MIDI data reduction cookbook
Contents
1. DPR Data Products FITS identifier EWS Expert Workbench Station FITS Flexible Image Transport System GUI Graphical User Interface IDL Interactive Data Language ITT Ind Inc IO Input output MACAO Multi Application Curvature Adaptive Optics MIA MIDI Interactive Analysis MIDI Mid Infrared Instrument PAF PArameter File RA Right Ascension SciOps Science Operations UT Unit Telecope VLT Very Large Telescope VLTI Very Large Telescope Interferometer MIDI data reduction cookbook VLT MAN ESO 15820 4039 2 1 4 Stylistic conventions The following styles are used bold in the text for commands etc as they have to be typed italic for parts that have to be substituted with real content box for buttons to click on teletype for examples and filenames with path in the text Bold and italic are also used to highlight words 1 5 Where to find help and learn about interferometry VLTI http www eso org projects vlti Optical Long Baseline Interferometry News OLBIN http olbin jpl nasa gov index html MIDI data reduction http www sc eso org chummel midi midi html MIA EWS http www strw leidenuniv nl nevec MIDI index html Jean Marie Mariotti Center JMMC http www mariotti fr MIDI data reduction cookbook VLT MAN ESO 15820 4039 3 2 Introduction 2 1 A few words about interferometry in the mid infrared The mid infrared can be seen both as a regime of the optical and radio techniqu2. 2 1 A few words about interferometry in the mid infrared 22 The interferometric signal lt AN oat PRA HEARERS NEEN e Zo Observing OTEUS ee thee a o A A ee A eR eS 24 Photometric normalizationg 2 2 5 4422 de eee eG eRe e 3 MIDI FITS Information ek MIDI tile eege kk Sh cra ri OE ER EAA REE ERE 32 Extracting FITS Information e ss sioro s skuret di HEE SAGER s oo Viale FITS Tables s s sec eee ee pee ed aer Pa eee bE ee A 34 Bile DOS EE e Re RA Ae A OR DIe ARA 3 0 Telescope Keywords 204 2528 644 8 bee aia we ERY Da 36 os lt lt lt 4 64 ace ENEE EN A ARTE we 3 7 Observation Block Keywords 2 46 44 6 o ee is oe Template Keywords i 2 24h dbase SSSA RRS RRR EE cu 3 8 1 Keywords common to all templates 2 4 448 44284 44 eee KH 29 ality Control Produces lt saurs d era oe EN ee we Ee HO 310 ae Table eek ere ee eke ee GOR ee we A Ree BE SE Be ee 4 The MIDI Pipeline ay ER ce oe ye eb A Shee he ee ee OE Ee ALI Downloading and installing fas ie bed A a EC 5 MIDI interactive reduction Bk MDA so le ae Ye AR AA EE da den Oe PWS 65 cohen Ted ea odo de O46 SS bE SE DEE eA ES Ee D3 rie kee Ree RR AA OE OH EOE ES ES A RE RH 6 Issues to pay attention to 7 lt A few words on MIDI data analysis NN A k oh E ewww D i CON JJ JJ On CO On OO al MIDI data reduction cookbook VLT MAN ESO 15820 4039 1 1 Preamble 1 1 Purpose This document is intended to get astronomers star
3. There are many tools to extract and parse FITS headers One convenient way of extracting FITS information and displaying it on a terminal or re directing it to a text file is to use two stand alone programs called dfits and fitsort Both are included into the eclipse distribution http www eso org eclipse dfits dumps a FITS header on stdout You can use it to dump the FITS headers of many files to allow the parsing of the output Example dfits fits grep TPL ID Usually you want to get the value of a list of given FITS keywords in a list of FITS files fitsort reads the output from dfits classifies the keywords into columns and prints out in a readable format the keyword values and file names Example dfits fits fitsort NAXIS1 NAXIS2 BITPIX fitsort also understands the shortFITS notation where e g HIERARCH ESO TPL ID is short ened to TPL ID A classification example could be both commands are equivalent since fitsort is case insensitive dfits fits fitsort TPL ID DPR TYPE MIDI data reduction cookbook VLT MAN ESO 15820 4039 6 dfits fits fitsort tpl id dpr type The output from this combination is something like FILE TPL ID DPR TYPE MIDI 2006 09 27T02 13 43 000 fits MIDI_starintf_obs_fringe TRACK OBJECT DISPERSED MIDI 2006 09 27T02 14 57 506 fits MIDI starintf obs fringe TRACK OBJECT DISPERSED MIDI 2006 09 27T02 16 12 013 fits MIDI_starintf_obs_fringe TRACK OBJECT DISPERSED MIDI 2006 09 27T02 18 23 000
