VERY LARGE TELESCOPE MIDI User Manual
Contents
1. lt Choppins Pude ees 6 2 Baselines 2 2 2 0 002 eee ee ee ee 6 2 1 UT Baselines o o 6 2 2 AT baselines o o 6 3 Delay Mes 024 soa kare nr Pe bes 12 ates estates ab tein ie e wld 12 Delos oe co de EEN 12 i teh ag E test at 13 EE a Saas hse 14 15 SEN cette Mi tui aie test slay Deg el 15 late a A er aech 15 Dts oP dee ite EEN 15 A A EEN 17 ven Bde tah AEA de vg Bias abe 17 e eet Aer tee ra ae ds 18 20 EEN 20 A A ce ae Ate a 21 d Seege Noe 21 a a a 22 Date 8 March 2006 Ko A MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 5 of 36 CA TIRIS aes se cs go oe space tte a heh chy ce e cae BE ake wp oe Sacer e ood AE hy a Gees 28 DS FINITO A e ER ASR e As bls GE Bk ek ih a ie ia a 30 6 6 Organization of VLTI observations 2 2 2 ee 30 7 PHASE 1 PROPOSAL PREPARATION WITH MIDI 31 del Target brightness gee isita a a a Bae ed dw ee ia Bey bata we Aa 31 TZ Vime of Observation if EE e a aa 31 137 GEOM soa vo be go ke had aw nate ded O ean hat e Ba 32 7 4 Guaranteed time observation objects 32 TS Galibfator stars 42 50 ao dee hes E Se we oh dey cw hs Peay ee 32 8 MIDI OBSERVATIONS 33 8 1 Observation sequence 33 8 1 1 Target acquisition 33 8 1 2 Fringe Search e 34 8 1 3 Fringe measurement in Fourier mode 34 8 1 4 Photometty se a etches is a E a Ne Set Re ge Sete 34 8 2 Total sequence mung 34 8 3 The VLT software env2. 5 4 Fringe exposure The only fringe exposure mode available for P78 is the Fourier mode Fig 7 In this mode fringes are scanned over an OPD of several to get interferograms as fringe packets featuring 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 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 Either the position of the maximum in the fringe enveloppe is used to compute the current OPD white fringe tracking or a Fourier transform of the dispersed fringes is used to measure the OPD group delay tracking The group delay tracking has been proved to be more sensitive and will likely be used for P78 observations The data reduction process is the same whatever the fringe tracking mode is A raw uncalibrated visibility estimation requires a few hundred scans The parameters for fringe measurement in Fourier mode in P78 are the following indicative values and subject to minor changes e One 1 detector frame per OPD sample e Five 5 OPD samples for fringe e Eight 8 fringes
3. On MIDI the thermal background is dominated by 27 reflections in the optical train at ambient temperature which in combination give a 35 reflection and almost radiate like a black body The thermal background El n Date 8 March 2006 O MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 13 of 36 therefore is dependent on the sky and interferometer temperature lower in winter for instance and also on 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 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 Substracting in each detector frame the area illuminated by one interferometric channel from the area illuminated by the other will therefore cancel out the thermal backgrou
4. Spectral mode prism or grism with 200 um 0 52 arcsec on sky slit e Detector readout integrate then read 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 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 these filters More data transmission vs wavelength plots and ASCII tables are available from http www eso org instruments midi inst filters html Name Central wavelength um FWHM um Nband 10 34 5 24 SiC 11 79 2 32 N8 7 8 64 1 54 Ari 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 El n Date 8 March 2006 O MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 21 of 36 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 orde
5. Organisation Europ enne pour des Recherches Astronomiques dans l H misphere Austral ES EUROPEAN SOUTHERN OBSERVATORY Europaische Organisation fiir astronomische Forschung in der s dlichen Hemisph re VERY LARGE TELESCOPE MIDI User Manual Doc No VLT MAN ESO 15820 3519 Version 78 1b 8 March 2006 Prepared EE E REENEN Name Date Signature This page was intentionally left blank Date 8 March 2006 Ko MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 3 of 36 Change Record Issue Date Section Parag Reason Initiation Documents Remarks affected 78 1b 8 March 2006 6 1 2 8 8 1 Corrected values of off target guiding distance and limiting magnitude with ATs 78 1 17 February 2006 All Small changes for P78 77 2 7 December 2005 6 4 Paragraph on IRIS 77 1 31 August 2005 All Small changes for P78 76 2 30 May 2005 All Revision for phase 2 of P76 new version number ing 1 6b 7 March 2005 6 1 2 Corrected value of radius of off taget guiding for ATs 1 6 1 March 2005 All Global revision for P76 1 5 17 December 2004 1 2 1 4 1 5 Modified OB constraints User Support Group 5 6 1 6 4 7 1 User Support Department new fringe tracking de 7 3 8 3 scription of AO and guiding in 6 1 other minor changes 1 4b 3 September 2004 6 1 1 Added UT1 baselines info 1 0 22 August 2003 All First version Editor S bastien Morel ESO Paranal Science Operations smorelf
