Very Large Telescope Paranal Science Operations MIDI User Manual
Contents
1. i from telescope i Beam compressors 80 gt 18 mm beamwidth e A Field stop r Spatial filter ic Beamsplitter 30 70 Camera e from telescope 2 Optics T 40K Detector A lt Ga Detector T 5K 240 x 320 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 with nominally 50 of beam A being transmitted and nominally 50 of beam B being reflected The remaining light 50 reflection of beam A and 50 transmission of beam B is superimposed too and directed towards the detector thanks to 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 Fig 4 The intensity extracted by the photometric beamsplitters for each beam is around 30 of the incoming intensity MIDI has two focusing mechanisms th2. For P83 the available setups consist of 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 e Fringe type dispersed normal or white fringe astrometry mode VM only e Spectral mode prism or grism with 200 ym 0 52 arcsec on sky 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 described in detail in Sect 4 3 Below are some details about the modes for the beam combiner the dispersive element and the fringe exposure 5 1 Acquisition As a rule IRIS see Sect is used for acquisition If IRIS cannot be used or the use specifically requests a MIDI acquisition image for scientific purpose the MIDI imaging mode MIDI User Manual VLT MAN ESO 15820 3519 14 will be used for acquisition instead The imaging mode requires the selection of a spectral filter The N8 7 N band short wavelengths filter normally yields the better signal to noise ratio in the exposures obtained from acquisition Nevertheless the user has now the possibility to select the filter of his her choice for the acquisition This is important if for example the target shows strong absorption around 8 7 um The table 1 gives the characteristics of
3. on obser vation constraints 77 7 December 2005 6 4 Paragraph on IRIS Editor Thomas Rivinius S bastien Morel ESO Paranal Science Operations triviniu eso org MIDI User Manual VLT MAN ESO 15820 3519 iv Contents 1 INTRODUCTION PEA EE EEE TRATA 1 2 Acknowledgements e 4s ieee ea ee eee a Ip L3 loss A ae bead ea eed b dE Dd PEERS PoE EEE ESE t4 Contacts 3 2 4 4 044 ooh kw Seed Re Ew a Rae Ree Re N hh pa 2 A FEW WORDS ON INTERFEROMETRY 2 1 Introduction 0 0 0 00 2 2 2 How an interferometer work 2 3 Interferometric observablegl ee ee 2 4 Visibility estimators 2 0 200 2 ean Deg a eee ee 3 OBSERVING IN THE INFRARED 3 1 Atmospheric transmission 2 2 0 0 0 ee 3 2 Background emission 44 es bb fee a beeen eed ee 3 3 Atmospheric turbulence lt ki A Beda ek OS ad AA KS a ee Soe SE OMS e aa a ra A aa aa re Ww a o DO OH 4 MIDI OVERVIEW LL A Dit ot ISE 2 i o a A AR A a e De 12 Optical Lay t e se nad at ode A e A Bs 42 1 ol Opel lala o A A ee ee ES 4 2 2 Warm opties ooa E ER 11 4 2 3 Dispersio o aaora a a eate n ea ie E Eer E EN 11 43 Detector 4 4 66 6 EECH 11 o oo o A MIDI IN PERIOD 83 13 Sl Acgu isition e a da 13 de flee ee de a a Ge a 14 e uer aa ok a a ra A 14 E SA SAS bee 15 5 4 1 Dispersed Fourier model 15 5 4 2 Field astrometry Mode e 04 REEL ER LEE EE AL E Ee 16 6 THE VLTI ENVIRONMENT IN P
4. 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 D along the y axis and 1 pixel per A D along the x axis dispersion direction 5 4 Fringe exposure In P83 two fringe modes are offered the classical dispersed Fourier Mode and the new field astrometry mode 5 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 a MIDI User Manual VLT MAN ESO 15820 3519 16 between the two recombined beams see Sect 2 1 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 mod
5. 6 to 5 arcsec with the ATs Fringes can be obtained from any target within the FOV e A reduction of the thermal background related to VLTI optics In P83 we guarantee that the VCMs are used for MIDI observations with the ATs only To compensate OPD drifts due to uncertainty of the array geometry as well as atmospheric differential piston position offsets can be applied at high rate to the moving delay line by an OPD controller The OPD controller receives commands either from the science instrument itself self fringe tracking or from a fringe tracker 6 4 IRIS IRIS is the infrared field stabilizer of the VLTI Its purpose is to perform field stabilization on the telescopes by measuring the low frequency tip tilt from the VLTI laboratory IRIS guarantees therefore the correct alignment of the beam during the observations IRIS normally operates in K band for MIDI operations The users are requested to give the H magnitude in the MIDI OBs this value allows IRIS to work at its best performances thanks to an adaptive integration time algorithm An approximation of the H magnitude can be found from the V magnitude and the spectral type of the target using the plot on Fig As another advantage IRIS allows to skip the acquisition by MIDI imaging reducing con siderably the execution overheads If the target is too faint for IRIS then the acquisition MIDI User Manual VLT MAN ESO 15820 3519 23 Spectral ty