4. DITs Data product category Data product type Data product acquisition technique GRISM or PRISM Beam combiner SCILPHOT or HIGH_SENS Shutter AOPEN or BOPEN Acquisition filter MIDI data reduction cookbook VLT MAN ESO 15820 4039 9 4 The MIDI Pipeline 4 1 Purpose Pipelines can serve two purposes First to allow a quick and automatic reduction of data as it comes from the instrument in order to make an assessment of the data quality Sec ond a science grade pipeline produces optimally calibrated products which can be analyzed scientifically without any further manipulation The MIDI pipeline is of the first type i e final data reduction for scientific analysis will be done using other reduction packages which will be described in the following 4 1 1 Downloading and installing The MIDI pipeline is not currently available for download For up to date information please check http www eso org projects dfs dfs shared web vlt vlt instrument pipelines html MIDI data reduction cookbook VLT MAN ESO 15820 4039 10 5 MIDI interactive reduction Software packages have been written which provide a lot more parameters to optimize the reduction The methods used fall into two distinct categories coherent and incoherent analysis The former method aims to improve the signal to noise ratio of the visibility amplitudes by implementing an off line fringe tracking algorithm in order to align the interferograms before co addi
5. pipeline is mostly based on this keyword value DPR CATG Data Product category SCIENCE CALIB DPR TYPE Data Product type OBJECT SKY DPR TECH Data Product acquisition technique e g IMAGE SPECTRUM TPL NEXP Number of scheduled exposures within the template 3 1 TPL EXPNO Exposure number within template A template may produce several different frame types Frames are discriminated by the value of the DPR keywords DPR CATG DPR TYPE and DPR TECH take different values depending on the observed frame type 3 9 Quality Control Products Quality control outputs are produced into PAF files A PAF file is a PArameter File containing formatted ASCII data looking like a FITS header They are made so that human beings can read them easily and so that they can also be parsed by software means Keywords belonging to the realm of quality control products are currently being defined in a dictionary for later acceptance by the DICB board at ESO Expect changes in the months following the release of this document See the chapter dedicated to MIDI pipeline products for a full description of all possible output keywords MIDI data reduction cookbook VLT MAN ESO 15820 4039 3 10 Summary Table FITS keyword Meaning ARCFILE DEC DET DIT DET NDIT DPR CATG DPR TYPE DPR TECH INS GRIS INS OPTI1 INS SHUT INS FILT NAME Archive file name Declination J2000 in degrees Detector Integration Time Number of averaged
6. EUROPEAN SOUTHERN OBSERVATORY Organisation Europ ene pour des Recherches Astronomiques dans l H misph re Austral Europ ische Organisation ftir astronomische Forschung in der siidlichen Hemisph re ESO European Southern Observatory Karl Schwarzschild Str 2 D 85748 Garching bei M nchen Very Large Telescope Paranal Science Operations MIDI data reduction cookbook Doc No VLT MAN ESO 15820 4039 Issue 87 0 Date 23 03 2011 C Hummel S Morel T Rivinius E e ENEE EE Date Signature A Kaufer PRO Cd A a IA RE E Date Signature C Dumas Released ERE Date Signature MIDI data reduction cookbook VLT MAN ESO 15820 4039 This page was intentionally left blank MIDI data reduction cookbook VLT MAN ESO 15820 4039 111 Change Record Issue Rev Date Section Parag affected Reason Tnitiation Documents Remarks 87 0 23 03 201 Edition for approval 81 0 29 08 2007 all Period update 78 0 30 11 2006 all creation MIDI data reduction cookbook VLT MAN ESO 15820 4039 This page was intentionally left blank MIDI data reduction cookbook VLT MAN ESO 15820 4039 Contents 1 Preamble DL EEN e 9 e Ae br Ce er E EC A ER et PR A E 1 2 Reference documents lt bs kee WEIEN EEN EE RA A 1 3 Abbreviations and acronyms 1 EE dea A EE A EI Er 14 Stylistic CONVENTIONS seco tcm AE Oe a AA A 1 5 Where to find help and learn about interferometry 2 Introduction
