated flux but have correlated fluxes below them In such cases i e when only the CF is below the limiting magnitude for the mode but still above the fringe tracking limit see correlated flux mode for UTs fo ATs the limit is estimated by CF 0 5 x PF observations will be moved to visitor mode as such observations usually require real time decisions on observing strategy All service observing modes rely on IRIS for acquiring the target and thus must not be fainter than the IRIS limiting magnitude see the VLTI manual If the target is invisible in IRIS but above the PF flux limit it can only be done in visitor mode including the additional acquisition overhead see 7 2 Still even a bright target may be so extended and therefore has such a low visibility that no fringe tracking is possible on it In this case MIDI cannot obvserve it The visual brightness will determine whether MACAO or STRAP can be used on the target or not details are found in the VLTT user manaual MIDI User Manual VLT MAN ESO 15820 3519 20
8. The software tool used to build OBs is called the P2PP tool It is distributed by ESO and can be installed on the personal computer of the user See http www eso org observing p2pp Service Mode SM In service mode opposite of the visitor mode see below the observations are carried out by the ESO Paranal Science Operation PSO staff Observations can be done at any time during the period depending on the CS given by the user OBs are put into a queue schedule in OT which later sends OBs to the instrument Template An elementary sequence of operations to be executed by the observation software OS of the instrument The OS dispatches commands written in templates not only to instrument modules that control the detector and motors but also to the telescopes and VLTI subsystems Template signature file TSF The file which contains input parameters for a template Some of these parameters can be set by the user Visitor Mode VM The classical observation mode The user is on the site to supervise his her program execution 1 4 Contacts The authors hope that this manual will help to get acquainted with the MIDI instrument before writing proposals especially to scientists who are not used to interferometric observations This manual is continually evolving and needs to be improved according to the needs of observers If you have any question or suggestion please contact the ESO User Support Department http www eso org s
9. e Acquisition imaging mode several spectral filters available e Beam combination HIGH_SENS high sensitivity no simultaneous pho tometric channel or SCI_ PHOT science with photometry simultaneous photometric channels CORR FLUX correlated flux no photometry on target just on calibrators e Fringe type dispersed normal e Spectral mode prism or grism with 200 ym 0 52 arcsec on sky with UTs 2 29 arcsec with ATs slit e Detector readout integrate then read e Fringe exposure Fourier mode long scan with dispersion self fringe tracking The Integrate Then Read mode has been introduced in detail in Sect In the following details about the modes for the beam combiner the dispersive element and the fringe exposure are described 4 1 Acquisition As a rule IRIS see Sect is used for acquisition If IRIS cannot be used or the user specifically requests a MIDI acquisition image for scientific purpose the MIDI imaging mode will be used for acquisition instead or in addition The imaging mode requires the selection of a spectral filter The N8 7 N band short wavelengths filter normally yields the best signal to noise ratio in the exposures obtained from acquisition Nevertheless the user has the possibility to select the filter for the acquisition This is important if for example the target shows strong absorption around 8 7 um Table 2 gives the characteristics of these filters More d
10. observations For each science target a calibrator star must be provided by the user with the submission of the phase 2 material To help the user to select a calibrator a tool called CalVin is provided by ESO CalVin can be used from any web browser Like VisCalc CalVin can be used on the web from http www eso org observing etc The users have also the possibility to use spectrophotometric calibrators as calibrators of their targets if they wish to later perform a calibration of the correlated flux A list of spectrophotometric calibrators MIDI consortium calibrators that are also referenced in the ISO catalogs is available at http www eso org sci facilities paranal instruments midi tools spectrophot_std html The standard observation consists of a SCI CAL pair The additional time for the CAL SCI CAL sequence has to be included in the phase 1 proposal 6 6 Observation constraints The Moon constraint is irrelevant for mid IR observations However if the Moon is too close to the target the scattered moonlight may prevent adaptive optics from working correctly The VLTI astronomers make sure that the OBs in service mode are executed when the Moon is far enough from the targets In visitor mode users should carefully schedule their night time using Moon ephemeris to avoid problems of scattered moonlight The seeing condition has no impact on the MIDI image quality as long as the seeing and the To allow madaptive opt
11. starts an OB on the instrument the acquisition template begins The sequence of this template starts by a preset the target coordinates Le A and the target proper motions are sent to the telescopes and the delay lines so they can slew to the position corresponding to the target coordinate at preset time Once the telescopes are tracking and Coud guiding i e the adgaptive optics loops are closed on both telescoped the target can be seen in the FOV of MIDI in imaging mode To ensure beam interference the images from both beams must be overlapped The beam overlap is performed by either e Using the IRIS guiding system see 5 2 IRIS will offset the telescopes to bring the target photocenter onto the reference pixels defined for the IRIS vs MIDI alignment of its detector IRIS guarantees that the beam overlap is kept by sending corrections to the telescopes Experience has shown an acquisition accuracy pf the order of 0 15 pixels with IRIS This method will be used as default e Using the MIDI acquisition image setup and its associated script that repeats several iterations of the sequence star photocenter measurement then offsets calculated and sent to the telescope to bring the star image on a reference pixel of the detector The MIDI User Manual VLT MAN ESO 15820 3519 24 loop stops when the error vector for both beams is smaller than a pre defined threshold 0 7 pixel This method is now considered as obsolete and is used on