6. 8 March 2006 O MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 8 of 36 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 observing proposals index 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 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 instrumen
7. FOV of MIDI in imaging mode To ensure beam interference the images from both beams must be overlapped This is done by several iterations of the sequence star centroid measurement then offsets calculated and sent to the telescope to bring the star image on a reference pixel of the detector 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 resp V 17 and within a 1 arcmin resp 20 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 A seeing constraint has to be indicated El n Date 8 March 2006 O MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 34 of 36 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 case scanning the OPD with the MIDI piezo mounted mirrors will yield interferograms e 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 8 1 3 Fringe measur
8. VLT MAN ESO 15820 3519 v 78 1b Page 11 of 36 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 6 1o 0 1 V 0 cos QrOPDO 4 0 Here is the wave number 1 2 Jo o 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 9 o and Lo do not vary strongly with wavelength Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 12 of 36 09 c 9 8 4 E 4 a J E E 0 6 J o 1 Ki 4 H 0 4 4 g Zo N Q W 0 0 5 10 15 20 25 30 Wavelength um Figure 2 Mid Infrared atmospheric transmission A OBSERVING IN THE INFRARED Interferometric observations strongly depend on the turbulence properties of the terrestrial atmosphere Ob serving in the MIR implies
9. consortium is allowed to observe these objects with MIDI until the end of the guaranteed time period This guaranteed time period covers the full P78 To make sure that a target has not been reserved already the list of GTO objects can be downloaded from http www eso org observing proposals gto midi index html 7 5 Calibrator stars High quality measurements require that the observer minimizes and calibrates the instrumental losses of visi bility 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 the calibrator is observed after the science target using the same templates 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 Cal Vin can be used on the web from http www eso org observing etc El n Date 8 March 2006 O MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 33 of 36 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 P78 can be split in th
10. 74 C X Cy where X is the pixel position 1 left edge column 320 right edge column in the displayed image The coefficients are C2 0 0001539 C 0 009491 Co 15 451905 The accuracy of the above formula with these coefficients is 0 2 um Future MIDI calibrations should refine these values The estimated spectral resolution A AA of the grism is R 230 at A 10 6 um Exact value to be measured Its Co C1 and C2 coefficients see above for definition are Co 1 21122x 107 Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 22 of 36 09 Ci 0 0231223 Co 7 54423 The accuracy of the above formula with these coefficients is 0 1 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 um which corresponds to 0 52 arcsec on sky with the 8 m unit telescopes and to 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 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
11. 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 position whatever the delay line position is 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 transfering the pupil are e An optimal field of view 5 arcsec with the ATs Fringes can be made from any target within the FOV e A reduction of the thermal background related to VLTI optics In P78 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 VLTI infrared field stabilizer Its purpose is to perform field stabilization on the telescopes by mea suring the low frequency tip tilt from the VLTI laboratory IRIS guarantees therefore the correct alignment of the beam during the observations It is also used with MIDI to perform faster acquisition For P78 IRIS is supposed a
12. additional constraints which are not encountered in optical observations 3 1 Atmospheric transmission The thermal infrared 8 to 25 um atmospheric transmission is dominated by various molecular species includ ing O3 9 6 um 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 timescales 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 distur bance Nevertheless new detector technology and the extremely dry mountain top of Cerro Paranal improves data quality Transmission spectra are shown in Fig 2 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