7. e A MIDI data set consists of a fringe exposure and of two photometry exposures beam A only and beam B only in HIGH_SENS mode There is normally one file per exposure but the files are limited to 100 MB If the amount of data in an exposure is bigger than this limit it is split into two or more files each one not bigger than 100 MB Each file is in FITS format and contains several HDUs header data units that can be visu alized using fv see Sect 3 3 The structure of a MIDI file is the following e HDU1 Primary FITS header containing information about the observation e HDU2 Header table of instrument setup description e HDU3 Header table of instrument train e HDU4 Header table containing the frame data each line contains for each frame the matrices of pixel values in the detector windows and the information about the frame like time local OPD e HDU5 Header table giving for each frame the local OPL Ancillary data attached to MIDI files are all written into FITS headers This chapter lists the most important keywords and describes commands on how to retrieve them For ease of reading keywords are shortened from HIERARCH ESO A B C to A B C shortFITS notation Note that all of this information is present in the ESO dictionaries available from the ESO archive Web site http archive eso org DICB This chapter only tries to summarize the most important information 3 2 Extracting FIT S Information
8. e instrument workstation ARCFILE is the archive file name 3 5 Telescope Keywords Here is a non exhaustive list of telescope keywords RA Right ascension J2000 in degrees Notice that the comment field indicates the value in hh mm ss s format DEC Declination J2000 in degrees Notice that the comment field indicates the value in hh mm ss s format ADA POSANG Position angle on sky as measured from North to East degrees TEL AIRM START Airmass at start TEL AIRM END Airmass at end TEL AMBI FWHM START Astronomical Site Monitor seeing at start Note that this value might differ significantly from the MIDI image quality which is usually better TEL AMBI FWHM END Astronomical Site Monitor seeing at start Note that this value might MIDI data reduction cookbook VLT MAN ESO 15820 4039 7 differ significantly from the MIDI image quality which is usually better 3 6 Instrument Keywords Here is a non exhaustive slit of instrument keywords INS GRIS GRISM or PRISM INS OPTI1 Beam combiner SCLPHOT or HIGH_SENS INS SHUT Shutter AOPEN or BOPEN INS FILT NAME Acquisition filter 3 7 Observation Block Keywords OBS PROG ID Program ID OBS NAME Name of the OB as prepared with P2PP OBS TARG NAME Target package name as prepared with P2PP 3 8 Template Keywords 3 8 1 Keywords common to all templates TPL ID contains an unique identifier describing the template which was used to produce the data Frame selection in the
9. edge due to the closed cycle cooler of MIDI can introduce strong systematic effects on the extracted photometry More on this can be found at http www sc eso org chummel midi midi html following the link to the observations with the ATs in P76 e Chopping jitter If one browses through individual chopped frames for example using MyMidiGui s photometry GUI some jitter of the target can be seen normally close to the moment when MACAO reaquires the target after a sky cycle In HIGH_SENS mode this does not cause any problems while one needs to pay attention to this in SCLPHOT due to the fact that chopping is on during the fringe exposure possible causing overlap problems MIDI data reduction cookbook VLT MAN ESO 15820 4039 13 7 A few words on MIDI data analysis The reduction of the MIDI data should conclude with the calibrated visibility data written to disk in a suitable format such as the OIFITS exchange format http www mrao cam ac uk jsy1001 exchange This format is currently supported by MyMidiGui OYSTER and EWS but not MIA General analysis software will be based on this format due to its adoption by the IAU Working Group on Interferometry For an overview of this software see for example http olbin jpl nasa gov software index html It is a good idea to obtain all calibrator data of the same night as the science target from the public archive of ESO in order to study the consistency of their results Thi