12. table files Because of the high frame rate of the MIDI detector the amount of produced data is bulky One should expect at least 1 2 Gbyte of raw data for each calibrated visibility measurement To ease data handling an exposure is split into several 100 Mbyte files if the exposure is larger than this size and a file containing the information about the organization of the exposure in multiple files is generated 7 4 2 The pipeline Information concerning the pipeline quick data processor to assess the validity of exposure data and the data quality control can be found on the web at http www eso org observing dfo quality For the data reduction there are several IDL packages MIA developed by MPIA EWS de veloped by Leiden Observatory OYSTER an interface for MIA and EWS More information can be found at MIDI User Manual VLT MAN ESO 15820 3519 26 http www mpia hd mpg de MIDISOFT mia html http www eso org chummel midi midi html 7 4 3 Data distribution For any information on the ESO data distribution policy please check the webpage http www eso org sci observing phase2 DataRelease html MIDI User Manual VLT MAN ESO 15820 3519 oho 27
13. timescales similar or shorter than the temporal separation of the observation MIDI User Manual VLT MAN ESO 15820 3519 16 Since faint targets at 0 2 Jy cannot be acquired with MIDI itself correlated flux mode targets have to be sufficiently bright in K band to be acquirable with IRIS This mode is currently offered for UTs and in visitor mode only MIDI User Manual VLT MAN ESO 15820 3519 17 5 THE VLTI ENVIRONMENT IN PERIOD 90 This section only deals with aspects of the VLTI that are unique to MIDI obser vations For a complete overview see the VLTI user manual 5 0 3 Chopping Performing mid IR observations requires to discriminate the faint stellar signal against a strong and variable background that mostly comes from the sky and optical train thermal emission see Sect DA The standard technique consists in moving the secondary mirror of the tele scope M2 at a rapid frequency of the order of one Hertz for the VLTI The maximum chopping throw of a UT is 30 arcsec more than needed for the 2 arcsec interferometric field of the VLTI Chopping against an empty part of the sky reduces the background signal to zeroth order and normally removes the temporal background variations An intensity gradient sometimes strong remains in the background subtracted data It is due to the light paths through the system which are slightly different for the on and off positions Chopping is always used for MIDI image acquisition
14. to perform the beam overlap but not during fringe exposure in HIGH_SENS setup The power spectrum of the background variations should not affect the fringe visibility down to the offered limiting mag nitude 1 Jy in service mode with the prism and HIGH_SENS beam combiner for the UTs 5 1 Delay lines The VLTI delay lines are based on a cat s eye optical design featuring a variable curvature mirror VCM at its center The aim of the VCM is to perform a pupil transfer to a desired location whatever the delay line position In the case of MIDI the optimal pupil position is the cold stop that is located inside the cryostat after the shutter The advantages of transferring the pupil are e An optimized field of view For precise parameters see the VLTI manual Fringes can be obtained from any target within the FOV e A reduction of the thermal background related to VLTI optics Although VCMs are installed on all delay lines and are likely to be used In P90 we guarantee that the VCMs are used for MIDI observations with the ATs only 5 2 IRIS IRIS is the infrared field stabilizer of the VLTI Next to other advantages explained in the VLTIE manual IRIS allows to skip the acquisition by MIDI imaging reducing considerably the execution overheads If the target is too faint for IRIS then the acquisition by MIDI will be executed Users still have to indicate which MIDI filter has to be used for acquisition but it does not mea
15. 1 EEE 3 2 Optical Layout ss sa ss sa eee we A A Bee oe be ee ae E GG Gk a d Ee De ee E eg ya ROP ae ee Ra Skee eR Oe EO P VEN ee der A Bd Se Ae BUS e OH RE E Be Sh we ee 10 Bio RE PEI 10 A A Y 4 MIDI IN PERIOD 90 12 41 Acquisition REESEN 12 4 2 _ Beam ombination s os soes es sea be eeoa AAA A 12 4 3 Spectral dispersion e 2 a e E EE Ok ES HOE ES e A 13 4 4 Fringe exposure oe ee eee bk ee hehe ew ee eee re 14 4 4 1 Dispersed Fourier model o e 14 4 4 2 Correlated Flux mode 15 5 THE VLTI ENVIRONMENT IN PERIOD 90 17 5 0 3 EEN ose seh de oe See Hed ads e ES 17 5 1 Delay lies 644s odo ah edo oe SOO a a Aa eee de eS ee 17 ATRAE AA Ee ESS Ce RES RS 17 6 PHASE 1 PROPOSAL PREPARATION WITH MIDI 19 6 1 Target brightness deeg eee A Ede ey Se Be Woe Ba we bee 19 62 Time of observation e 20 A AAA 20 6 4 Guaranteed time observation obiectgl o 21 MIDI User Manual VLT MAN ESO 15820 3519 vi 6 5 Calibrator Star ce ae a SR Sle a RS ee ee ee ee A 21 BER Observation constraints seas cee Oe Gb DESDE ee Ree hee BS 21 Rb Teed EE ES E ees ee et ees eS ee E 22 7 MIDI OBSERVATIONS 23 a AS A o e es ee eS Re ee ee 23 7 1 1 Target acquisition oscense bee Ree ERR EE DDG ES 23 ACA SI 24 apa 24 TL4 Photometry cor gue ad dw Sok a Be ee ee E 24 7 2 Total sequence timing o Yer pha ee hea eee E es 24 7 3 The VLT software environment for phase2