13. closer than 20 arcsec to the target to be observed on MIDI 6 1 3 Telescope constraints The Moon constraint is irrelevant for mid IR observations However if the Moon is to 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 is far enough from the targets In visitor mode users should carefully schedule their nightime using Moon ephemeris to avoid problems of scattered moonlight 6 1 4 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 The maximum chopping throw of a UT is 30 arcsec more than needed for the 2 arcsec interferometric field of the VLTI El Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 26 of 36 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
14. e 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 the first one actuates M1 to focus on the spectroscope slit if dispersion is used or on future spatial filters and the second one translates the detector to the focus of the camera Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 17 of 36 09 DISPERSIVE MS DETECTOR PHOTOMETRIC CHANNELS INPUT KUSS WINDOW Figure 4 Lightpath in MIDI cold optics germanium set of lenses Baffling within the instrument is mainly done by a cold pupil stop just downstream to t
15. e 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 then Coud MACAO or STRAP guiding 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 interfero grams obtained by OPD scans 6 Integrate exposures for photometry in the same instrument configyration but with beam A only then 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 template begins The sequence of this template starts by a preset the target coordinates o Al 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
16. ement in Fourier mode Once the delay line offset yielding zero OPD has been found out a batch of interferograms can be recorded as a series of scans to form an exposure The position of the central fringe is extracted in real time for each scan and used to correct the position of the tracking delay line If the SCI_PHOT 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 interferograms itis 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 the MIDI shutter at different positions beam A only then beam B only Similar exposures are also taken sometimes in SCI_LPHOT mode to measure the splitting ratios of the beam combiner 8 2 Total sequence timing As said in Sect 7 2 for MIDI in P78 the required 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 Visitor mode is usually allocated for targets that require a non standard acquisition procedure i e very faint target no Coud guide star exisiting target that is embedded in a complex structure in IR e
17. erlands Kapteyn As tronomical Institute Groningen The Netherlands Astronomical Institute Utrecht University The Nether lands Astronomical Institute University of Amsterdam The Netherlands Netherlands Foundation for Re search in Astronomy Dwingeloo The Netherlands Space Research Organization Netherland Utrecht and Groningen The Netherlands Th ringer Landessternwarte Tautenburg Germany Kiepenheuer Institut f r 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 reimaged 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
18. g 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 not good enough Therefore from P78 observations without MACAO will be carried out in VM only Users are requested to come with an alternative 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 Tp larger than 1 5 ms e Airmass less than 2 e Distance from the optical axis less than 57 5 arcsec Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 25 of 36 Figure 8 A unit telescope left and an auxiliary telescope right 6 1 2 The Auxiliary Telescopes and STRAP Each AT is equipped with the tip tilt corrector called STRAP It consists of a avalanche photodiode quadrant which measures the tip tilt of the incoming wavefront The measured tip tilt is compensated by acting on the M6 mobile mirror When at the limit M6 is offloaded to the alt az axes of the telescope The sensitivity of STRAP on the ATs is V 13 If the target to observed is fainter than V 13 it is possible to perform off target Coud guiding provided a suitable guide star exists This guide star must be brighter than V 13 and
19. he entrance window of the cryostat 4 2 2 Warm optics OPD modulation is performed in the warm 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 determined 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 MLA and MLB one for each arm One piezo is used for OPD modulation A translation stage underneath one of them MLA 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 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 KRSS 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 Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 18 of 36 09 To Cryostat From Laser Master Alignment Pla