6. GA 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 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 loop stops when the error vector for both beams is smaller than a pre defined threshold MIDI User Manual VLT MAN ESO 15820 3519 29 0 8 pixel This method is now considered as obsolete and is used only if the target is too faint for IRIS As stated in Sect 6 1 the user has a possibility to use a guide star for the Coud systems different from the target He she will have to indicate the coordinates of this star which for the UTs resp the ATs should be brighter than V 13 5 resp V 17 and within a 57 5 arcsec resp 57 5 arcsec radius from the target If no guide star exists Coud guiding will not be used which may have an impact on the fringe SNR Such observations have to be requested in visitor mode and the proposal has to indicate a seeing constraint 8 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
7. Sect 1 4 that are used to control MIDI and the telescope The templates are extensively described in the template manual available from the MIDI webpage at beginning of phase 2 proposal preparation for P83 MIDI User Manual VLT MAN ESO 15820 3519 9 4 MIDI OVERVIEW 4 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 insti tutes Max Planck Institut fiir Astronomie Heidelberg Germany Netherlands 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 Insti tute University of Amsterdam The Netherlands Netherlands Founda
8. 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 The limiting magnitudes for MIDI observations on unresolved objects in P83 are the following Telescopes Beam combiner Dispersion Limiting mag N Equivalent in Jy at 12 um UT HIGH_SENS PRISM 4 1 UT HIGH_SENS GRISM 2 8 3 UT SCLPHOT 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 For MIDI in P83 the correlated flux is defined by the uncorrelated flux multiplied by the estimated visibility i e the product brightness x visibility In case of extended objects which are bright enough to meet the above limits for uncorrelated photometric flux PF but have correlated fluxes CF below those limits fringe tracking is still possible to somewhat lower values than noted above e Without FINITO CF 0 5 x PF e With FINITO CF 0 1 x PF Still a bright target may be so extended and therefore has such a low visibility that no fringe tracking is possible on it The visual brightness will determine whether MACAO or STRAP can be used on the target or not As said before the brightness limit is V 17 for MACAO on the UTs and V 13 5 for STRAP on the ATs The field astrometry mode is offered for the UTs
9. 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 to perform the beam overlap but not during fringe exposure in HIGH_SENS setup The power spectrum of the background MIDI User Manual VLT MAN ESO 15820 3519 20 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 6 2 Baselines 6 2 1 UT Baselines For P83 All the four unit telescopes of the VLT are available for MIDI The following table gives the characteristics of the possible on ground baselines is the vector component over the East direction and N over the North direction Name E m N m On ground baseline length m UT1 UT2 24 8 50 8 56 5 UT1 UT3 54 8 86 5 102 4 UT1 UT4 113 2 64 3 130 2 UT2 UT3 30 30 7 46 6 UT2 UT4 88 3 13 5 89 3 UT3 UT4 58 3 22 2 62 4 Note that we cannot guarantee that these six baselines will actually be offered in P83 The final subse
10. 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 8 1 3 Fringe measurement in Fourier mode Once the delay line offset that yields the zero OPD is found a batch of interferograms can be recorded in a series of scans to form an exposure Group delay measurements are used to correct the position of the tracking delay line 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 8 1 4 Photometry With the HIGH_SENS setup and in order to compute the fringe visibility from interfero grams it is necessary to measure the flux from each beam through the beam combiner using the same optical set up as for fringe measurement Two exposures are therefore taken with
11. feeding optics for a CO test laser A 10 6 um spatially incoherent blackbody target and alignment target plates 4 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 5 3 4 3 Detector The detector is a 320x240 pixel Raytheon Si As Impurity Band Conduction IBC array also called BIB blocked impurity band with the following characteristics MIDI User Manual VLT MAN ESO 15820 3519 12 To Cryostat From Laser Master Alignment Plate MAP DLB B1 i Laser Alignment Plate LAP Black Screen Slave Alignment Plate GAP Beam A Beam B From VLTI Figure 5 MIDI warm optics with individual elements labeled Array dimensions 320 x 240 Pixel Size 50 um x 50 um Peak Quantum efficiency 34 Dark current 10 3 pix s at 10 K Operating temperature 4 12 K Well capacity 1 1e7 RON 800e The standard operating mode of the detector is called Integrate Then Read ITR In this mode snapshot mode before the start of integration a reset is performed simultaneously on the whole chip and further accumulation of signal is stopped by a bias voltage at the end of integration The frame rate in snapshot mode is determined by the su
