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AMBER User Manual - Observation and Data Reduction with the
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1. 8 AMBER in P76 AMBER combines most of the aspects that usually exist independently in several astronomical instruments It involves visibility measurements interferometry spectral dispersion spec troscopy and background level corrections Hence AMBER in its final configuration will feature a large number of modes selectable by the user However most of the modes are still under development In P76 the only modes offered will be the medium resolution K band MR K and the low resolution K band LR HK with a spectral resolution A AA of approximately 1500 and 35 respectively In LR HK mode the K band will be acquired simultaneously with the H band however the quality of the H data cannot be guaranteed at present In MR K mode the limit AMBER User Manual P76 11 on DIT will translate to a maximum of 20 spectral channels available covering approximately 0 03 um In the case of MR K 2 configurations are available with central wavelengths of 2 1 and 2 3 um and a wavelength coverage of 1 926 2 275 um and 2 126 2 474 um respectively 8 1 Service and Visitor Modes For P76 AMBER is offered in service mode and in visitor mode see Sect 12 During all the period the unique contact point at ESO for the user will be the User Support Depart ment email usd help eso org and homepage http www eso org org dmd usg The visitor mode is more likely to be offered for proposals requiring non standard observation procedures The
2. During this period the potential users are invited to prepare and submit scientific proposals For more infor mation http www eso org observing proposals index html Phase II Proposal Preparation P2PP Once proposals have been approved by the ESO Observation Program Committee OPC users are notified and the Phase II begins In this phase users are requested to prepare their accepted proposals in the form of OBs and to submit them by Internet 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 the observations are carried out by the ESO Paranal Science Operation staff PSO alone 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 send OBs to the instrument Template A template is a sequence of operations to be executed by the instrument The observation software of an instrument dispatches commands written in templates not only to instrument modules that control its motors and the detector but also to the telescopes and VLTI sub systems Template signature file TSF File which contains template input parameters Visitor Mode VM The classic observation mode The user is on site to supervise his her program e
3. Ola Complex elds ec meee ee a ee en A ee Hew Pw oe ewe ee a 9 2 Choice of AMBER configuration 0000 eee ee ee MAL Instrument setup ek ee Pee ORES EDS Bee Oe ee Se 922 Observing modes s so AAR ROE ORR ERROR ERE ERE ES Oe Asem te 2b ve eee hea Deedes Ae we we ee ee 9 2 4 Calibrating the background emission 0 9 2 5 Standard calibration the instrumental visibility Std 10 Introducing Observation Blocks OBs 10 1 Standard observation UBS Sta ee ee a eRe ER Pw EES MILI Observing Cycle o wee ee bP ee ER Oe EY be eRe Re oe SE 1072 Tomine tme OVE EGAGE o c sees ok ee ARA EE ES 10 3 Check list 11 Bibliography 12 Glossary vil 11 12 12 12 12 13 13 13 13 14 14 14 14 14 15 15 15 15 16 16 AMBER User Manual P76 1 1 INTRODUCTION AMBER the near infrared red focal instrument of the VLTI operates in the bands J H and K ie 1 0 to 2 5um The instrument has been designed to be used with two or three beams thus enabling also the use of closure phase techniques The magnitude limit of AMBER is K 7 with Low Resolution LR on UTs K 4 with Medium Resolution MRK on the UTs It should be emphasized that these values are very conservative estimates pending further commissioning and characterisation and that ultimately much better sensitivities are expected The specific magnitude limits for the ATs have not yet been determined 1 1 Scope of this manual This docum
4. gto amber index html 9 1 6 Calibrator Stars High quality measurements require that the observer minimizes and calibrates the instrumental losses of visibility To get a correct calibration the user should use appropriate calibrator stars in terms of target proximity calibrator magnitude and apparent diameter In the case of AMBER 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 phase2 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 9 1 7 Field of View AMBER is a single mode instrument and therefore the field of view FoV is limited to the Airy disk of each individual aperture i e 250 mas for the ATs in K and 60mas for the UTs in K For most observations this will not come into effect but can be limiting to observations of binaries with large separations larger than the FoV The observer should be aware that if the binaries are separated by more than the FoV only one of the components will be seen by AMBER 9 1 8 Complex fields For normal observations of single objects there are no special constraints on the seeing it is sufficient that MACAO is working within the normal constraints see section 9 1 3 When observing complex fields w