20. image acquisition 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 magnitude 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 P78 All the four unit telescopes of the VLT are available for MIDI The following table gives the characteristics of the possible on ground baselines E is the vector component over the East direction and N over the North direction Name E m N m On ground baseline length m UTI UT2 24 8 50 8 56 5 UT1 UT3 54 8 86 5 102 4 UTI UT4 113 2 64 3 130 2 UT2 UT3 30 35 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 P78 The final subset 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 zero OPD cannot be tracked for the region of the sky indicated with dots in Fig 9 o Da
21. included in the FITS header of the data files that will be delivered to the user after observation El n Date 8 March 2006 O MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 32 of 36 7 3 Geometry Important parameters of the instrument to be taken into account for the preparation of the observing sched ule are the VLTI geometry during observation u v coverage The selection 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 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 we encourage the user to collect as much information on their target as possible before submitting a MIDI proposal For instance a proposal for a pre imaging study on the TIMMI2 instrument at the ESO 3 6 m telescope of La Silla Observatory 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 MIDI
22. ironment for phase 2 o 000 2 eee 34 8 4 Post observation procesg oiaoi k e i a a a E ea E a E a a G eaa 35 S41 Data hands a a ta Sie A E 35 8 42 The pipeline fui Sl iaa 35 8 4 3 Data distribution a aT A TA E enke aie ae Eee 35 List of Figures 1 Principle of beam combination in long baseline interferometry 10 2 Mid Infrared atmospheric transmission e 12 3 Principle of MIDI optics For simplicity some of the mirrors are illustrated as lenses 16 4 Lightpath in MIDI cold optics 2 o o 17 5 MIDI warm optics with individual elements labeled o o 18 6 Image of dispersed fringes obtained in laboratory by MIDI with its grisM 19 7 Fourier mode of MIDI for fringe exposure and associated fringe tracking modes 23 8 A unit telescope left and an auxiliary telescope right o oo 25 9 Limitation of sky coverage for the UT1 UT4 baseline e 27 10 Theoptpealpoahmtbevil 00000000004 27 11 Layout of VLTI telescope locations ooo 29 12 Difference of magnitude between V and H bands depending on the spectral type 29 BS El O Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 6 of 36 List of Abbreviations AT CS DIT DRS ESO FOV FWHM IRIS MACAO MIDI MIR OB OD OPC OPD OPL OS OT P2PP QC SM SNR STRAP TSF USD UT VCM VLT VLITI VM Auxiliar
23. lways be used with the UTs IRIS is a H band sensor and requires from the user the H magnitude of the targets in order to work at its best performance 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 12 o Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 29 of 36 ross Track Instrumentation Laboratory Unit Telescopes Delay Lines Es e Long Track VLTI Stations Figure 11 Layout of VLTT telescope locations Spectral type Figure 12 Difference of magnitude between V and H bands depending on the spectral type El Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 30 of 36 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 During P78 FINITO is still under commissioning and is not offered with MIDI 6 6 Organization of VLTI observations For P78 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 Support Department see Sect 1 5 For the phase 2 USD is still the contact point for service mode a
24. m of the object brightness angular distribution O x y The sampled point in the Fourier plane is u B hk v B h By By are the coordinates of the projected baseline V corresponds to the visibility Imax Imin Umax 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 co axial 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 i 11 2V V Ialpisin 2nOPD A 0 h Iw IBp 2V VInIgpsin 2rO0PD A 0 where Jr and J are the intensities from the two outputs of the combiner 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 El Date 8 March 2006 MIDI User Manual
25. nd in the frame 3 3 Atmospheric turbulence Atmospheric turbulence is a major contributor to the difficulty of optical and infrared interferometry from the ground Rapid atmospheric variations of the OPD between the two arms of an optical or infrared interferome ter if uncorrected will cause smearing of the fringe pattern and strong discorrelation 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 ap proximately ten centimeters in optical is not as sharp as one would expect 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 timescales The speckle image is in fact a result of interference between wavefronts of randomly appear ing moving and disappearing subapertures These subapertures 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 parameter defined as follows the resolution of seeing limited images obtained through an atmosphere with turbulence characterized by a Fried parame
