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Data Flow System

Document Title: Document Number: Issue: Date:
Document Prepared by: Document Approved by: Document Reviewed by: Document Released by: Peter Bunclark & Simon Hodgkin (CASU) Mike Irwin (CASU Manager) William Sutherland (VISTA Project Scientist) Jim Emerson (VDFS Project leader)

VISTA Infra Red Camera Calibration Plan VIS-SPE-IOA-20000-0002 1.4 pre 1 2006-11-13
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Change Record
Issue 0.5 1.0 1.1 1.2 1.3 Date 2004-04-08 2004-12-15 2005-05-03 2005-05-11 2006-09-28 Sections All All All All All Remarks New Document FDR release post-FDR revision Final FDR fixes Rework of photometry reflecting WFCAM experience, procedure changes following pipeline prototyping, hardware references following actual build details. QC table

1.4pre 1

2006-11-13

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Notification List
The following people should be notified by email that a new issue of this document is available. IoA: RAL: QMUL ATC W Sutherland G Dalton J Emerson Malcolm Stewart Steven Beard


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Table of Contents
Change Record ............................................................................................................... 2 Notification List ............................................................................................................. 3 Table of Figures ............................................................................................................. 6 1 Introduction ............................................................................................................ 7 1.1 Purpose ........................................................................................................... 7 1.2 Scope .............................................................................................................. 7 1.3 Applicable Documents ................................................................................... 7 1.4 Reference Documents .................................................................................... 8 1.5 Abbreviations and Acronyms ........................................................................ 8 1.6 Glossary ......................................................................................................... 9 2 Overview .............................................................................................................. 11 2.1 Hardware ...................................................................................................... 11 2.2 Observing Modes ......................................................................................... 14 2.2.1 Imaging Mode Description .................................................................. 14 2.2.2 Calibrations .......................................................................................... 14 2.2.3 High Order Wave Front Sensor (HOWFS) Mode ............................... 14 2.2.4 Calibrations .......................................................................................... 15 2.3 Pipeline ........................................................................................................ 15 2.4 Operation...................................................................................................... 15 3 Calibration Accuracy ........................................................................................... 17 3.1 Overview ...................................................................................................... 17 3.2 Astrometric Error ......................................................................................... 17 3.3 Photometric Error......................................................................................... 17 3.3.1 RMS ......................................................................................................... 18 3.3.2 Additive systematics ................................................................................ 18 3.3.3 Multiplicative systematics ....................................................................... 18 3.3.4 Extinction monitoring .............................................................................. 19 4 Calibration Data for Instrumental Signature Removal ........................................ 20 4.1 Purpose ......................................................................................................... 20 4.2 Reset Frames ................................................................................................ 22 4.3 Dark Frames ................................................................................................. 22 4.4 Dome flats .................................................................................................... 23 4.5 Detector Noise ............................................................................................. 24 4.6 Linearization Measurements ........................................................................ 24 4.7 Twilight Flats ............................................................................................... 25 4.8 Illumination Correction Measurement ......................................................... 26 4.9 Image Persistence Measurements ................................................................ 27 4.10 Electrical Cross-Talk Measurements ........................................................... 27 5 Data for Photometric Calibration ......................................................................... 29 5.1 Introduction .................................................................................................. 29 5.2 Calibration from 2MASS ............................................................................. 29 5.3 Calibration from Standard Star Fields ......................................................... 29 5.4 Standard Fields............................................................................................. 31 6 Calibration Data Derived from Science Data ...................................................... 32 6.1 For Instrument Signature Removal .............................................................. 32