5. the J and H pupil apertures In the MR K mode the limit on DIT will translate to a maximum of 20 spectral channels available covering approximately 0 04 um Two configurations are available with central wave lengths of 2 1 and 2 3 ym and a wavelength coverage of 1 926 2 275 um and 2 126 2 474 um respectively The observer can select any observational central wavelength within these wave length ranges Any change of the spectral configuration requires an internal calibration i e spectral calibra tion and P2VM calibration Any change of the neutral densities the polarizers the BCD the ADC the position of the fiber heads i e all elements located before the spatial filters does not require internal calibrations They can be used or not depending on the source characteristics 9 2 2 Observing modes The observing mode is characterized by the detector integration time DIT Currently only a fixed DIT 25 ms is offered for standard observations and DIT 50 ms for differential phase observations see section 2 2 3 This longer DIT isn t suitable for standard observations as there is a lowering of the contrast 9 2 3 Calibration cycle 9 2 4 Calibrating the background emission It is necessary to measure the sky and the instrumental emission in order to subtract this background to the science images The procedure consists in observing a source free region This observation is performed with the same set up as the science observatio
6. EUROPEAN SOUTHERN OBSERVATORY Organisation Europ ene pour des Recherches Astronomiques dans H misph re Austral Europ ische Organisation ftir astronomische Forschung in der s dlichen Hemisphare ESO European Southern Observatory Karl Schwarzschild Str 2 D 85748 Garching bei Miinchen Paranal Observatory Science Operations AMBER USER MANUAL VLT MAN ESO 15830 3522 Issue 1 3 Period P76 Date 2005 08 05 Fredrik Rantakyr 2005 08 05 rs a Ia serch hace ci aes a gh dice de Rn Oo dann eS BAR wee aks Approved Let A A A it Oe ee ee a Be AMBER User Manual P76 This page was intentionally left blank AMBER User Manual P76 111 Change Record Issue Rev Date Sect Prg Remarks 0 99 2005 02 24 T06 24 26 Thu all First Draft for P76 1 00 2005 03 08 T11 04 00 Tue all Update with COUDE guiding info 1 2 2005 06 09T13 00 00 Thu 9 1 3 Update on MACAO performance 1 3 2005 08 04T15 00 00 Thu all Updated FoV wavelength ranges differential phase AMBER User Manual P76 lv ACRONYMS AND ABBREVIATIONS AD AMBER AO AT CfP CS DI DIT DDL DL DRS ESO ETC FINITO FT LR MR MIDI MIR NDIT NIR OD OB OT OPC OPD OPL Phase I P2PP QC REF SM SNR STRAP TBC TBD TSF UT VIMA VINCI VISA VLT VLTI VM MACAO Applicable document Astronomical Multi BEam Recombiner Adaptive optics Auxiliary telescope 1 8m Cal
7. 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 9 Preparing the observations Submission of proposals for AMBER should be done through the ESOFORM It is important to carefully read the following information before submitting a proposal as well as the ESO FORM 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 AMBER could reveal inter esting features the first thing to do is to determine whether this target can be observed with AMBER or not At this point AMBER is offered with conservative performance estimates The limiting magnitude will be K 4 in MR K and K 7 in LR HK Note that this is the limit for an unresolved source ie with a calibrated visibility of unity Resolved sources will have lower limiting magnitudes correspondingly to their visibility 9 1 Choice of the VLTI configuration 9 1 1 Telescopes The only available telescopes for AMBER for P76 are the 8 2 m Unit Telescopes UTs all telescopes are equipped with the MACAO Adaptive Optics system 9 1 2 Baselines AMBER can be used in Period 76 using all available triplets of UT baselines e UT1 UT2 UT3 e UT1 UT2 UT4 e UT1 UT3 UT4 e UT2 UT3 UT4 AMBER User Manual P76 12 Baseline lengths with Unit Telescope UT are e U
8. T2 UT3 47m e UT1 UT2 57m e UT3 UT4 62m e UT2 UT4 89m e UT1 UT3 102m e UT1 UT4 130m 9 1 3 Coude Guiding 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 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 offoaded 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 2 2m In practice with AMBER MACAO can be used with V 17 Note There is also the additional constraint that the object should be fainter than V 1 for MACAO to work properly If the target to observed is fainter than V 17 it is possible to perform off target Coude guiding provided a suitable guide star exists This guide star must be brighter than V 17 and closer than 57 5 arcsec to the target to be observed with AMBER There are also a few weather condition constraints for proper MACAO performance e Seeing lt 1 5 arc