16. EUROPEAN SOUTHERN OBSERVATORY ES Organisation Europ ene pour des Recherches Astronomiques dans l H misphere Austral Q Europ ische Organisation f r astronomische Forschung in der s dlichen Hemisph re ESO European Southern Observatory Karl Schwarzschild Str 2 D 85748 Garching bei M nchen Very Large Telescope Paranal Science Operations MIDI User Manual Doc No VLT MAN ESO 15820 3519 Issue 90 Date 06 03 2012 W J de Wit Prepared A Date Signature A Kaufer Approved EE RE Ree Date Signature C Dumas Released AN Date Signature MIDI User Manual VLT MAN ESO 15820 3519 This page was intentionally left blank MIDI User Manual VLT MAN ESO 15820 3519 Change Record Issue Date Section Parag Reason Initiation Documents Remarks affected 90 06 Mar 2012 Version for P90 89 22 Nov 2011 Version for P89 88 24 Feb 2011 4 4 2 6 1 Version for P88 correlated flux mode for UTs 87 23 Aug 2010 Version for P87 FINITO mode decomm 25 min per OB 86 27 Feb 2010 Version for P86 85 20 Aug 2009 4 4 5 3 Field mode decomm FINITO w AT only 84 31 Jan 2009 All Split of manual into VLTI general and MIDI specific part therefore Sect number ing changed 83 31 Aug 2008 All Presence of off axis guide star should be in dicated in Phase 1 URLs updated for new ESO web structure parameters wavelength solutio
17. ata transmission vs wavelength plots and ASCII tables are available from http www eso org sci facilities paranal instruments midi inst filters html 4 2 Beam combination As already explained in Sect the high sensitivity beam combination HIGH_SENS is realized by using the R T 50 50 combining plate alone In order to obtain photometric information exposures with one beam only first A then B through the beam combiner with the same optical path are recorded after the fringe observation has been performed MIDI User Manual VLT MAN ESO 15820 3519 13 The SCILPHOT setup uses the same combining plate as HIGH_SENS but two beamsplitting plates with R T 30 70 are inserted in the optical path of each beam before it reaches the combining plate in order to extract the photometric signal 4 3 Spectral dispersion Because of the domination of the thermal background in detector frames it is necessary to spread the incoming light onto a large zone of the detector to increase the DIT detector integration time without saturating the pixels The sampling time of one fringe has however to be adjusted to stay within the atmospheric coherence time at 10 wm 100 ms typically Re specting this rule it has been noticed that fringe dispersion on MIDI yields a better sensitivity than undispersed fringes Obviously dispersion in interferometry allows visibility calculation for different spectral chan nels In this case the minimum fringe si
18. bsence of turbulence Scintillation is less important in the mid infrared where fluctuations of sky emission sky noise dominate 2 6 Conclusion These observational difficulties mostly resulting from high background variations have led to the development of specific observation techniques These techniques are included in the templates see Sect that are used to control MIDI and the telescopes The templates are extensively described in the template manual available from the MIDI webpage at beginning of phase 2 proposal preparation for P90 MIDI User Manual VLT MAN ESO 15820 3519 7 3 MIDI OVERVIEW 3 1 A bit of history MIDI belongs to the first generation of VLTI instruments Conceptual studies for a VLTI mid infrared instrument started in 1997 The Final Design Review of MIDI was passed in early 2000 for the hardware and mid 2001 for the software The integration took place at the Max Planck Institut fiir Astronomie Heidelberg Germany After Preliminary Acceptance in Europe in September 2002 MIDI was shipped to Paranal and re assembled there in November 2002 where it obtained its first fringes with the UTs on the 12 December 2002 Since 1 September 2003 MIDI is offered to the worldwide community of astronomers for observations in service mode or in visitor mode The MIDI consortium who built and commissioned MIDI consists of several european in stitutes Max Planck Institut fiir Astronomie Heidelberg Germany Ne
19. ci observing email usd help eso org The web page dedicated to the MIDI instrument is accessible at the following URL http www eso org sci facilities paranal instruments midi MIDI User Manual VLT MAN ESO 15820 3519 3 I Telescope A OPD Combining plate Incoming OPD wavefront Figure 1 Principle of beam combination in long baseline interferometry 2 INTERFEROMETRY IN THE INFRARED 2 1 Interferometric observables An interferometer measures the coherence between the interfering light beams The primary observable at a given wavelength A is the complex visibility T Ve Olu v In this ex pression u v is the Fourier transform of the object brightness angular distribution O x y The sampled point in the Fourier plane is u B A v B A Bz By are the coordinates of the projected baseline V corresponds to the visibility Imax min Lmax Imin of the fringes its phase is related to their position The visibility can be observed either as the fringe contrast in an image plane or by modulating the internal delay and detecting the consequent temporal variations of the intensity as has always to be done in the case of coaxial beam combination This latter mode is used in the MIDI instrument In this case the light from two telescopes A and B is combined by a half transparent plate and the combiner has two output channels 1 and 2 Fig 1 The total observed intensity is ideally g