10. es Heterodyne technology applied in the radio has been used to build an interferometer working at 10 microns http isi ssl berkeley edu The main characteristic of this regime is the high background against which the object flux has to be extracted A radio interferometer uses the fact that the background is uncorrelated and therefore drops out during the correlation process In MIDI the two outputs of the beam combiner are subtracted from each other removing the background fluctuations while preserving the interference fringes due to their 180 degree phase difference between the outputs The photometry necessary to normalize the correlated flux is obtained by using standard chopping techniques which provide the differential measure between target and background Nodding is not applied with MIDI 2 2 The interferometric signal To derive the interferometric signal i e the intensity of the combined light as a function of optical path length difference between the two beams OPD assume two fully coherent y 1 monochromatic wavelength A beams A and B of light with electrical field amplitudes of An and Ag If interfered their electric field vectors will add and the total amplitude will vary as a function of OPD between A Ag and A Ag Since intensity of light is the square of the electric field vectors integrated over a time interval it will vary between JR AR 241 Ag AZ and J AZ 24 Ag AZ Therefore using the def
11. excludes regions on the detector containing most importantly emission from outside the field of view such as tunnel emmission If the science target is too faint to be used to define the mask location the calibrator can be used for this purpose It is important to know that the perfect beam overlap is not guaranteed even though during the acquisition strict criteria are applied Drifts can happen and can in principle be monitored and corrected for with the SCI PHOT mode Only the overlapping regions of the beams can contribute to the correlated flux and therefore the normalizing flux is computed by multiplying the chopped photometric frames corresponding to the two telescope beams first before masking the frames and integrating the flux to obtain the spectra e Sky background MIA EWS photometric routines estimate a baseline for the spectrum extraction by looking at the background in two windows on either side of the signal from the target Despite the chopping small residual baseline offsets may remain which can be corrected for in this way However the field of view of the ATs is much smaller than the one of the UTs in terms of the width of the Airy disk and thus the area containing the sky background is smaller and more difficult to define The location of the windows can be set using the dSky parameter the location is shown in the mask GUI of MIA e Edge of field of view effects If the target is close to the FOV the jitter of its
12. extraction and the remainder is MIDI data reduction cookbook VLT MAN ESO 15820 4039 4 interfered to produce two channels with the interferometric signal If the target is faint the HIGH_SENS combiner directs all light into the interferometric channels requiring the pho tometry to be done following the fringe exposure The telescopes are chopping during the photometry in HIGH_SENS mode and during the combined fringe and photometry exposures in SCIPHOT mode They are not chopping during the fringe exposure in HIGH_SENS mode 2 4 Photometric normalizations To compute the normalization in the HIGH_SENS mode two photometric exposures are done one with shutter A open the other with shutter B The resulting spectra Jup 12 can be used directly to compute the normalization factor yi Atlpia yI A2lp 2 by which the correlated flux computed from the interferogram J Iz has to be divided In SCI PHOT mode the same photometric exposures this time on any bright target are used to compute the coefficients of the k matrix as follows kia h 13 k24 b L h and so forth Therefore it does not matter whether the photometry exposures are done using the HIGH_SENS or SCI PHOT beam combiners The matrix coefficients can then be used to compute the IAg 12 from the spectra Pa p determined from the photometric channels MIDI data reduction cookbook VLT MAN ESO 15820 4039 5 3 MIDI FITS Information 3 1 MIDI file structur