12. fringes consist of a spatially modulated pattern on the detector If the two beams are superimposed coaxially as in the case of MIDI the fringes show a temporal modulation of the signal when scanning the optical path difference OPD between the two beams The angular resolution that the interferometer can achieve depends on the wavelength of observation and on the length of the projected baseline the projected baseline vector is the projection of the on ground baseline vector onto a plane perpendicular to the line of sight The projected baseline changes over the night because of Earth rotation The smallest angular separation that can be resolved is proportional to the quantity A B where A is the wavelength of the observation and B is the projected baseline of the interferometer This is equivalent to the expression for diffraction limited spatial resolution in single telescope observations where B would be the telescope diameter MIDI User Manual VLT MAN ESO 15820 3519 4 LA Telescope A CA OPD LE Combining plate ba Telescope B vax Incoming OPD wavefront gt Figure 1 Principle of beam combination in long baseline interferometry 2 3 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 O u v In this ex pression u v is the Fourier transform of the object brightn
13. 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 mode 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 MIDI User Manual VLT MAN ESO 15820 3519 2 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
14. 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 MACAO or STRAP correction the MIDI images are diffraction limited 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 SCI 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 MIDI User Manual VLT MAN ESO 15820 3519 28 8 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 8 1 Observation sequence An observation with MIDI in P83 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 te
15. only with the limiting correlated magnitude of 5 Jy If future commissioning runs prove that this limit can be lowered waiver for observing fainter targets with this mode will be considered Highest achievable precision for calibrated visibilities require observations taken in SCI PHOT mode with both target and calibrator brigther than 15 Jy with the UTs and 200 Jy with the ATs MIDI User Manual VLT MAN ESO 15820 3519 26 7 2 Time of observation It is important to know that in P83 slots of 60 minutes in service mode and 90 minutes in visitor mode per calibrated visibility vs wavelength curve at a given u v point will be allocated regardless of the correlated magnitude in N band of the target Since the DIT detector integration time of MIDI is usually determined by the level of the thermal background illuminating the detector the user will have no freedom on this parameter Paranal Science Operation can adjust parameters of fringe exposures for faint objects 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 7 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 selection of the baseline requires the knowledge of both the geometry of the VLTI and of that of
16. 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 3 2 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 therefore 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 temperat
17. use appropriate calibrator stars in terms of target proximity calibrator magnitude and apparent diameter In the case of MIDI the calibrator is observed after the science target using the same templates MIDI User Manual VLT MAN ESO 15820 3519 27 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 typical observation consists of a SCI CAL pair It is possible to request a CAL SCI CAL sequence but the additional time has to be included in the phase 1 proposal 7 6 Observation constraints The Moon constraint is irrelevant for mid IR observations However if the Moon is too close to the target less than 5 deg typically the scattered moonlight may prevent MACAO from working correctly The VLTI astronomers make sure that the OBs in service mode are executed when the Moon