20. e mode 4 4 2 Correlated Flux mode In HIGH_SENS or SCI_PHOT mode source photometry is taken simultaneously or immediately after the fringe track Due to the background being fully correlated i e subtractable in the fringe exposure without residuals photometry is harder to obtain at good quality than fringes This is the effective limiting factor in the modes relying on photometry to calibrate data Under certain conditions this limitation can be worked around by obtaining fringes on the source only i e the measured correlated flux and comparing them to the measured vs the known correlated flux of the calibrator This way the calibrated correlated flux of the source can be determined An observing sequence to obtain a correlated flux with MIDI consists of a calibrator obser vation with photometry a science source observation without photometry and again the same calibrator with photometry This means that the science target can be as faint as the MIDI fringe tracking limit which is 0 2 Jy at the UTs but the calibrator must be brigther than the PRISM HIGH SENS UT limit of 1 Jy The use of the same calibrator for all correlated flux measurements of a given target eliminates potential problems due to not well determined calibrator fluxes In order to compare the correlated target fluxes to each other which is necessary to infer target geometry as they are not absolutely calibrated file visibilities the target must not be variable on
21. gnal for each spectral channel must be more or less equivalent to the limiting correlated magnitude yielding a visibility over the whole N band see Sect 7 1 The estimated spectral resolution A AA of the prism is R 30 at A 10 6 ym The relationship between wavelength in micron and detector pixel on MIDI with the prism is given by XAG OX EG where X is the pixel position 1 left edge column 320 right edge column in the displayed image The coefficients are Ca 0 0001539 C 0 009491 Co 15 451905 The accuracy of the above formula with these coefficients is 0 2 wm Future MIDI calibrations should further refine these values The estimated spectral resolution A AA of the grism is R 230 at A 10 6 wm Its Co Ci and C coefficients see above for definition are Table 2 Characteristics of the MIDI filters Name Central wavelength um FWHM um Nband 10 34 5 24 SiC 11 79 2 32 N8 7 8 64 1 54 ArII 9 00 0 13 SIV 10 46 0 16 NelI 12 80 0 21 N11 3 11 28 0 60 MIDI User Manual VLT MAN ESO 15820 3519 14 C 1 21122 x 10 6 Ci 00 0232192 Co 6 61739 The accuracy of the above formula with these coefficients is 0 05 um Future MIDI calibrations should refine these values In spectral dispersion a slit is inserted in the intermediate focus inside the cold optics of MIDI The width of this slit is 200 wm which corresponds to 0 52 arcsec on sky with the 8 m unit telescopes and to
22. h period Ao 1 OPD appear in the channelled spectrum provided that Jo o and Die do not vary strongly with wavelength 2 3 Atmospheric transmission The thermal infrared 8 to 25 um atmospheric transmission is dominated by aerosol particles and by various molecular species including Oz 9 6 wm CO 14 um and H20 lt 7 um which absorb large parts of the infrared spectrum Some of these components are constant for many hours at a time and over the whole sky others may be variable on quite short time scales or over short distances in the sky Ground based mid infrared observations can only be made in two atmospheric transparency windows the N band wavelengths between 7 5 and 14 um and the Q band 16 to 28 um but even in these windows the atmospheric absorption and radiation are a major disturbance Nevertheless new detector technology and the extremely dry mountain top of Cerro Paranal can improve the data quality Transmission spectra are shown in Fre D 2 4 Background emission The atmosphere and telescope thermal radiation causes a high background that makes the observation of faint astronomical targets difficult It is not unusual to observe objects which are thousands of times fainter than the sky The mid IR 8 to 15um sky background is primarily due to thermal emission from the atmosphere i e it is equivalent to 1 transmission multiplied by a blackbody spectrum at a temperature of about 250K The transmission and t
23. herefore the emission varies with atmospheric water vapor content and air mass On MIDI the thermal background is dominated by 27 reflections when the UTs are used or 24 when the ATs are used in the optical train at ambient temperature which in combination give a 35 reflection and almost radiate like a blackbody The thermal background therefore is dependent on the sky and interferometer temperature lower in winter for instance and also on possible dust particles on the numerous mirrors used to relay the beams Under these conditions it has become a standard procedure to observe the target together with the inevitable sky and subtract from it an estimate of this sky background obtained by MIDI User Manual VLT MAN ESO 15820 3519 5 Atmospheric Tronsmission 5 10 15 20 25 A0 Wavelength um Figure 2 Mid Infrared atmospheric transmission fast switching between target and an empty sky position The method used with MIDI to deal with this correction is called chopping The characteristic time of such a correction is dependent on the sky variability which is fast and on the weather Sky subtraction has the additional advantage that detector artifacts are removed In interferometric setup without the photometric channels chopping is not employed To cancel out the background we rely on the fact that between the interferometric channels of MIDI the background is almost totally correlated whereas the interferometric signals
24. ics correction the MIDI images are diffraction limited MIDI User Manual VLT MAN ESO 15820 3519 22 The sky transparency constraint is relevant only in special cases in HIGH_SENS mode thin cloud conditions may cause variations of flux between fringe track and photometry exposures Also if a faint off axis guide star is used thin cloud conditions may have impact on the MIDI image quality On the other hand thin cloud conditions are not a problem in SCT PHOT mode if the guide star is brighter than V 15 if the UTs are used or V 12 if the ATs are used The difference between photometric and clear conditions are irrelevant for MIDI operations 6 7 Visitor vs service mode For P90 MIDI is offered in service mode and in visitor mode see Sect 1 3 For the phase 1 of a period the unique contact point at ESO for the user is the User Sup port Department see Sect DA For the phase 2 USD is still the contact point for service mode and the Paranal Science Operation department is the contact point for visitor mode http www eso org sci facilities paranal sciops VA_GeneralInfo html1 The visitor mode observations are likely to be required for proposals adopting non standard or complicated observation procedures for instance complex structures in the field of view of MIDI The OPC will decide whether a proposal should be observed in SM or VM As for any other instrument ESO reserves the right to transfer visitor programs to service a