9. a matrix with AMBER User Manual P76 10 the values of the carrying wavesc k and d k The matrix M k is the so called pixel to visibilities matrix P2VM During calibration one can measure the P2VM and then inverse it so that for each pixel we get the visibility 6 Instrument performances 6 1 Instrumental contrast The contrast of the instrument is measured during the P2VM calibration procedure that occurs every time that we change the spectral set up of the instrument after the fibers It depends on the wavelength 7 Instrument features and problems to be aware of The AMBER instrument is not yet fully commissioned Therefore the following features are not proposed for the moment e No ADC This prevent to observe at low altitude in low resolution mode with the UTs with a large spectral coverage e Vibrations have been found in the VLTI arm which have been partially fixed Residual vibrations may still exist e OPD model may not be completely optimized and time can be lost to find fringes in particular in LR mode e Due the the absence of a fringe tracking system FINITO the spectral coverage can be severely limited to a dozen of pixels e Differential visibilities and phases can be used but differential modes are not yet available e AMBER is a single mode instrument and therefore the field of view is limited to the Airy disk of each individual aperture i e 250 mas for the ATs in K and 60 mas for the UTs in K
10. always model dependent Different images can lead to similar visibilities and discriminating between models usually requires measures with high accuracy In general the phase of the fringes cannot be related to the phase of the source Fourier transform because of the atmospheric phase jitter Only relative phase measurements are possible 2 2 2 Relative visibility V f A V f Ao In some cases one are interested in the variation of the target spatial intensity distribution with the wavelength This is the case when observing a structure which is present in a spectral AMBER User Manual P76 4 line whereas the continuum corresponds to an unresolved structure On can then calibrate the measurement in the line by those in the continuum and the knowledge of the absolute visibility is not required just the ratio between the visibility at a given wavelength and a reference channel 2 2 3 Relative phase variation with wavelength If the instrument is operated simultaneously at different wavelengths then one can measure the variation of the phase with the wavelength The principle is exactly the same as in astrometry except that the reference is the source itself at a given wavelength The most remarkable aspect of this phase variation is that it yields angular information on objects which can be much smaller than the interferometer resolution limit This features comes from the possibility to measure accurately phase variations much smaller th
11. an 27 When the object is non resolved the phase variation f A f Ao yields the variation with wavelength of the object photocenter e A e Ap This photocenter variation is a powerful tool to constrain the morphology and the kinematics of objects where spectral features result from large scale relatively to the scale of the source spatial features Note that if this is attempted over large wavelength ranges the atmospheric effects has to be corrected in the data reduction 2 2 4 Closure phase and phase reconstruction If fringes are present at all three baselines and the fringes for all baselines are analyzed si multaneously then we obtain a relation called closure phase The closure phase relations are independent from any antenna based atmospheric or instrumental phase offsets affecting the beams before arriving to the telescopes If all spatial frequencies have their phases in partially redundant closure phase relations an iterative algorithm allows to compute all phases step by step Then it is therefore possible to reconstruct the image if the u v plane is well filled or to constrain the models if only some closure phases are available 2 3 AMBER characteristics The main capabilities of AMBER are summarized in Table 1 3 AMBER within the VLT interferometer 3 1 VLTI infrastructure AMBER is the final stage of an overall infrastructure It is part of a VLTI well defined plan The general concept of the VLTI is to pro
12. e OBs should be identical in instrument setup having only different target coordinates Moreover with single telescope instruments any OB can be performed during the night In the case of interferometric instrument the instant of observation define the location of the observation in the u v plan 10 1 Standard observation OBS_Std The same exposure cycle can be used for two or three telescopes If only two telescopes are available 2 UTs the third pupil is closed up in the spectrograph cold pupil plane in order to reduce the background The correction of instrumental biases is based on the use of a reference star and the sequence of operations is as presented in Fig 6 10 1 1 Observing cycle A standard observation with AMBER in P76 can be split in the several subtasks 1 Configuration Setup of the desired spectral resolution wavelength range and DIT 2 Internal calibration of the chosen instrument configuration P2VM see sec 4 2 4 3 Acquisition Slew telescopes to target position on sky and slew the delay lines to the expected zero OPD position and bring the DLs in tracking state pre defined sidereal trajectory a As stated in Sec 9 1 3 the user has the possibility to use a guide star for the Coude systems different from the target He she will have to indicate the coordinates of this star which for the UTs should be brighter than V 17 and fainter than V 1 and within a 1 arcmin radius from the science targe