18. DI The presence of a suitable guide star should be noted in the proposal If V gt 15 there is a risk that Coud guiding cannot be performed depending on the off axis distance and on the sky conditions seeing To If no guide star exists it is still possible to go through with the target acquisition in order to ensure beam overlap by directly offsetting the Nasmyth guide probe from the MIDI obser vation software causing a motion of M2 which is offloaded to the alt az axes In this case only the field stabilization by the Nasmyth probe is enabled and the image quality on MIDI is usually limited by the seeing Therefore a lower data quality may be expected However this requires an exceptional seeing In the past we have noticed that many attempts to observe in SM failed because the seeing was never good enough Therefore observations without MACAO will be carried out in VM only Users are requested to come with an alterna tive back up observing program since the conditions to carry out OBs for which MACAO cannot be used are likely to not be encountered We guarantee that the MACAO loop is closed under the following conditions e Seeing less than 1 5 arcsec e Coherence time in visible 77 larger than 1 5 ms e Airmass less than 2 e Distance from the optical axis less than 57 5 arcsec MIDI User Manual VLT MAN ESO 15820 3519 19 Figure 9 A unit telescope left and an auxiliary telescope right 6 1 2 The Auxiliary T
19. ERIOD 83 6 1 Telescopes and adaptive optics The available telescopes for MIDI in P83 are the 8 m Unit Telescopes UTs of the VLT and the movable 1 8 m Auxiliary Telescopes ATs 6 1 1 The Unit Telescopes and MACAO Each UT is equipped with an adaptive optics system called MACAO It consists of a Roddier wavefront curvature analyzer using an array of 60 avalanche photodiodes This analyzer applies a shape correction on the M8 deformable mirror of the UT The M8 is mounted on a tip tilt correction stage In this case the telescope is tracking in field stabilization mode In this mode the Nasmyth guide probe camera tracks on a selected guide star observable within the 30 arcmin Nasmyth FOV which is centered on the science target by applying tip tilt correction to M2 When reaching the limit the M2 position is offloaded to the alt az axes of the telescope The tip tilt mount of the M8 is offloaded by offsetting the Nasmyth guide probe position and therefore by offsetting the M2 or the alt az axes The sensitivity of MACAO is V 16 for a 20 Strehl at A 2 24m In practice with MIDI MACAO can be used with V 17 To get diffraction limited images on MIDI it is mandatory that MACAO is used If the target to observed is fainter than V 17 but a suitable guide star exists it is possible to perform off target Coud guiding The guide star must be brighter than V 17 and closer than 57 5 arcsec to the target to be observed on MI
20. ERIOD 83 18 6 1 Telescopes and adaptive optics 2 18 6 1 1 The Unit Telescopes and MACAO o o 18 6 1 2 The Auxiliary Telescopes and STRAP 19 6 1 3 Chopping o s s odisea A A A BS 19 6 2 Baselines e EE EE aaa 20 MIDI User Manual VLT MAN ESO 15820 3519 Vv 62 1 UT B seli es y voca een a Da Dee ee ee ee we ES 20 6 2 2 A baselines es esd eee dete AA Rese BS 21 KE EE EE EE A E EE ee ee eee ee E ee A 21 Aaa ias oa aaa Se 22 dE AA e a e 23 LIA AAA AA a 23 7 PHASE 1 PROPOSAL PREPARATION WITH MIDI 25 a Gace Bees See sado we See Ge ok oe ee 25 E RATE E ee OS ee Ye OS Re On ee ee BE 26 T3 Geometry s es s dos soadh u a oe Diino ERR Ree Ree EA eee Po 26 e odo ee a E Ee ee ech 26 7 5 Calibrator Stars iris am pare e Ae e E E RR A 26 7 6 Observation constraints ee res eee Dee ede das 27 8 MIDI OBSERVATIONS 28 fo ed hake OES BOE SE LESS RL ESSE 28 8 1 1 Target acquisition ars wo slag 2 ae ed Ee Beh HO Ea de we 28 8 1 2 Fringe search fo dk ge oh a De RR BE BE ES A 29 ee ee ee ee eee Se ER 29 8 1 4 Photomeliyj s s sok A we ee ees ea ee eee 29 8 2 Total sequence timing e 30 8 3 The VLT software environment for phase 2 0 4 30 AAA eee ee 30 8 4 1 Data handling mem q eo eed ee ed Be Ee ee Ee ee mC 30 pp wate ee ok ee Boe ee ebe Be eee Bee al SA Sok re Gets a Gs Beye we Gd Aca ee se eae ee Be 31 List of Fig
21. EUROPEAN SOUTHERN OBSERVATORY ES Organisation Europ ene pour des Recherches Astronomiques dans H misph re Austral Q Europ ische Organisation f r astronomische Forschung in der s dlichen Hemisph re ESO European Southern Observatory Karl Schwarzschild Str 2 D 85748 Garching bei M nchen Very Large Telescope Paranal Science Operations MIDI User Manual Doc No VLT MAN ESO 15820 3519 Issue 83 Date 31 08 2008 Th Rivinius S Morel os A Date Signature A Kaufer Approved A dE e Date Signature 0 Hainaut Released REA Date Signature MIDI User Manual VLT MAN ESO 15820 3519 This page was intentionally left blank MIDI User Manual VLT MAN ESO 15820 3519 111 Change Record Issue Date Section Parag Reason Initiation Documents Remarks affected 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 solution 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 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