26. nd 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 proce dures 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 e Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 31 of 36 El O 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 Target brightness The brightness of the target in the N band will determine whether it is observable at all in self fringe tracking mode The limiting magnitudes for MIDI observations on unresolved objects in P78 are the following Telescopes Beam combine
27. per scan in white fringe tracking four 4 fringes per scan in group delay track ing 0 Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 23 of 36 Intensity integrated over A A lt q measured point of One scan zero OPD OPD white fringe bout tracking dee A t OPD Repeat scans Process of scans gt visibility gt OR Wi group delay tracking t OPD one frame Intensity FT 2 wavenumber Repeat the same on several frames gt OPD sign Fringe peakposition gt OPD Figure 7 Fourier mode of MIDI for fringe exposure and associated fringe tracking modes 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 El Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 24 of 36 6 THE VLTI ENVIRONMENT FOR MIDI IN PERIOD 78 6 1 Telescopes and adaptive optics The available telescopes for MIDI in P78 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 pho
28. pupil stops 3 first off axis paraboloids M1 Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 16 of 36 00 Principle of MIDI the MID infrared Interferometer for the VLTI z Se 240 x 320 Delay line variation bymov bis 00 JANE EEN EXE SS sae SS Shs Sas See See SSeS aS SESS i DEE el i 70 K radiation shield 1 SI Cold box la g Sa E Dewar i Intermediate E I foc window 1 1 Off axis og Off axis i B ZnSe 11 15 paraboloids Ggs ii Hii P Ges l 1 2 1 A alee a ae pe gt i I i gt A ARA P ik TT ZZ gt ii Beam compressors T f e 1 80 gt 18 mm beamwidth In i gt q im A sli Ke A a ER br me ke l e E Dr Beamwidth D a Ha Pupil Field stop Wun Filter eTA f A 2 La 1 l stop Spatial filter 1 i 8 i 1 ii 5 T IS 3 q vd Photometric GrisnvPrism Hy B rr II Beamsplitter II E LS it 30 70 Camera 22 733 p 721327322 1 S e EH 1 HH SS y TA i VU H II mt i VA I a Optics T 40K Detector A lt Wy La I i j i Wi mirrors on Piezo stages Figure 3 Principle of MIDI optics For simplicity some of the mirrors are illustrated as lenses 4 diaphragms in intermediate focus spatial filters 5 second off axis paraboloids M2 6 folding flat mirror M3 7 photometric beam splitter plates 8 folding flat mirrors for on
29. r Dispersion Limiting magnitude N Equivalent in Jy at 12 um UT HIGH_SENS PRISM 4 1 UT HIGH_SENS GRISM 2 8 3 UT SCI_PHOT PRISM 3 2 2 UT SCI_PHOT GRISM 2 6 AT HIGH_SENS PRISM 0 25 50 AT SCI_PHOT PRISM 1 100 Note that in the case of MIDI in P78 the N band limiting magnitude refers to the correlated unresolved flux i e to the product brightness x visibility A bright target may be so extended and therefore has such a low visibility that no fringe tracking is possible on it The GRISM is not offered with the ATs in P78 because no valid calibrator star would be brighter than the MIDI limiting magnitude with this setup 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 17for MACAO on the UTs and V 13for STRAP on the ATs 7 2 Time of observation It is important to know that in P78 slots of 60 minutes in service mode and 90 minutes in visitor mode per calibrated visibility point measurement will be attributed 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
30. r readout reduces the amount of data to be processed in the further steps of data handling Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 19 of 36 09 TT mr A A A mg Figure 6 Image of dispersed fringes obtained in laboratory by MIDI with its grism 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 El Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 20 of 36 5 MIDI IN PERIOD 78 MIDI combines most of the aspects that usually exist independently in several astronomical instruments It involves visibility measurements interferometry spectral dispersion spectroscopy 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 However most of the modes are still under development For P78 the available setups consist of Acquisition imaging mode several spectral filters available e Beam combination HIGH_SENS high sensitivity no simultaneous photometric channel or SCLPHOT science with photometry simultaneous photometric channels Fringe type dispersed only