13. e proportional to the central wavelength of the band Therefore the Airy disk and the fringes have the same size for all central wavelength This allows to use the same spectrograph achromatic optics for all bands and to have the same sampling of all the central wavelengths Then the beams enter the cylindrical optics anamorphoser OPM ANS before entering the spectrograph SPG through a periscope used to align the beam produced by the warm optics AMBER User Manual P76 8 Figure 3 Photography of AMBER at Paranal AMBER User Manual P76 9 and the spectrograph 4 2 2 Spectrograph The spectrograph has an image plane cold stop a wheel with cold pupil masks for 2 or 3 telescopes The separation between the interferometric and photometric beams is performed in a pupil plane inside the spectrograph after the image plane cold stop 4 2 3 Detector After dispersion the spectrograph chamber sends the dispersed image on the detector chip DET 4 2 4 Calibration unit The Calibration and Alignment Unit OPM CAU contains all calibration lamps and can emulate the VLTI in the integration test and calibration phases The matrix calibration system OPM MCS is set of plane parallel plates which can be introduced in the beam sent by the OPM CAU in order to introduce the A 4 delays in one beam necessary to calibrate the matrix of the pixel to visibility linear relation Several components of the AMBER instrument such as the dich
14. ent summarizes the features and possibilities of the Astronomical Multi BEam combineR AMBER of the VLT as it will be offered to astronomers for the six month ESO observation period number 76 P76 running from 1 October 2005 to 31 March 2006 Since AMBER is a recent instrument a very limited number of instrument modes are offered Hence only the features that are supported by ESO for P76 are given in this document The bold font is used in the paragraphs of this document to put emphasis on the important facts regarding AMBER in P76 1 2 What s new in this issue of the AMBER User Manual There is an added discussion on observations of binaries size of the Field of View and minor corrections to the text 1 3 Acknowledgments The editor thanks Fabien Malbet LAOG Grenoble who delivered the document which formed the first first version of this manual in February 2005 The editor also thanks Markus Wittkowski at ESO Garching for his comments 1 4 On the contents of the AMBER User Manual Section 1 of this manual is aimed at users who are not familiar with the AMBER instrument and who are interested in a quick overview of its capabilities Section 4 provides the de scription of the instrument the instrument layout sec 4 2 the expected performances sec 6 and a reference to instrument features to be kept in mind while planning the obser vations or reducing the data sec 7 It can be consulted by users who want to prepare an Observi
15. example the phase difference i e the difference between the phase in each spectral channel and the phase in a reference channel This is the main purpose of Differential Phase observations the closure phase when used with three beams for the following spectral resolutions 35 1500 and 10000 and a spectral coverage containing the K H and J bands AMBER User Manual P76 3 The scope of this manual is limited to the measure and calibration of single or triplets u v points and do not address the use these measurements to constrain astrophysical models or the image reconstruction process 2 2 Science accessible with the different observables Thanks to the combination of instrument performance choice of baselines closure phase capability and the photon collecting power of the VLTI a wide range of astronomical sources can be targeted What follows is a brief presentation of the major objectives which are in no way a full listing of all scientific possibilities of the instrument Most of these objectives need the PRIMA astrometry fringe stabilizer and dual feed facility or FINITO fringe tracker to realise the objectives to their full extent but even without these AMBER will be able to make great advances in several areas including the following areas e Hot extrasolar planets Determination of planetary mass orbital parameters and the spectra of the planet and the star e Active Galactic Nuclei Spatially reso