22. 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 P83 running from 1 April 2009 to 30 September 2009 The bold font is used in the paragraphs of this document to put emphasis on the important facts regarding MIDI in P83 1 2 Acknowledgements The editor thanks Olivier Chesneau Observatoire de la Cote 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 editor also thanks Monika Petr Gotzens Andrea Richichi and Markus Wittkowski at ESO Garching for their comments as well as Markus 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
23. e we now systematically use group delay tracking the OPD is measured from the position of a fringe peak in 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 P83 are the following indicative values and subject to minor changes e Dispersion by either prism or grism see Sect 5 3 e One 1 detector frame per OPD sample Five 5 OPD samples for fringe 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 fringe mode 5 4 2 Field astrometry mode This mode is only interesting when it is known that the scientific target consists of a cluster of several not connected sources in a narrow field of view The aim is therefore to observe several interference fringe packets each one corresponding to one of the sources We consider a S the pointing vector of a source that is centered in the MIDI field of view and S the pointing vector of a nei
24. e 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 MIDI User Manual VLT MAN ESO 15820 3519 11 DISPERSTUE MS ELEMENT FILTER DETECTOR PHOTOMETRIC CHANNELS INPUT FOCUS WINDOW Figure 4 Lightpath in MIDI cold optics 4 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 de termined 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 performed 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 Fig 5 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
25. ed 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 signal 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 MIDI User Manual VLT MAN ESO 15820 3519 15 A AO Co 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 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 C 1 21122 x 107 Ci 0 0232192 Co 6 61739 The accuracy of the above formula with these coefficients is 0 05 wm 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 2
26. elescopes and STRAP Each AT is equipped with the tip tilt corrector called STRAP It consists of four avalanche photodiode quadrants which measure the tip tilt of the incoming wavefront The measured tip tilt is compensated by acting on the M6 mobile mirror When reaching the limit the M6 position is offloaded to the alt az axes of the telescope The sensitivity of STRAP on the ATs is V 13 5 If the target to observed is fainter than V 13 5 it is possible to perform off target Coud guiding provided a suitable guide star exists This guide star must be brighter than V 13 5 and closer than 57 5 arcsec to the target to be observed on MIDI The presence of a suitable guide star should be noted in the proposal If V gt 12 there is a risk that Coud guiding cannot be performed depending on the off axis distance and on the sky conditions seeing To Note that unlike the UTs the ATs have no possibility of guiding if they cannot guide with the Coud Therefore it is mandatory to use a suitable Coud guide star either the target itself or an off axis guide star 6 1 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 Sec 3 2 The standard technique consists in moving the secondary mirror of the telescope M2 at a rapid frequency 0 5 Hz typically The maximum chopping throw of
27. ess 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 MIDI User Manual VLT MAN ESO 15820 3519 5 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 a 1 V 0 cos 2mOPDo p 0 Here is the wave number 1 A o a 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 see Sect 3 3 and the incoming beams have the same intensity From this equation we see that fringes with period Ao 1 OPD appear in the channelled spectrum provided that o 0 and Die do not vary strongly with wavelength MIDI User Manual VLT MAN ESO 15820 3519 6 0 8 0 6 Atmospheric Transmission Ivo H d 5 10 15 29 25 30 Wavelength um Figure 2 Mid Infrared atmospheric transmission 3 OBSERVING IN THE INFRARED Interferometric observations strongly depend on the turbulence properties of the terrestrial atmosphere Observing in the MIR implies additional constraints which are not encountered in optical observations 3 1 Atmospheric transmission The thermal infrared 8 to 25 wm atmospheric transmission is dominated by aerosol
28. ess 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 Imar min Lmas 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 given by h Tay Ii 2Vy Tu fgisin 2r0PD A ol L Ia ISS 2V y Ial gosin 2rOPD A o 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 2 4 Visibility estimators Due to atmospheric fluctuations see Sect 3 3 the fringe pattern to be observed is often in rapid motion and the phase of the complex visibility cannot be estimated It is often better to work with the square of the visibility V rather than V itself because V estimators are l