13. fits MIDI_starintf_obs_fringe PHOTOMETRY OBJECT MIDI 2006 09 27T02 19 37 485 fits MIDI_starintf_obs_fringe PHOTOMETRY OBJECT MIDI 2006 09 27T02 21 27 000 fits MIDI_starintf_obs_fringe PHOTOMETRY OBJECT MIDI 2006 09 27T02 22 41 506 fits MIDI starintf obs fringe PHOTOMETRY OBJECT This kind of table is useful in getting an idea of what is present in a directory or list of direc tories Loading such a summary table into a spreadsheet program also makes it conveniently readable In this example we see files related to an observation with MIDI in HIGH_SENS mode including fringe exposure and two photometry exposures one for shutter A and one for shutter B MIDI detector files are cut into more manageable sub files of about 100 MByte in size Therefore in the above example just three exposures are present 3 3 Visualizing FITS Tables Similarly a FITS table can be visualized on the command line using the dtfits command Al ternatively we recommend using fv available from http heasarc gsfc nasa gov docs software ftools fv dfits and fitsort will help you classify tables and see ancillary data attached to them but dt fits will display all information contained in the table itself in ASCII format on the command line There are various options to help make the output readable on a terminal or by a spreadsheet program See the dtfits manual page to get more information 3 4 File Names ORIGFILE contains the name of the file on th
14. ing MIA currently in MIA EWS 1 5 start and compile its code by typing mia sh Type f midigui and select by clicking in the left most column a file of kind OBS_FRINGE_TRACK_DISPERSED as well as two photometry files of type DEFAULT_CHOP immediately following the fringe expo sure The click Select Type x xmdv f dwim and MIA will reduce the selected files Once ready type x gt visibility and you will get a listing of the visibility amplitudes The first column is the wavelength in microns the second the wavelength bin the third the visibility amplitude and the fourth the photometric flux used for normalization If the same is done for a calibrator dividing the target amplitudes by the calibrator amplitudes will calibrate the visibility spectrum provided the calibrator is unresolved for the baseline used Otherwise a correction factor has to be applied To see the methods associated with the object x type help x obj To find out more one has to look at the source code in MIA xmidispvisi pro MIDI data reduction cookbook VLT MAN ESO 15820 4039 11 5 2 EWS EWS routines can be used from within the same IDL session as MIA However EWS saves all relevant intermediate results on disk as FITS files To run the EWS pipeline select the files as before f midigui define a string variable to be used as a prefix for all filenames with intermediate results e g tagsci HD8660 and type midi
15. inition of the visibility amplitude as V 1 7 Jex Tim the maximum visibility of two interfering beams of intensities J and Ig will be V 2V Jalp Ua Ip This visibility will of course be less if the beams are only partially coherent y lt 1 and the bandwidth of the light is wide From now on we write V yV The intensity of light in the two interferometric channels of MIDI can be written as here for I only h Is Igea 0 90 IP V sin 2rOPD A where 1 2 1 195 the intensity variation due to interference is equal to 2 Ia 1 Ig Due to the beam splitter IA 1 k1 ALa and so forth defining the coefficients of the so called k matrix An interferogram i e the intensity as a function of OPD of a single scan across the fringe packet free from background variations is derived as mentioned above by computing J la The background will be greatly reduced to Ia 1 A Kaa Q k1B K2B where a is the ratio of the beam intensities from the two telescopes The fringe signals however will add to become 2V Ia 1181 2V La 11B 2 3 Observing procedures The variability of the background with time and airmass makes it desirable to measure all necessary quantities for the visibility determination simultaneously These include the instan taneous total and correlated flux For this purpose the SCIPHOT beam combiner extracts from the two input beams the photometry channels 30