16. iguration telescopes and baselines Choose the AMBER configuration Select calibrator Build sequence of OBs ie each science target OB is paired with one calibrator OB Compute time to be requested including overheads Bibliography Observing with the VLT Interferometer Les Houches Eurowinter School Feb 3 8 2002 Editors Guy Perrin and Fabien Malbet EAS publication Series vol 6 2003 EDP Sciences Paris The Very Large Telescope Interferometer Challenges for the Future Astrophysics and Space Science vol 286 editors Paulo J V Garcia Andreas Glindemann Thomas Hen ning Fabien Malbet November 2003 ISBN 1 4020 1518 6 reference documents templates calibration plan maintenance manual science technical operation plan AMBER User Manual P76 17 12 Glossary Constraint Set CS List of requirements for the conditions of the observation that is given inside an OB OBs are only executed under this set of minimum conditions Observation Block OB An Observation Block is the smallest schedulable entity for the VLT It consists of a sequence of Templates Usually one Observation Block include one target acquisition and one or several templates for exposures Observation Description OD A sequence of templates used to specify the observ ing sequences within one or more OBs Proposal Preparation and Submission Phase I The Phase I begins right after the Call for Proposal CfP and ends at the deadline for CfP
17. ith several objects within a few arcseconds the situation is more complex For fields with several objects within 1 to 3 arcseconds it is not guaranteed that MACAO will perform properly It is therefore recommended to use a guide star in this situation For fields with objects with separations less than an arcsecond MACAO will resolve the ob jects down to 0 1 0 15 arcsec Due to the way that light is injected into AMBER injection procedure only maximises the flux injected into the fiber it cannot be guaranteed in the case of separations smaller than 0 2 0 3 arcsec that the proper target has been injected into the fiber These kind of observations will have to follow a non standard extensive procedure to perform the injection adjustment and will require the presence of the PI Visitor Mode AMBER User Manual P76 14 9 2 Choice of AMBER configuration 9 2 1 Instrument set up The instrument set up is defined by the spectral configuration of the instrument and the 3T configuration In each 3T configuration the spectral configuration can be R 35 Full KHJ bands R 1500 Full KH bands R 1500 Full JH bands R 10000 One spectral window either in the K H or J bands In addition one can use the low or medium spectral resolution with only one band or one pair of bands i e HK in R 35 or K alone in R 1500 These configurations for the K band alone allow to reach a better reduction of the background noise by closing
18. l for proposals Constrain set Differential Interferometry Detector Integration Time Differential Delay line Delay line Data Reduction Software European Southern Observatory Exposure Time Calculator VLTI fringe tracker Fringe tracker Low Resolution Multiple Application Curvature Adaptive Optics Medium Resolution MID infrared Interferometric instrument Mid InfraRed 5 20 microns Number of individual Detector Integration Near InfraRed 1 5 microns Observation Description Observation Block Observation Toolkit Observation Program Committee Optical path difference Optical path length Proposal Preparation and Submission Phase II Proposal Preparation Quality Control Reference documents Service Mode Signal to noise ratio System for Tip tilt Removal with Avalanche Photo diodes To be confirmed To be defined Template Signature File Unit telescope 8m VLTI Main Array array of 4 UTs VLT INterferometric Commissioning Instrument VLTI Sub Array array of ATs Very Large Telescope Very Large Telescope Interferometer Visitor mode AMBER User Manual P76 This page was intentionally left blank AMBER User Manual P76 Contents 1 INTRODUCTION LL Sope or tie manual s e coss ds gh a eRe ee Ee 1 2 What s new in this issue of the AMBER User Manual 1 3 Acknowledgments oaoa a 1 4 On the contents of the AMBER User Manual 1 5 Contact Information Capabilitie
19. lve the Broad Line Region and to constrain its ge ometry and kinematics The ionized disks around the putative Massive Black Hole can be studied to constrain its morphology size and velocity and density field Measuring the wavelength dependence of the central point source the shape and size of circum nuclear dust structures as well as additional structures e g the inner region of jets circumnuclear starburst regions or bars in order to test AGN models e Circumstellar material in hot cold and young old stars Constraints on the size and morphology of the disk including velocity and density fields Similarly jets and bipolar outflows can be studied obtaining sizes morphology and velocity and density fields e Binaries Direct measurement of actual orbital motions and the masses of the stars e Stellar structure Measurements of the radius ellipticity surface activity and limb darkening effects 2 2 1 Absolute visibility V f A If the source is bright or if a bright reference star is close enough it is possible to obtain an un biased estimate of the source visibility amplitude from the fringe contrast A visibility measure for a single baseline can constrain the equivalent size of the source for an assumed morphology Visibility measures for several spatial frequencies obtained through Earth rotation different wavelength different baseline constrain severely the models However the interpretation of the results remains