29. 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 interferometers strongly depends on the seeing i e on the Fried param 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 large
30. ghbor source in the MIDI field of view Fig 8 Both sources will yield fringes separated by the optical path difference AOPD B S S where B is the VLTI baseline used by MIDI In field astrometry mode the OPD scanning range is 140 um the maximum allowed by the PZT Since the fringe packets of the two sources can be observed only simultaneously in a scan if the two sources have a separation angle of the order of AOPD B a fraction of arcsec Therefore this mode is reserved to observe very close sources The field astrometry does not use spectral dispersion The observations can be performed with any of the filters that are used for acquisition see Sect 5 1 MIDI User Manual VLT MAN ESO 15820 3519 17 Intensity integrated over A lt q measured point of zero OPD One scan OPD white fringe tracking about 100 um t OPD OR group delay tracking t OPD one frame Intensity FT 2 gt wavenumber Repeat the same on several frames gt OPD sign Repeat scans m i Process of scans gt visibility Fringe peakposition gt OPD Figure 7 Dispersed Fourier mode of MIDI The white fringe tracking is no longer used for observations VLTI telescope 1 VLTI telescope 2 MIDI field of view de 2 arcsec maxi AOPD Figure 8 Field astrometry mode of MIDI MIDI User Manual VLT MAN ESO 15820 3519 18 6 THE VLTI ENVIRONMENT IN P
31. he 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 observing p2pp Manual For a detailed description of the MIDI templates please refer to the P83 MIDI Template Manual This document will be available online with the announcement of observing time web letters 8 4 Post observation process 8 4 1 Data handling Data from the MIDI instrument will be stored as FIT S binary 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 8 4 2 The pipeline Information concerning the pipeline quick data processor to assess the validity of exposure data and the data quality con
32. lescopes in Nasmyth then Coud MACAO or STRAP guiding mode use of a guide star for field stabilization if feasible 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 but 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 8 1 1 Target acquisition When the operator starts on the instrument an OB received from the OT the acquisition tem plate begins The sequence of this template starts by a preset the target coordinates a 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 with MACAO or STRAP 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
33. ls from the following links e http www eso org sci meetings IW2002 index html proceedings of ESO Chile Interferometry Week 2002 e http olbin jpl nasa gov intro index html Optical Long Baseline Interferome try News tutorials e http www eso org sci facilities paranal telescopes vlti VLTI general de scription and tutorials e http www mariotti fr obsvlti obsvlti book html proceedings of EuroWinter school Observing with the VLTT e http www mpia hd mpg de FRINGE tutorials 01 tutorial_01 html tutorial in Ger man 2 2 How an interferometer works An optical interferometer samples the wavefronts of light emitted by a remote target Sampling is performed at two or more separate locations see Fig 10 and 11 for the case of the VLTI The interferometer recombines the sampled wavefronts to produce interference fringes In the MIDI context the interferometer uses two telescopes as light collectors The telescopes are separated on the ground by a baseline vector The wavefronts add constructively or destructively depending on the path difference between the wavefronts and produce a fringe pattern that appears as bright and dark bands with the bright bands being brighter than the sum of intensities in the two separate wavefronts A pathlength change in one arm of the interferometer by a fraction of a wavelength causes the fringes to move If the beams from the telescopes are combined at a small angle the
34. m of the integration time and the readout time On the other hand ITR mode allows to select an integration as short as 0 2 ms per frame The minimum integration 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 selec tion reduces the time needed for readout Windowing by software by column selection after readout reduces the amount of data to be processed in the downstream steps of data handling Fig 6 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 13 Figure 6 Image of dispersed fringes obtained in laboratory by MIDI with its grism 5 MIDI IN PERIOD 83 MIDI combines most of the aspects that usually exist independently in several astronomical instruments It involves 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 in its final configuration will offer a large possibility of setups selectable by the user