16. ng them A differential visibility phase can be obtained too using this method In the incoherent method the power spectral density function of each scan across the interferogram is integrated to produce the squared visibility amplitude which is then integrated over time The package doing the coherent analysis is EWS Expert Work Station written by W Jaffe University of Leiden The other incoherent packages are MIA MIDI Interactive Analysis written by R Kohler et al Universities of Leiden and Heidelberg and the Meudon Pack age distributed by the JMMC http www mariotti fr data_processing_midi htm MIA and EWS are distributed in a single installation available at http www strw leidenuniv nl nevec MIDI index html 5 1 MIA This software is based on IDL ITT Industries Inc http www ittvis com index asp also making use of a library of functions written in C The C code is common to EWS and is used for IO and CPU intensive tasks such as computing the chopped frames IDL provides the other computational services as well as a graphical user interface A pecularity of the MIA implementation is object oriented approach such that the results of the reduction are encoded in an IDL object containing both data and methods Since a good description of the use of MIA and EWS written by O Chesneau is available at http vltischool obs ujf grenoble fr we will just give a quick recipe for reducing a visibility spectrum After install
17. pipe tagsci file f Do the same for a calibrator e g tagcal HD4128 and then calibrate the science target using midicalibrate tagsci tagcal Look for PostScript plot files in the working directory summarizing the results To read the FIT S file containing the reduced visibility amplitudes use v oirgetvis tagsci redcal fits You can also read FITS files and binary extension tables using mrdfits which is part of the IDLASTRO distribution http idlastro gsfc nasa gov 5 3 MyMidiGui This IDL based GUI front end for the MIA EWS software was originally written for Paranal Science Operations by C Hummel to support an alternative to the MIDI ESO pipeline for the quick look data quality assessment It has since been integrated into the OYSTER inter ferometry analysis software http www sc eso org chummel oyster oyster html which provides astrometry modelling and plotting capabilities MyMidiGui can be found at http www sc eso org chummel midi mymidigui mymidigui html including HTML documentation MIDI data reduction cookbook VLT MAN ESO 15820 4039 12 6 Issues to pay attention to Interferometry has always been a technique subject to many systematic errors and MIDI is no exception Especially the photometric calibration can be tricky due to the high and variable background The reader should pay attention to the following issues e Masks and beam overlap The spectra are extracted using a mask which
18. s is important for the assessment of the science reduction in terms of systematic errors The analysis of the MIDI results in terms of astrophysical models can then be performed in a meaningful way MIDI data reduction cookbook VLT MAN ESO 15820 4039 oo 14
19. ted on the reduction of their MIDI data describing the various formats delivered by MIDI and the software available Observational procedures are also discussed This document does not describe the MIDI instrument its modes of operations how to acquire data the offered templates or the various issues attached to Phase II Proposal Preparation The reader is assumed to have read the MIDI User s Manual beforehand and have a basic knowledge of interferometric and mid infrared data reduction Furthermore this document makes extensive use of references to on line material and re sources At the time of writing MIDI data reduction is still in a state of rapid evolution especially with respect to the fine tuning of parameters for the data obtained with the ATs Therefore as the external sources are more easily kept up to date the purpose of this document shall be restricted strictly to providing guidance to the users as well as useful references 1 2 Reference documents 1 MIDI User Manual http www eso org instruments midi doc 2 MIDI Calibration Plan http www eso org instruments midi doc 3 ESO DICB Data Interface Control Document GEN SPE ESO 00000 0794 http archive eso org DICB 1 3 Abbreviations and acronyms The following abbreviations and acronyms are used in this document AT Auxilliary Telescope ESO European Southern Observatory Dec Declination DICB Data Interface Control Board DIT Detector Integration Time
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