25. ine If the SCILPHOT setup is used chopping synchronized with the scans is performed in order to remove at data reduction time the thermal background from the photometric channels This does not affect the fringe tracking 7 1 4 Photometry With the HIGH_SENS setup and in order to compute the fringe visibility from interfer ograms it is necessary to measure the flux from each beam through the beam combiner seperately using the same optical set up as for fringe measurement Two exposures are there fore taken with the MIDI shutter at different positions beam A open only then beam B open only Similar exposures are also taken in SCLPHOT since they can be used the refine the pho tometry measured in the photometric channels during the fringe exposure For P90 the number of frames for the photometry can be adjusted by the user See the MIDI template manual for details 7 2 Total sequence timing As said in Sect 6 2 for MIDI in P90 the average time to get a calibrated visibility point is 50 minutes in service mode SCI CAL Hence the time to complete the above tasks is 25 minutes This value includes all the overheads The variable duration of the photometry exposures set by the user has a minor impact on the OB execution time MIDI User Manual VLT MAN ESO 15820 3519 25 Visitor mode is usually allocated for targets that require a non standard acquisition procedure De very faint target no Coud guide star exisiting
26. ion of signal The frame rate in snapshot mode is determined by the sum of the integration time and the readout time ITR mode allows to select an integration as short as 0 2 ms per frame The minimum inte gration time is given by the time needed to propagate the reset signal The time required to readout a full frame in ITR mode is about 6 ms However in many cases there is no need to read out the full detector array it may be advantageous to use sub array readout windowing Windowing of the detector by line selection reduces the time needed for readout hardware windowing Software windowing by column selection after readout reduces the amount of data to be processed in the downstream steps of data handling Fig o shows the type of data that is collected by MIDI if a dispersive element is inserted in the optical path after the beam recombination dispersed fringes with phase opposition from the two recombined beams in the image plane MIDI User Manual VLT MAN ESO 15820 3519 12 4 MIDI IN PERIOD 90 MIDI combines many aspects that usually exist independently in several astronomical in struments This includes visibility measurements interferometry spectral dispersion spec troscopy imaging with a detector array imaging and background level plus fluctuation measurement thermal infrared imaging techniques Hence MIDI offers a large possibility of setups selectable by the user For P90 the available setups consist of
27. iven by h Isi T Igi 2V y TIaitpi sin 270PD A T o Io IR 2V y I yolpo sin 270PD A ol where J and I are the intensities from the two outputs of the combiner Isy the intensity from telescope x mixed for the channel y of the combiner and OPD the path difference introduced by the atmospheric turbulence and by the optomechanical modulator used to produce interferograms MIDI User Manual VLT MAN ESO 15820 3519 4 2 2 Visibility estimators Due to atmospheric fluctuations the fringe pattern is usually in rapid motion and the phase o of the complex visibility I cannot be estimated It is often better to work with the square of the visibility V rather than V itself because V estimators are less affected by the smearing of the moving fringe pattern Averaging the squared visibility introduces a noise bias which however can be taken into account properly Performing a spectral dispersion of the output of a two element pupil plane interferometer produces the so called channeled spectrum i e a fringe modulated spectrum I o Ip o 1 V 0 cos 2mOPDo p 0 Here is the wave number 1 1 Zo 0 the spectrum of the light target and OPD the total delay between the light path of the two telescopes Note that the above equation corresponds to an ideal case for which no atmospheric turbulence would cause OPD fluctuation and the incoming beams have the same intensity From this equation we see that fringes wit
28. kus Sch ller and Andreas Kaufer at ESO Paranal and Jean Gabriel Cuby formerly at ESO Paranal 1 3 Glossary Constraint Set CS List of requirements for the conditions sky transparency baseline of the observation that is given inside an Observation block see below which is only executed under this set of minimum conditions Observation Block OB The smallest schedulable entity for the VLT VLTI It consists of a sequence of templates see below Usually one Observation Block includes one target acquisition and one or several templates for exposures Observation Description OD Sequence of templates used to specify the observing sequences within one or more OBs Observation Toolkit OT Tool used to create queues of OBs for later scheduling and possible execution Proposal Preparation and Submission Phase 1 The phase 1 begins right after the Call for Proposal CfP and ends at the deadline for CfP During this period the potential users are invited to prepare and submit scientific proposals For more information see http www eso org sci observing phasel html Phase 2 Proposal Preparation P2PP Once proposals have been approved by the ESO Observation Program Committee OPC users are notified and the phase 2 begins In this phase users are requested to prepare their actual observations in the form of OBs and to submit them electronically in case of service MIDI User Manual VLT MAN ESO 15820 3519 2 mode