31. r 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 um 100 ms typically Respecting 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 channels 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 um Exact value to be measured The relationship between wavelength in micron and detector pixel on MIDI with the prism is given by N CoX
32. rials from the following links e http www sc eso org santiago science interf2002 html proceedings of ESO Chile Interferometry Week 2002 e y ttp olbin jpl nasa gov intro index htm1 Optical Long Baseline Interferometry News tutorials e http www eso org projects vlti general VLTI general description and tutorials e http mariotti ujf grenoble fr obsvlti proceedings of EuroWinter school Observ ng with the VLTT kat e http www mpia hd mpg de FRINGE tutorial available in German 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 tele scopes 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 in tensities 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 fringes appear as a
33. spatially modulated pattern on the detector If the two beams are superimposed co axially 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 Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 10 of 36 09 l Telescope A Combining plate h Telescope B PD BA OPD Incoming e O wavefront 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 u v In this expression O u v is the Fourier transfor
34. strument will be stored as FITS 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 control can be found on the web at http www eso org observing dfo quality 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 07 MIDI User Manual Date 8 March 2006 VLT MAN ESO 15820 3519 v 78 1b Page 36 of 36 oho
35. sue of the MIDI user manual This version manual covers significant changes in the offered possibilities for observing with MIDI e Some of the available baselines use the Auxiliary Telescopes ATs e Beam combination can be performed with simultaneous photometric channels SCI_PHOT setup or still without them HIGH_SENS setup For the SCI_PHOT setup the limiting correlated magni tudes are not exactly known yet We give preliminary values in Sect 7 1 1 3 Acknowledgements The editor thanks Olivier Chesneau Observatoire de la C6te 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 Scholler and Andreas Kaufer at ESO Paranal and Jean Gabriel Cuby formerly at ESO Paranal 1 4 Glossary Constrain 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 El n Date
36. t Template An elemenatry 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 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 5 Contacts The authors hope that this manual will help to get acquainted with the MIDI instrument before writing pro posals 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 org dmd usg index html email usd helpteso org The web page dedicated to the MIDI instrument is accessible at the following URL http www eso org instruments midi El n Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 9 of 36 2 AFEW WORDS ON INTERFEROMETRY 2 1 Introduction This section gives a short summary and a reminder of the principles of interferometry Astronomers interested in using the VLTI and MIDI but who are not familiar with interferometry yet can get tuto
37. tc 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 Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 35 of 36 09 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 observing p2pp Manual For a detailed description of the MIDI templates please refer to the P78 MIDI Template Manual This document will be downloadable at phase 2 time around January 2006 from the MIDI webpage at ESO 8 4 Post observation process 8 4 1 Data handling Data from the MIDI in
38. te 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 27 of 36 Nasmyth Focus Beam Combination Lab Coud Focus Interferometry fringes gt Cat s Fye Retro Reflector Delay line Optical delay Figure 10 The optical path in the VLTI El Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 28 of 36 6 2 2 AT baselines For P78 Some AT baselines are available for MIDI The following table gives the characteristics of the pos sible on ground baselines E is the vector component over the East direction and N over the North direction Name E m N m On ground baseline length m E0 G0 15 1 5 2 16 0 G0 H0 30 3 10 4 32 0 GO0 K0 60 5 20 8 64 0 With the commissioning of the variable curvature mirrors VCMs see following section on the different VLTI delay lines at different times and in order to have always the VCMs used during observations the name 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 equivalent baselines having the same vectors as the ones in the list 6 3 Delay lines The delay lines are used to compensate the OPD between the two telescopes from the incoming stellar wave plane to the instrument entrance Each telescope has