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6.1.1 Night-Sky Maps ................................................................................... 6.1.2 Sky Subtraction and Fringe Removal .................................................. 6.1.3 Jittering ................................................................................................ 6.1.4 Microstepping ...................................................................................... 6.2 For Astrometric Calibration ......................................................................... 6.2.1 Optical Distortion Effects .................................................................... 6.2.2 Final WCS Fit ...................................................................................... 7 Quality Control .................................................................................................... 7.1 Further Quality Control Data Derived from Science Frames ...................... 7.1.1 Object Extraction ................................................................................. 7.2 On line quality control (QC-0) ..................................................................... 7.3 Quality Control Parameters.......................................................................... 8 Templates ............................................................................................................. 8.1 Imaging Calibration Templates.................................................................... 8.1.1 Reset ..................................................................................................... 8.1.2 Dark...................................................................................................... 8.1.3 Dark Current ........................................................................................ 8.1.4 Acquire Dome Screen .......................................................................... 8.1.5 Dome Flat............................................................................................. 8.1.6 Detector Linearity ................................................................................ 8.1.7 Noise and Gain ..................................................................................... 8.1.8 Acquire Twilight Field ......................................................................... 8.1.9 Twilight Flat......................................................................................... 8.1.10 Persistence............................................................................................ 8.1.11 Astrometric Calibration ....................................................................... 8.1.12 Photometric Calibration Standard Fields ............................................. 8.1.13 Quick look ............................................................................................ 8.1.14 Cross-talk ............................................................................................. 8.1.15 Illumination .......................................................................................... 8.2 HOWFS mode calibration............................................................................ 8.2.1 HOWFS Acquire Dome Screen ........................................................... 8.2.2 HOWFS Reset ...................................................................................... 8.2.3 HOWFS Dark....................................................................................... 8.2.4 HOWFS Dome Flat.............................................................................. 8.3 Imaging Mode Science Templates ............................................................... 8.3.1 Acquire ................................................................................................. 8.3.2 Observe Paw ........................................................................................ 8.3.3 Observe Tile ......................................................................................... 8.3.4 Observe Offsets .................................................................................... 8.3.5 Observing a set of Tiles ....................................................................... 8.4 HOWFS mode data ...................................................................................... 8.4.1 HOWFS Acquire .................................................................................. 8.4.2 HOWFS Wave front ............................................................................ 8.4.3 HOWFS Expose ................................................................................... 8.5 Instrument Health Templates ....................................................................... 9 Technical Programs ............................................................................................. 9.1 TP-VIS1: Establishment of Secondary Standard Fields .............................. 10 Format of Data Frames ....................................................................................

32 32 33 34 34 34 35 37 37 37 37 38 43 47 47 47 47 47 47 48 48 48 49 49 50 50 50 50 51 51 51 51 52 52 52 52 53 54 55 55 56 56 56 56 57 58 58 60


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10.1 Principle ....................................................................................................... 10.2 Model FITS header ...................................................................................... Appendix A. 2MASS calibration Fields ................................................................ 11 Index ................................................................................................................

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Table of Figures
Figure 2-1 VISTA Focal plane: Each of the 4 groups of detectors in the Y direction (e.g. #s 1-4, 5-8, 9-12, 13-16) is read out by a separate IRACE controller. ........ Figure 2-2 VISTA Engineering Pawprint. ................................................................... Figure 2-3 Filter Transmission Curves for Reference Samples of Y, J, H, and Ks bands. ................................................................................................................... Figure 4-1 Cascade Diagram for producing Calibration Frames ................................. Figure 8-1 Hierarchy of VISTA IR Camera Templates............................................... Figure 8-3 Pre-selected twilight fields ......................................................................... Figure 9-1 Distribution of the 2MASS touchstone fields on the sky ........................... Table 10-1 FITS Example Header ............................................................................... 11 12 13 21 43 49 58 70


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1 Introduction
1.1 Purpose
This document forms part of the package of documents describing the Data Flow System for VISTA, the Visible and Infrared Telescope for Astronomy. As stated in [AD1] "The Calibration Plan is the prime document which describes the different instrument-specific components of the Data Flow System".