29. ly if the target is too faint for IRIS or on specific user request in case the acquisition images are of scientific value The user has a possibility to use a guide star for the Coud systems different from the target For details and limits see the VLTI user manual 7 1 2 Fringe search Once beams are overlapped fringes can be obtained provided VLTI delay line positions yield a zero OPD 30 um In this case scanning the OPD with the MIDI piezo mounted mirrors will yield interferograms In absence of an external fringe tracker fringe search for MIDI consists in doing several scans at different delay line position offsets These offsets are within a range around the expected zero OPD value given by an OPD model and the incremental step of the offset is adjusted for covering the whole fringe search range given by piezo scans The fringe search is normally executed in fast mode 500 um OPD steps between two scans and dispersion using the grism to get the maximum coherence length For targets with low correlated flux fringes can be searched in slow mode The search mode is decided by the instrument operator with view om the target parameters given by the user 7 1 3 Fringe measurement in Fourier mode Once the delay line offset that yields the zero OPD is found a batch of interferograms will be recorded in a series of scans to form an exposure Group delay measurements are used to correct the position of the tracking delay l
30. n FINITO limits 82 29 Feb 2008 7 1 Updates for P82 brightness requirement for precision visibilities 81 29 Aug 2007 all Updates for P81 sensitivities 80 25 May 2007 1 2 6 5 Notes on FINITO 8 1 4 Note on photometry frames 111 MIDI User Manual VLT MAN ESO 15820 3519 Issue Date Section Parag Reason Initiation Documents Remarks affected 80 1 March 2007 All Corrections in Sect 1 2 5 6 5 79 31 August 2006 All Global revision for P79 78 26 June 2006 All Corrected again values of off target guiding distance and limiting magnitude with ATs new format new paragraph 7 5 on obser vation constraints 77 7 December 2005 6 4 Paragraph on IRIS Editors Willem Jan de Wit Thomas Rivinius S bastien Morel ESO Paranal Science Operations wdewit eso org MIDI User Manual VLT MAN ESO 15820 3519 v Contents 1 INTRODUCTION EU OOD eos NA AS a et eee vet ere ot A 1 2 Acknowledgements cocer a a EEN yee a Bele A LA lee A ae Si ae ea PEERS A e e T4 Contactos EENHEETEN N hh Fi 2 INTERFEROMETRY IN THE INFRARED 2 1 Interferometric observables e 2 2 Visibility estimators 2 4 4344 4 6o ee OSS DEORE ee Oe Bee Oe ed es 2 3 Atmospheric transmission 2 2 NN e GRASP RR ERS EN BO 2 4 Background emission ad r A Peay be Mee A E E eR eS 2 5 Atmospheric turbulence oaoa aa A 4 4 444444442686 ee he Dex Ee Daa PE E Q a VG VG WwW D 3
31. n that MIDI acquisition exposures will be taken MIDI User Manual VLT MAN ESO 15820 3519 18 As a rule we do not guarantee that MIDI acquisition images are taken in normal operation However acquisition images with MIDI can specifically be requested by the user at phase 2 if they are to be used for scientific purposes MIDI User Manual VLT MAN ESO 15820 3519 19 6 PHASE 1 PROPOSAL PREPARATION WITH MIDI Submission of proposals for MIDI must be done using the ESOFORM LaTeX template It is important to carefully read the following information before submitting a proposal as well as the ESOFORM user manual The ESOFORM package can be downloaded from http www eso org sci observing proposals esoform html Considering a target which has a scientific interest and for which MIDI could reveal interesting features the first thing to do is to determine whether this target can be observed with MIDI or not Several criteria must be taken into account 6 1 Target brightness The brightness of the target in the N band will determine whether it is observable at all in self fringe tracking mode and whether meaningful photometry data can be obtained or only correlated flux can be measured The limiting magnitudes for MIDI observations on unresolved objects in P90 are the following Table 3 Sensitivity limits for MIDI Telescopes Beam combiner Dispersion Limiting N mag Equiv in JyQ12 um UT CORR_FLUX PRISM 5 7 0 2 UT HIGH_SENS
32. n the cryostat the elements of the cold optics appear in the following sequence along the beams 1 Shutter 2 Cold pupil stops 3 First off axis paraboloids M1 4 Diaphragms in intermediate focus spatial filters MIDI User Manual VLT MAN ESO 15820 3519 8 Principle of MIDI the MID infrared Interferometer for the VLTI Delay line variation by movable roof mirrors on Piezo stages gt dl meee il Il E IT ne i I S res See f l Et P i 70 K radiation shield S Hy 4 l H Cold box 40 K radiation shield i l a 21 3 i n 4 Dewar i Intermediate L d N li 1 4 E ki window 1 Bi focus Photometric channels pl ZnSe CL Off ae Off axis gt my ES parabdoloids A fe I GO W i l P paraboloids s AH i LI Beamcombiner 7 la E i ea i S Se i e E a E gt rel Le i gt H gt E pl Beam compressors MIRA Ze Se ne rie CH 80 gt 18 mm beamwidth ty E ee a a Y 50 50 Coating i i ce i a ps UN 11 gt ripe Ir b gt gt ane db li Y AA A l A 11 Beamwidth ee a ft Ur Pupil Field stop 10 mm File IT YY YY 2 Ey I i T Ten l a i stop Spatial filter 14 i i d I 3 ji i Photometric Gtismn Prism 2 MH pl Beamsplitter pl aie d 30 70 Camera 75 E C 1 l d d pl EE d Optics T 40K Detector A He Detector T 5K 240x320 e i i Es l