39. te MAP MLI 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 4 3 Detector The detector is a 320 240 pixel Raytheon Si As Impurity Band Conduction IBC array also called BIB blocked impurity band with the following characteristics Array dimensions 320 x 240 Pixel Size 50 um x 50 um Peak Quantum efficiency 34 Dark current 10 amp pix s at 10 K Operating temperature 4 12K Well capacity 1 1e7 e RON 8008 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 ac cumulation of signal is stopped by a bias voltage at the end of integration The frame rate in snapshot mode is determined by the sum 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 subarray readout windowing Windowing on the detector reduces the time needed for readout Windowing by software afte
40. ter r 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 ru are essentially diffraction limited The scaling of r with wavelength is favorable for MIDI since ro lt as 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 El Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 14 of 36 at any time brighter or fainter than the average intensity the ideal incoming cylinder of rays Scintillation is less important in the mid infrared where fluctuations of sky emission sky noise dominate 3 4 Conclusion These observational difficulties mos
41. teso org E MIDI User Manual Date 8 March 2006 VLT MAN ESO 15820 3519 v 78 1b Page 4 of 36 Contents 1 INTRODUCTION Lil Scope E siesta e reg tity Ste lt Hdl Ss 1 2 What s new in this issue of the MIDI user manual 1 3 Acknowledgements TA lt GlOSSany 33 dl Ba te ae tel IAE da ES COMAE ive EN a eae RA a A AN ewe ea A 2 A FEW WORDS ON INTERFEROMETRY 2 1 Introduction 2 2 How an interferometer works 2 3 Interferometric observables 2 4 Visibility estimators o ooo 3 OBSERVING IN THE INFRARED 3 1 Atmospheric transmission 3 2 Background emission o 3 3 Atmospheric turbulence o o 3 4 Conclusion ai a Ae ae e A 4 MIDI OVERVIEW 4 1 Abitofhistory 000 4 2 Optical Layout o o e 4 21 Cold optics a et Ee a Bes 4 2 2 Warmoptics o o 4 2 3 DISPEESION EE as Aias A A Ay Be ce a See ae ted es 5 MIDI IN PERIOD 78 DL ACQUISIION ee ee ee 5 2 Beam combination 5 3 Spectral dispersion o o Dd Fring EXDOSULS ia a 6 THE VLTI ENVIRONMENT FOR MIDI IN PERIOD 78 6 1 Telescopes and adaptive optics 6 1 1 The Unit Telescopes and MACAO 6 1 2 The Auxiliary Telescopes and STRAP 6 1 3 Telescope constraints OLA
42. tly resulting from high background variations have led to the develop ment of specific observation techniques These techniques are included in the templates see 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 P78 El Date 8 March 2006 MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b Page 15 of 36 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 in strument 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 institutes Max Planck Institut fiir Astronomie Heidelberg Germany Netherlands Graduate School for Astronomy NOVA Lei den The Netherlands Department of Astronomy Leiden Observatory The Neth
43. todiodes 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 tip tilting the M2 When at limit the M2 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 2 um 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 it is possible to perform off target Coud guiding provided a suitable guide star exists This guide star must be brighter than V 17 and closer than 1 arcmin to the target to be observed on MIDI If no guide star exists it is still possible to go through with the target acquisition in order to ensure beam overlap by directly offseting the Nasmyth guide probe from the MIDI observation 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 seein
44. y Telescope Constraint Set Detector Integration Time Data Reduction Software European Southern Observatory Field Of View Full Width at Half Maximum Infra Red Image Sensor Multi Application Curvature sensing Adaptive Optics MID infrared Interferometric instrament 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 Unit Telescope Variable Curvature Mirror Very Large Telescope Very Large Telescope Interferometer Visitor Mode LE Date 8 March 2006 O MIDI User Manual VLT MAN ESO 15820 3519 v 78 1b t Page 7 of 36 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 number 78 P78 running from 1 October 2006 to 31 March 2007 Since MIDI is a recent instrument a limited number of instrument modes is offered Hence only the features that are supported by ESO for P78 are given in this document The bold font is used in the paragraphs of this document to put emphasis on the important facts regarding MIDI in P78 1 2 What s new in this is
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