1.2 Scope
This document describes the VISTA DFS calibration plan for the output from the 16 Raytheon VIRGO IR detectors in the (Infra Red) camera for VISTA. The baseline requirements for calibration are included in the VISTA DFS Impact Document [AD2]. The major reduction recipes and algorithms to be applied to the data are described in the VISTA DFS Data Reduction Library Design [RD1]. Each camera exposure will produce a `pawprint' consisting of 16 non-contiguous images of the sky, one from each detector. The VISTA pipeline will remove instrumental artefacts, combine the pawprint component exposures offset by small jitters, and photometrically and astrometrically calibrate each pawprint. It will also provide Quality Control measures. It will not combine multiple adjacent pawprints into contiguous filled images, nor stack multiple pawprints at the same sky position. This document does not describe any calibrations or procedures relating to the CCD detectors that are also located within the camera and which interact with the Telescope Control System. This document covers only the Routine Phase of operations of VISTA's IR Camera. In particular it does not describe any calibrations or procedures that form part of the Commissioning Plan for VISTA, nor any procedures needed during routine Engineering Maintenance. [Except for HOWFS observations, which are made using the science detectors, and passed to the science archive.] Arrangements for processing any calibrations or procedures carried out under such categories are the responsibility of the VISTA Project Office.

1.3 Applicable Documents
The following documents, of the exact issue shown, form part of this document to the extent specified herein. In the event of conflict between the documents referenced herein and the contents of this document, the contents of this document shall be considered as a superseding requirement. [AD1] Data Flow for the VLT/VLTI Instruments Deliverables Specification, VLTSPE-ESO-19000-1618, issue 2.0, 2004-05-22. [AD2] VISTA Infra Red Camera DFS Impact, VIS-SPE-IOA-20000-00001, issue 1.3, 2005-12-25. [AD3] VISTA Infrared Camera Data Flow System PDR RID Responses with PDR Panel Disposition, VIS-TRE-IOA-20000-0006 issue 1.0


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[AD4] VISTA Infrared Camera Data Flow System FDR RID Responses VIS-TREIOA-20000-0013 issue 1.0 2005-12-25

1.4 Reference Documents
The following documents are referenced in this document. [RD1] VISTA Infra Red Camera Data Reduction Library Design, VIS-SPE-IOA20000-0010, issue 1.3, 2006-01-31. [RD2] Data Interface Control Document, GEN-SPE-ESO-19940-794, issue 3, 200502-01. [RD3] VISTA Operational Concept Definition Document, VIS-SPE-VSC-00000-0002 issue 1.0, 2001-03-28 [RD4] VISTA Infrared Camera Technical Specification, VIS-SPE-ATC-06000-0004, issue 2.0, 2003-11-20 [RD5] VISTA IR Camera Software Functional Specification, VIS-DES-ATC-0608100001, issue 2.0, 2003-11-12. [RD6] IR Camera Observation Software Design Description, VIS-DES-ATC-060840001, issue 3.2, 2005-02-24. [RD7] VISTA Science Requirements Document, VIS-SPE-VSC-00000-0001, issue 2.0, 2000-10-26 [RD8] A Global Photometric Analysis of 2MASS Calibration Data, Nikolaev et al., Astron. J. 120, 3340-3350, 2000 [RD9] 2MASS Calibration Scan Working Databases and Atlas Images, http://www.ipac.caltech.edu/2mass/releases/allsky/doc/seca4_1.html [RD10] A New System of Faint Near-Infrared Standard Stars, Persson et al., Astrophys. J. 116, 2475-2488, 1998 [RD11] JH standard stars for large telescopes: the UIRT Fundamental and Extended lists, Hawarden et al., Mon.Not.R.Soc. 325, 563-574,2001 [RD12] The FITS image extension, Ponz et al, Astron. Astrophys. Suppl. Ser. 105, 53-55, 1994 [RD13] Representations of world coordinates in FITS, Griesen, & Calabretta, A&A, 395, 1061.2002 [RD14] Representations of celestial coordinates in FITS, Calabretta & Griesen, A&A, 395, 1077, 2002 [RD15] Overview of VISTA IR Camera Data Interface Dictionaries, VIS-SPEIOA-20000-0004, 0.1, 2003-11-13 [RD16] Northern JH Standard Stars fro Array Detectors, Hunt et al Astr.J 115, 2594, 1998