33. nd vice versa MIDI User Manual VLT MAN ESO 15820 3519 23 7 MIDI OBSERVATIONS Once a MIDI proposal is accepted by the OPC it must be set in a form that it can be carried out by Paranal Science Operation Some knowledge of the observation sequence of MIDI is necessary before tackling phase 2 7 1 Observation sequence An observation with MIDI in P90 can be split in the several subtasks 1 Slew telescopes to target position on sky and slew one of the delay lines to the expected zero OPD position Bring them in tracking state pre defined sidereal trajectory 2 Bring telescopes in Nasmyth UT only then Coud MACAO or STRAP guiding mode use of a guide star for field stabilization 3 Adjust telescope positions so the beams from the target will overlap inside MIDI and be recombined 4 Search the optical path length OPL offset of the tracking delay line yielding fringes on MIDI actual zero OPD by OPD scans at different offsets 5 Go back to the OPL offset corresponding to zero OPD and start to record data of interest interferograms obtained by OPD scans fringe exposure 6 Integrate exposures for photometry in the same instrument configuration first with beam A only then with beam B only Usually the sensitivity is limited by the internal fringe tracking which has to be performed with integration times that are shorter than the atmospheric coherence time 7 1 1 Target acquisition When the operator
34. nisms the first one actuates the M1 mirrors to focus the beams on the spectroscope slits if dispersion is used or on future spatial filters and the second one translates the detector plane to bring it at the focus of the camera germanium set of lenses Baffling within the instrument is mainly done by a cold pupil stop just downstream of the entrance window of the cryostat 3 2 2 Warm optics OPD modulation is performed in the ambient temperature laboratory environment An overview on this so called warm optics is given in Fig 5 As said above the degree of coherence of the light target i e the object visibility at the actual baseline setting is mea sured by stepping the internal OPD rapidly over at least one wavelength within a time when the fringe has not moved more than 1 um on the average t lt 0 1s This operation is per formed by two dihedral mirrors mounted on piezo translators DLA and DLB one for each arm One piezo is used for OPD modulation A translation stage underneath one of them DLA allows to change the internal OPD by a larger amount As sketched by Pie the warm optics bench also includes ancillary devices for calibration and alignment that can be inserted in the optical path These devices are beamsplitter and feeding optics for a CO test laser A 10 6 um spatially incoherent blackbody target and alignment target plates MIDI User Manual VLT MAN ESO 15820 3519 10 To Cryostat Ma
35. ollect as much information on their targets as possible before submitting a MIDI proposal For instance a proposal for a pre imaging study on VISIR VLT instrument may help to know more about the feasibility on MIDI MIDI User Manual VLT MAN ESO 15820 3519 21 6 4 Guaranteed time observation objects proposers must check any scientific target against the list of guaranteed time observation GTO objects Only the GTO consortium is allowed to observe these objects with MIDI The GTO target list is submitted every period individually To make sure that a target has not been reserved already for P90 the list of GTO objects can be downloaded from http www eso org sci observing visas gto 6 5 Calibrator stars High quality measurements require that the observer minimizes and calibrates the instrumental losses of visibility To get a correct calibration the user should use appropriate calibrator stars in terms of target proximity calibrator magnitude and apparent diameter In the case of MIDI two calibration sequences are offered In the SCI CAL sequence the calibrator is as a rule observed after the science target using the same templates Exceptionally the calibrator may be observed before for operational reasons e g when the target is faint and pupil alignment is a mandatory step like after a baseline change Alternatively the user can request a CAL SCI CAL sequence where the science observation is sandwiched between calibrator
36. scription Observation Program Committee Optical Path Difference Optical Path Length Observation Software Observation Toolkit Phase 2 Proposal Preparation Quality Control Service Mode Signal to Noise Ratio System for Tip tit Removal with Avalanche Photodiodes Template Signature File User Support Department Unit Telescope Variable Curvature Mirror Very Large Telescope Very Large Telescope Interferometer Visitor Mode MIDI User Manual VLT MAN ESO 15820 3519 1 1 INTRODUCTION 1 1 Scope This document summarizes the features and possibilities of the MID infrared Interferometric instrument MIDI of the VLT as it will be offered to astronomers for the six month ESO observation period P90 running from 1 October 2012 to 31 March 2013 The bold font is used in the paragraphs of this document to put emphasis on the important facts regarding MIDI in P90 This manual is intended to be used together with the VLTI user manual The VLTI user manual contains important information on the VLTI subsystems relevant for MIDI observations 1 2 Acknowledgements The editors thank Olivier Chesneau Observatoire de la C te d Azur France who wrote the very first version of this manual in August 2002 and also contributed to the update by providing us with some important MIDI facts after commissioning runs The editors also thank Monika Petr Gotzens Andrea Richichi and Markus Wittkowski at ESO Garching for their comments as well as Mar
37. ster Alignment Plate MAP A EE Laser Feed Carriage BSL beamsplitter From Laser Slave Alignment Plate SAP Beam A Beam B From VLTI Black Screen Z Laser Alignment Plate LAP lt ___ Figure 5 MIDI warm optics with individual elements labeled 3 2 3 Dispersion MIDI has a spectroscopic mode based on either a NaCl prism or a KRS5 grism that are used along with beam combination The interest of such a mode besides the possibility to measure fringe visibility for different spectral channels in the N band is explained in Sect 3 3 Detector The detector is a 320x240 pixel Raytheon Si As Impurity Band Conduction IBC array also called BIB blocked impurity band The detector characteristics are summarized in Tablel1 Table 1 MIDI detector characteristics Array dimensions Pixel Size Peak Quantum efficiency Dark current per pixel Operating temperature Well capacity RON 320 x 240 50 ym x 50 um 34 104 e7 s at 10K 4 to 12K 1 1 x 107 e7 x 800 e7 MIDI User Manual VLT MAN ESO 15820 3519 11 Figure 6 Image of dispersed fringes obtained in laboratory by MIDI with its grism The standard operating mode of the detector is called Integrate Then Read ITR In this snapshot mode a reset is performed simultaneously on the whole chip before the start of integration The end of integration is set by applying a bias voltage that inhibits further accumulat