20. n and close in time about 5 minutes 9 2 5 Standard calibration the instrumental visibility Std It is necessary to determine the instrumental complex visibility that affects multiplicatively the measured visibility The procedure consists in observing a point like source or a target AMBER User Manual P76 15 which intrinsic visibility is known the reference object has to be close to the science object This observation has to be performed with the same set up as the science observation and close in time 10 Introducing Observation Blocks OBs For general VLT instruments an Observation Block OB is a logical unit specifying the telescope instrument and detector parameters and actions needed to obtain a single observation It is the smallest schedulable entity which means that the execution of an OB is normally not interrupted as soon as the target has been acquired An OB is executed only once when identical observation sequences are required e g repeated observations using the same instrument setting but different targets a series of OBs must be constructed An OB can contain only one target but can contain several telescope offsets to measure the sky for example In the case of interferometry instruments the situation is a little bit different since we need calibrator stars to assess the atmosphere instrument system visibility cf sec 9 2 3 Thus each science object OB should be accompanied by a calibrator OB Thes
21. ng Proposal Phase I but should definitively be read by those who have been granted observing time and have to prepare their observations Phase II Section 9 provides the basic information needed to prepare a program the configuration of the VLTI sec 9 1 the identification of the observing modes and of the standard settings sec 9 2 1 5 Contact Information The aim of this manual is to make the users get acquainted with the AMBER instrument before writing proposals In particular sections 3 1 4 and 5 are aimed at astronomers not AMBER User Manual P76 2 used to interferometric observations This document is evolving continually and needs to be updated and improved according to needs of the astronomers All questions and suggestions should be channeled through the ESO User Support Department email usd helpCeso org and homepage http www eso org org dmd usg The AMBER Home Page web page is found at the following URL http www eso org instruments amber Any user of the instrument should visit the web page on a regular basis to be kept informed about the current instrument status and developments 2 Capabilities of the instrument What follows is not intended to be perfectly accurate from a mathematical point of view but to remind what is accessible in practice In principle the contrast and phase of the fringes observed on a source with given baseline B and wavelength A yield the amplitude and phase of the Fourier tra
22. nsform of the source brightness distribution at the spatial frequencies f B A If this Fourier function is sufficiently sampled in the Fourier plane then an inverse Fourier transform yields an objective i e model independent image of the object at the wavelength A with an angular resolution A Bmax Besides the sensitivity limits two classes of problems make this imaging process quite difficult First calibrating the measurements i e deducing the object visibility and phase from the fringes contrast and position Second making measurements at a sufficiently sampled Fourier plane can be time consuming This is why although making images will actually be the goal of AMBER on the VLTTI in some cases it is worth examining what kind of astrophysical information can be extracted from any individual AMBER measurement for a given baseline configuration 2 1 What measures AMBER AMBER is a beam combiner for up to three beams feeding a spectrograph and a camera working in the near infrared from 1 to 2 5 microns It is a single mode instrument which means that each baseline give access to only one point in the frequency space per spectral channel For this baseline the instrument is designed to measure the absolute visibility in each spectral channel the relative visibility i e the ratio between the visibility in each spectral channel and the visibility in a reference spectral channel average of several other channels for
23. ode optical fibers 7 photometric beams Star separators which allows to extract two fields of 2 within a field of 1 called respec tively the primary and secondary fields not currently available Transportation of the primary and secondary beams from the telescopes to the focal lab Compensation by the delay lines DLs of the optical path difference due to the sidereal motion Correction of the slow lt 1 Hz tip tilt motion of the beams caused by tunneling seeing effects by means of a fast detector sensing the beam motions and sending corrections to the X Y table so that the beams are kept centered on the optical axis IRIS 3 2 Other VLT instruments Connection with other VLT instruments like spectrographs ISAAC CRIRES or adaptive optics NAOS CONICA and SINFONI AMBER yields information at scales between B and A D A single mode instrument like AMBER has therefore no direct access to structures larger then A D Like for radio interferometer one might need in certain cases information at small spatial frequencies in order to inject it with the data collected with AMBER The best suited instrument that can give access to this data is the NAOS CONICA and SINFONI instruments which measures diffraction limited images in the the same wavelength domain as AMBER MIDI instrument is similar to AMBER but operates two telescopes in the N band More infor mation can be found at the following web address http www eso