35. pe Figure 12 Difference of magnitude between V and H bands depending on the spectral type by MIDI will be executed Users still have to indicate which MIDI filter has to be used for acquisition but it does not mean that MIDI acquisition exposures will be taken 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 6 5 FINITO FINITO is the VLTI fringe tracker Its purpose is to compensate at high rate the atmospheric differential piston between the two telescopes used by the interferometric instruments in order to stabilize the fringes FINITO works in H band During P83 FINITO can be used with MIDI in VM in HIGH_SENS mode with the ATs only The interest of FINITO with MIDI is limited for several reasons 1 For many targets the limiting magnitude of MIDI comes from the quality of the pho tometry exposures 2 Many MIDI targets are over resolved in H band The interest of FINITO would be to observe targets with a high visibility in H and a low visibility in N Therefore FINITO is interesting for very specific MIDI programs e g extended disks around stars The performances of FINITO are e Limiting magnitude in H 5 Limiting magnitude in K for IRIS fast guiding 5 Limiting visibility in H 15 Seeing better than 0 6 Airmass les
36. r than rg 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 ro x A5 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 MIDI User Manual VLT MAN ESO 15820 3519 8 average intensity the cylinder of rays that would ideally feed each telescope in absence of turbulence Scintillation is less important in the mid infrared where fluctuations of sky emission sky noise dominate 3 4 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
37. s than 1 5 MIDI User Manual VLT MAN ESO 15820 3519 24 6 6 Organization of VLTI observations For P83 MIDI is offered in service mode and in visitor mode see Sect 1 4 For the phase 1 of a period the unique contact point at ESO for the user is the User Sup port Department see Sect 1 5 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 www eso org observing p2pp VisitorMode html The visitor mode is more likely to be offered for proposals requiring non standard observation procedures like 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 and vice versa MIDI User Manual VLT MAN ESO 15820 3519 25 7 PHASE 1 PROPOSAL PREPARATION WITH MIDI Submission of proposals for MIDI should be done through 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 observing proposals 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 7 1
38. 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 observa tions 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 De partment http www eso org org dmd usg index html 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 2 A FEW WORDS ON INTERFEROMETRY 2 1 Introduction This section gives a short summary and a reminder of the principles of interferometry As tronomers interested in using the VLTI and MIDI but who are not familiar with interferometry yet can get tutoria
39. t of realized baselines will depend on the number of requests for each baseline Therefore users might be asked later to switch to the next best baseline They can already indicate an alternative baseline in their proposals as a comment to the interferometric table For the longest baseline UT1 UT4 there are limitation for the direction of pointing in the sky related to the mechanical range of the delay lines The VisCalc tool see Sect 7 3 gives the possible limits MIDI User Manual VLT MAN ESO 15820 3519 21 a bh j d SZ Zenith A 7 o s Telescope 224 lescope 2 f S i Baseline y Nasmyth Focus el Nasmyth focus Train Beam Combination Lab Coud Focus Coud Focus Interferometry tringes gt Nat s Fye Retro Reflector Figure 10 The optical path in the VLTI 6 2 2 AT baselines For P83 A large set of AT baselines is available for MIDI To get the list please refer to the webpage http www eso org sci facilities paranal telescopes vlti configuration With the ongoing commissioning of the variable curvature mirrors VCMs see following sec tion on the different VLTI delay lines at different times and in order to have the VCMs always in operation during observations the names of the stations that will be used and therefore the actual names of the baselines may not be the one given in the list However the observations will be carried out on equi
40. the MIDI shutter at different positions beam A open only then beam B open only Similar exposures are also taken in SCILPHOT since they can be used the refine the pho tometry measured in the photometric channels during the fringe exposure For P80 the number of frames for the photometry can be adjusted by the user See the MIDI template manual for details 8 2 Total sequence timing As said in Sect 7 2 for MIDI in P83 the average time to get a calibrated visibility point is 60 minutes in service mode Hence the time to complete the above tasks is 30 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 30 Visitor mode is usually allocated for targets that require a non standard acquisition procedure De very faint target no Coud guide star exisiting 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 90 minutes 8 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 A constraint set A list of intervals of the local sidereal times at which t