38. ters strongly depends on the seeing i e on the Fried param MIDI User Manual VLT MAN ESO 15820 3519 6 eter defined as follows the resolution of seeing limited images obtained through an atmo sphere with turbulence characterized by a Fried parameter ro is the same as the resolution of diffraction limited images taken with a telescope of diameter ro Observations with telescopes much larger than ro are seeing limited whereas observations with telescopes smaller than ro are essentially diffraction limited The scaling of r with wavelength is favorable for MIDI since l ro x A which at 10 um and for a 0 8 arcsec seeing in the visible leads to r 4 6 m It is therefore much easier to achieve diffraction limited performance at longer wavelengths and simple tip tilt correction already will give correct results with large apertures An interferometer correctly works only if the wavefronts from the individual telescopes are coherent i e have phase variances not larger than 1 rad Scintillation is another consequence of inhomogeneities in the atmosphere at altitudes of some kilometers Phase gradients due to the pressure gradients of the turbulent air cause mild deflections of the direction of travel of the wavefront The cross section of the resulting cone of rays intercepted by the telescope pupil is at any time brighter or fainter than the average intensity the cylinder of rays that would ideally feed each telescope in a
39. the Fourier transform of the frame showing dispersed fringes Scanning the OPD remains necessary to determine the OPD sign and also to provide a better quality of the visibility in each spectral channel at data reduction time A raw uncalibrated visibility estimation requires a few hundred scans The parameters for fringe measurement in dispersed Fourier mode in P90 are the following indicative values and subject to minor changes e Dispersion by either prism or grism see Sect 4 3 e One 1 detector frame per OPD sample e Five 5 OPD samples for fringe MIDI User Manual VLT MAN ESO 15820 3519 15 Intensity integrated over A lt q measured point of zero OPD One scan OPD white fringe tracking AabOUt ds ea re 100 um t OPD Repeat scans gt Process of scans gt visibility OR o group delay tracking t OPD one frame Intensity Fringe peakposition gt OPD FT 2 wavenumber Repeat the same on several frames gt OPD sign Figure 7 Dispersed Fourier mode of MIDI The white fringe tracking is no longer used for observations e 8 fringes per scan gt OPD scanning range 80 um e Zero OPD offset 0 or 50 um After completion of the exposure each interferogram can be individually processed by Fourier transform to yield a raw visibility The algorithm used for this computation usually involves a Fourier transform of the interferograms hence the name of this fring
40. therlands Graduate School for Astronomy NOVA Leiden The Netherlands Department of Astronomy Leiden Observatory The Netherlands Kapteyn Astronomical Institute Groningen The Nether lands Astronomical Institute Utrecht University The Netherlands Astronomical Institute University of Amsterdam The Netherlands Netherlands Foundation for Research in Astron omy Dwingeloo The Netherlands Space Research Organization Netherland Utrecht and Groningen The Netherlands Thiiringer Landessternwarte Tautenburg Germany Kiepen heuer Institut fiir Sonnenphysik Freiburg Germany Observatoire de Paris Meudon Meudon France Observatoire de la Cote d Azur Nice France 3 2 Optical Layout Inside MIDI the beam combination occurs in a plane close to the re imaged pupil and the signal is detected in an image plane infinity MIDI in the interferometric laboratory is composed by two main parts the warm optics on the MIDI table and the cold optics in the cryostat In addition in the adjacent Combined Coud Laboratory are located an infrared CO laser used for calibration measurements the control electronics and the cooling system 3 2 1 Cold optics Since radiation at 10 um is dominated by thermal emission from the environment most of the instrument optics has to be in a cryostat and to be cooled to cryogenic temperatures 6 K to 12 K for the detector 40 5 K for the cold bench 77 K for the outer radiation shield I
41. ually determined by the level of the thermal background illuminating the detector the user will have no freedom on this parameter Paranal Science Operations can adjust parameters of fringe exposures for faint objects but will do so only during VM observations on request of the visiting astronomer at the console The values of these parameters which are not visible for the user from the templates are included in the FITS header of the data files that will be delivered to the user after observation 6 3 Geometry Important parameters of the instrument to be taken into account for the preparation of the observing schedule are the VLTI geometry during observation the u v coverage The selec tion of the baseline requires the knowledge of both the geometry of the VLTI and of that of the target To assess observability of a target with VLTI it is mandatory to use the VisCalc software The front end of VisCalc is a comprehensive web based interface VisCalc can be used from any browser from the URL http www eso org observing etc It is important to check that the altitude of the object is not below 30 during the observations which is the operational limit of the VLTI Since we had problems in service mode in the past with over resolved targets which appeared resolved in imaging mode at the acquisition or for which no fringes were found or with low precision IR coordinates which could not be seen on MIDI we encourage the users to c
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