24. org instruments midi AMBER User Manual P76 7 4 AMBER overview 4 1 AMBER principle Figure 1 summarizes the key elements of the AMBER concept AMBER has a multi axial beam combiner A set of collimated and parallel beams are focused by a common optical element in a common Airy pattern which contains the fringes 1 in Fig 1 The output baselines are in a non redundant set up i e the spacing between the beams is selected for the Fourier transform of the fringe pattern to show separated fringe peaks at all wavelengths The Airy disk needs to be sampled by many pixels in the baseline direction an average of 4 pixels in the narrowest fringe i e at least 12 pixels in the baseline direction while in the other direction only one pixel is sufficient To minimize detector noise each spectral channel is concentrated in a single column of pixels 3 in Fig 1 by cylindrical optics 2 in Fig 1 The fringes are dispersed by a standard long slit spectrograph 4 in Fig 1 on a two dimensional detector 5 in Fig 1 For work in the K band with resolutions up to 10 000 the spectrograph must be cooled down to about 60 C with a cold slit in the image plane and a cold pupil stop In practice we found it simpler to cool it down to liquid nitrogen temperature To produce high accuracy measurements it is necessary to spatially filter the incoming beams to force each one of them to contain only a single coherent mode To be efficient the s
25. patial filter must transmit at least 10 more light in the guided mode than in all the secondary modes For the kind of imperfect AO correction Strehl ratios often lt 50 available for the VLTI the single way to achieve such high filtering quality with decent light transmission is to use single mode optical fibers 6 in Fig 1 The flux transmitted by each filter must be monitored in real time in each spectral channel This explains why a fraction of each beam is extracted before the beam combiner and sent directly to the detector through a dispersive element 7 in Fig 1 The instrument must also perform some beam cleaning before entering the spatial filter such as correcting for the differential atmospheric refraction in the H and J bands or in some cases eliminating one polarization 4 2 AMBER layout Figure 2 shows the global implementation of AMBER with the additional features needed by the actual operation of the instrument 4 2 1 Warm optics There are three spatial filters one for each spectral band because of the limited wavelength range over which a fiber can remain single mode The three spatial filters inputs are separated by dichroic plates For example the K band spatial filter OPM SFK is fed by dichroic which reflect wavelengths higher than 2 ym and transmit the H and J bands After the fiber outputs a symmetric cascade of dichroics combines the different bands again but the output pupil in each band has a shap
26. roics the fibers the filters the beam splitter the cryostat window are optimized for only one polarization Then the other polarization will provide only a small gain in flux but can produce a substantial loss in contrast To avoid this one polarization can be eliminated by movable polarization cubes OPM POL 5 From images to visibilities The raw data produced by AMBER are images of the overlap of the 3 beams dispersed by a prism Because of the beam splitter one get in addition 3 photometric outputs corresponding to each beam An image of the detector image is displayed on Fig 4 The fringes are processed for each wavelength individually In fact 3 fringe system are present in the interferometric output and the first action consists in separating them apart Dur ing the calibration the carrying wave corresponding to each baseline are recorded and the interference term of the base ij is for the pixel k mig k 2y PiP ciz k Viz cos i k di k Viz sin k 1 The quantities c k and d k are called the carrying waves and are displayed on Fig 5 These waves are in quadrature so that each pixel is sensitive to a complex number Therefore we can write the photometry subtracted interferogram corr k as Leoni S my lk 2 j gt i M k xC 3 where C is a vector of the values Rij corresponding respectively to the real and imaginary part of the correlated flux 24 P P Vi for all baselines and M k is