1.5 Abbreviations and Acronyms
2MASS CDS DAS DFS FITS HOWFS ICRF 2 Micron All Sky Survey Correlated Double Sampling Data Acquisition System Data Flow System Flexible Image Transport System High Order Wave-Front Sensor International Coordinate Reference Frame


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IMPEX IR IWS LOWFS OB OS OT PI QC-0 QC-1 SDT TCS URD VDFS VIRCAM VISTA VPO WCS WFCAM ZPN

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Import Export (P2PP ASCII files) Infra Red Instrument Workstation Low Order Wave-Front Sensor Observation Block Observing System Observing Tool Principal Investigator Quality Control, level zero Quality Control, level one Survey Definition Tool Telescope Control System User Requirements Document VISTA Data Flow System VISTA Infra Red Camera Visible and Infrared Survey Telescope for Astronomy VISTA Project Office World Coordinate System Wide Field Camera (on UIRT) Zenithal Polynomial

1.6 Glossary
Confidence Map An integer array, normalized to a median of 100% which is associated with an image. Combined with an estimate of the sky background variance of the image it assigns a relative weight to each pixel in the image and automatically factors in an exposure map. Bad pixels are assigned a value of 0, 100% has the value 100, and the maximum possible is 32767 (negative values are reserved for future upgrades). The background variance value is stored in the FITS header. It is especially important in image filtering, mosaicing and stacking. DIT Digital Integration Time. Separate readouts are summed digitally. Exposure The stored product of many individual integrations that have been co-added in the DAS. Each exposure is associated with an exposure time. Integration A simple snapshot, within the DAS, of a specified elapsed time DIT seconds. This elapsed time is known as the integration time. Jitter (pattern) A pattern of exposures at positions each shifted by a small movement (<30 arcsec) from the reference position. Unlike a microstep the non-integral part of the shifts is any fractional number of pixels. Each position of a jitter pattern can contain a microstep pattern.


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Mesostep A sequence of exposures designed to completely sample across the face of the detectors in medium-sized steps to monitor residual systematics in the photometry. Microstep (pattern) A pattern of exposures at positions each shifted by a very small movement (<3 arcsec) from the reference position. Unlike a jitter the non-integral part of the shifts are specified as 0.5 of a pixel, which allows the pixels in the series to be interleaved in an effort to increase sampling. A microstep pattern can be contained within each position of a jitter pattern. Movement A change of position of the telescope that is not large enough to require a new guide star. Offset A change of position of the telescope that is not large enough to require a telescope preset, but is large enough to require a new guide star. Pawprint The 16 non-contiguous images of the sky produced by the VISTA IR camera, with its 16 non-contiguous chips (see Figure 2-2). The name is from the similarity to the prints made by the padded paw of an animal (the analogy suits earlier 4chip cameras better). Preset A telescope slew to a new position involving a reconfiguration of the telescope control system and extra housekeeping operations that are not necessary for a movement or an offset. Tile A filled area of sky fully sampled (filling in the gaps in a pawprint) by combining multiple pawprints. Because of the detector spacing the minimum number of pointed observations (with fixed offsets) required for reasonably uniform coverage is 6, which would expose each piece of sky, away from the edges of the tile, to at least 2 camera pixels. The pipeline does not combine pawprints into tiles.


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2 Overview
2.1 Hardware
VISTA is a wide field alt-az telescope designed for a single purpose, surveys, and which does not have a conventional focus. It can only be used with a purpose built camera, and is delivered with an IR camera. Thus it is the performance and pointing of the telescope-camera system that is important. The telescope by itself front sensing. The IR Autoguider CCDs and two CCDs, operating in has no capability to lock onto a guide star or carry out wave Camera therefore contains, as well as 16 IR detectors, two two low order wave front sensor (LOWFS) units, each with the I band, as shown in Fig 2-1. Two autoguiders, on opposite

+Y

+X

Figure 2-1 VISTA Focal plane: Each of the 4 groups of detectors in the Y direction (e.g. #s 1-4, 58, 9-12, 13-16) is read out by a separate IRACE controller.