41. 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 observed object is never below 30 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 collect 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 7 4 Guaranteed time observation objects It is important to 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 has to be submitted every period individually To make sure that a target has not been reserved already for P83 the list of GTO objects can be downloaded from http www eso org sci observing visas gto 7 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
42. these filters More data transmission vs wavelength plots and ASCII tables are available from http www eso org sci facilities paranal instruments midi inst filters html Name Central wavelength ym FWHM um Nband 10 34 5 24 SiC 11 79 2 32 N8 7 8 64 1 54 Ar 9 00 0 13 SIV 10 46 0 16 Nell 12 80 0 21 N11 3 11 28 0 60 Table 1 Characteristics of the MIDI filters 5 2 Beam combination As already explained in Sect 4 2 1 the high sensitivity beam combination HIGH_SENS consists in 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 The SCI_PHOT 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 5 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 notic
43. tion for Research in Astronomy Dwingeloo The Netherlands Space Research Organization Netherland Utrecht and Groningen The Netherlands Th ringer Landessternwarte Tautenburg Germany Kiepenheuer Institut fiir Sonnenphysik Freiburg Germany Observatoire de Paris Meudon Meudon France Observatoire de la C te d Azur Nice France 4 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 4 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 In 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 10 Principle of MIDI the MID infrared Interferometer for the VLTI
44. trol can be found on the web at http www eso org observing dfo quality MIDI User Manual VLT MAN ESO 15820 3519 31 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 http www mpia hd mpg de MIDISOFT mia html http www eso org chummel midi midi html 8 4 3 Data distribution For any information on the ESO data distribution policy please check the webpage http www eso org org dmd usg DataDist html MIDI User Manual VLT MAN ESO 15820 3519 oho 32
45. ure which in combination give a 35 reflection and almost radiate like a blackbody The thermal background therefore MIDI User Manual VLT MAN ESO 15820 3519 7 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 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 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 3 3 Atmospheric turbulence Atmospheric turbulence is a major contributor to the difficulty of optical and infrared interfer ometry
46. ures neo ok 4 EPIA se ee eg ae ye 6 EECHER EECHER 10 4 Lightpath in MIDI cold Optiesd 14 ec 2 02 drid aa 11 5 MIDI warm optics with individual elements labeled 12 gt 13 DRECHEN 17 8 Field astrometry mode of MILL 17 9 A unit telescope left and an auxiliary telescope right 19 10 The optical path in the VLTI 24 4 8 44 ob 4 6 eo Oe eee eee he ees 21 MIDI User Manual VLT MAN ESO 15820 3519 vi 11 Layout of VLTI telescope locations o 22 12 Difference of magnitude between V and H bands depending on the spectral type 23 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 VET 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 Description 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
47. valent baselines having the same vectors as the ones in the list For some AT stations shadowing by the UTs prevents observations for some hour angles particulalry at northern declinations Plots of sky accessibilty are available at http www eso org observing p2pp AMBER v1lti_pointing restrictions html 6 3 Delay lines The delay lines are used to compensate the OPD between the two telescopes from the incoming stellar waveplane to the instrument entrance Each telescope has a dedicated delay line One is fixed whereas the other continuously moves in order to compensate OPD for apparent sidereal motion 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 MIDI User Manual VLT MAN ESO 15820 3519 22 FE Cross Track Unit Telescopes r i Instrumentation CG dE Laboratory mm TE Ke Seet e s d N ae i H Es GE A UT2 SS z7 d hi ura d D j Delay Lines CON Long Track e VLTI Stations Figure 11 Layout of VLTI telescope locations transferring the pupil are e An optimized field of view 1
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