27. s of the instrument 2 1 What measures AMBER 2 2 Science accessible with the different observables 2 2 1 Absolute visibility V f A 2 2 2 Relative visibility V f A V f o 2 2 3 Relative phase variation with wavelength 2 2 4 Closure phase and phase reconstruction 2 3 AMBER characteristics 6 644 be a AMBER within the VLT interferometer 3A VETI infrastructure ss se a eoe rd ee how le 3 2 Other VLT instruments AMBER overview ad AMBER principle oag y ioro ori a POSS 4 2 AMBER layout aoaaa 0 Ge 4d ea LLL Warm Optics caoba s rea te aeae neg ABD Opectroptaph ess koet ereen ra e A IDETEGIOE o e ece aa Ea Kb oe ESS 42A Calibration Unit es e eer ipee Ee ee From images to visibilities Instrument performances 6 1 Instrumental contrast 0 84 Instrument features and problems to be aware of AMBER in P76 8 1 Service and Visitor Modes Preparing the observations 9 1 Choice of the VLTI configuration 9 1 1 Telescopes 2 oie os cesar e vi See Fe pa a aR RwWwWwWnrhDy Q e O OO NNNOOQA AMBER User Manual P76 LA SST oc ia sa AE AA Se E E we ES Bl Coude Guiding lt lt lt nicas ia AA AN DA OO aa a ad ES A A RA 9 1 5 Guaranteed time observation objects o O10 Calbrator SUIS 4 a rd AAA 917 Pieldof View lt e evened aa eed on Be Oe eS he ESSERE KES RS
28. sec e 7 gt 1 5ms e Airmass lt 2 9 1 4 Geometry Important parameters of the instrument to be taken into account for the preparation of the observing schedule 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 with VisCalc that the altitude of the observed object is never below 40deg 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 an AMBER proposal AMBER User Manual P76 13 9 1 5 Guaranteed time observation objects It is important to check any scientific target against the list of guaranteed time observation GTO objects Only the AMBER consortium is allowed to observe these objects with AM BER until the end of the guaranteed time period This guaranteed time period covers the full P76 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
29. t 4 Injection Adjustment Adjust telescope positions so the beams from the target will center on the injection fibers in AMBER AMBER User Manual P76 16 5 Fringe Search Search the optical path length OPL offset of the tracking delay lines yielding fringes on AMBER actual zero OPD by OPD scans at different offsets When fringes are found the atmospheric piston is calculated and the OPL offsets corresponding to zero OPD are applied Observations Start to record data of interest with suitable DIT In P76 it is foreseen to only use a DIT of 25 ms for standard observations and a DIT of 50 ms for differential phase observations The longer DIT of 50ms causes a lowering of the contrast but is acceptable for differential phase observations For standard observations this loss of contrast is not desirable and thus the longer DIT should only be used for differential phase observations 10 2 Computing time overheads The user should assume that 120 minutes are required for one calibrated visibility point ie a measurement of the science object and a measurement of an interferometric calibrator star This applies to LR HK and to MR K for one spectral setting Users interested in obtaining visibility measurements at several spectral positions inside the K band should add 30 minutes for each additional spectral band A maximum of 3 bands per observation is allowed 10 3 Check list 1 2 3 4 5 11 Choose the VLTI conf
30. vide an interferometric focus to the instruments like modern telescopes provide almost diffraction limited beams to their instruments Therefore the VLTI infrastructure works like a general facility which supplies the following functions Sampling of the u v plane with 4 fixed UTs and 3 movable ATs with baselines ranging from 8m to 200m Collection of light with 4 large telescopes UTs 8m and 3 small telescopes ATs 1 8m Wavefront correction at the telescopes in the first phase adaptive optics for the UTs MACAO and tip tilt correction for the ATs STRAP AMBER User Manual P76 Table 1 AMBER characteristics and observing capabilities Description Specification Number of beams Two or Three Spectral coverage JHK 2 2 3 um Spectral resolution in K R 35 R 1500 R 10000 Spectral resolution in J amp H as it results from K Instrument contrast 0 8 Visibility accuracy 9 30 Optical throughput 2 in K 1 in J and H Detector size Detector read out noise Detector quantum efficiency 1024 x 1024 detector array 11 377 0 8 Observables V F A V F A V F Ao D F A F o P123 A AMBER User Manual P76 6 Figure 1 Basic concept of AMBER 1 multi axial beam combiner 2 cylindrical optics 3 anamorphosed focal image with fringes 4 long slit spectrograph 5 dispersed fringes on 2D detector 6 spatial filter with single m
31. xecution 000 AMBER User Manual P76 18 tudun RealTime Displays 1 0 03 04 14 es Lille BMULI E 3 imaga Mint ramara IAMGS_RTR DUNA x a Mi Walne li raz RES PEC fala Sab Cul Levak j Pluma cales cubs Festival magn tow 7 AZ 2 2 WS ME iaren T tec poticra onw Fi Fea pat Frama uu Int E rip H ware Bo selec ojec AO al wre AO esse 3 Corio see cic Figure 4 Image of the fringes recorded by AMBER in medium resolution around 2 1 microns using an artificial light source The wide stripes are the photometric spectrum of the 3 beams and the band with narrow stripes is the interferometric channel with the fringes AMBER User Manual P76 19 Ck12 4 dk32 a N Cko3 amp dkog O gt 0 2 10 20 30 40 el Es 0 2 LO 03 0 0 a cD lt 0 2 10 20 30 40 Pixels Figure 5 Example of carrying wave c k solid green line and d k dashed red line AMBER User Manual P76 Figure 6 Standard observation mode Std OBS Std 20
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