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edges of the focal plane, are used in order to meet the sky coverage requirements, although only one is allowed to apply corrections to the telescope axes at any given time. The LOWFSs measure aberrations that are used by the external active optics control process to adjust the position of the 5 axis (x, y, z, tip, tilt) secondary mirror support system and some aspects of the M1 surface to maintain image quality. The LOWFS operates roughly every 1 minute during tracking and needs exposures of ~40 sec to average out seeing effects. Although the Autoguiders and LOWFSs are physically located within the IR camera, both are considered part of the TCS from a software point of view. This is primarily to maintain consistency with existing VLT software and standards. The VISTA pipeline receives no data from these CCDs. The

Figure 2-2 VISTA Engineering Pawprint.


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CCDs therefore do not impact on the VISTA pipeline, except in so far as the pointing and image quality of the camera are dependent on their proper operation. A high order wave front (curvature) sensor (HOWFS) uses some of the science detectors to determine occasional adjustments to the primary mirror support system. (This is done perhaps once at the start of the night and once around midnight.) Processing the signals from the HOWFS is done within the Instrument Workstation, and so the pipeline will not have to deal with the HOWFS at all. However all data from the IR detectors, including HOWFS data, is passed to the science archive, so the necessary calibration templates for the HOWFS are covered here. Within the IR Camera are 16 Raytheon 2048x2048 VIRGO detectors arranged in a sparse array. Each camera exposure produces a pawprint consisting of 16 noncontiguous images of the sky. An example display of a complete FITS file consisting of a VISTA "pawprint" is shown in Figure 2-2. The VISTA IR camera has only one moving part, the filter wheel which has 8 filter holders, each filter holder containing 16 filters, one for each IR detector. There are further auxiliary (beam splitting) filters for use with the high order wave front sensor.

Figure 2-3 Filter Transmission Curves for Reference Samples of Y, J, H, and Ks bands.

One of the filter holders contains a set of completely block the detectors from incoming thermal emission) which are used for taking delivered with 6 filter sets (Z, Y, J, H, s and

16 cold blanks (metal units which sky radiation, and produce negligible dark frames. The instrument will be a narrow-band at 1.185 - Figure 2-3)


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and a further set of cold blanks, which can be replaced with other filters in due course. The position angle of the camera axis can be controlled by the instrument rotator. Single integrations are taken by a Reset-Read-Read procedure with the difference of the two Reads being performed within the DAS.

2.2 Observing Modes
IMAGING is the only mode in which science data will be acquired, but the science array is used to acquire data for internal wave-front analysis.

2.2.1 Imaging Mode Description
The sky target position is acquired and tracked and in parallel (for observing efficiency) the required filter set is placed in the beam. The LOWFS provides the necessary updates to the M2 and M1 support units. A set of exposures, each of which may consist of a number of integrations, are taken and are usually jittered by small offsets, to remove bad pixels and determine sky background. The set of exposures produced is combined in the pipeline to create a single pawprint, in which the jitters from all detectors are included.

Six such pawprints, taken at appropriate offsets, can be combined to produce an almost uniformly sampled image of a contiguous region, each bit of sky, except at the edges, having been observed by at least two pixels. The individual exposures making up each pawprint may be made on a jitter or a microstep pattern. Microstep patterns are interleaved rather than combined, so the calibration procedures are unchanged, though the data volume increases.

2.2.2 Calibrations
The calibrations are of four sorts: i. those that characterize the properties of the transfer function (image in, electrons out) of the end-to-end system (telescope, camera, IR detector system including associated controllers, etc.) so that instrumental effects can be removed from the data. As VISTA has a wide field of view, particular attention must be paid to variations across the field; ii. those that characterize the astrometric distortions of the images; iii. those that characterize the photometric zero points and extinction coefficients corresponding to the images; iv. those that generate Quality-Control measures.

2.2.3 High Order Wave Front Sensor (HOWFS) Mode
The HOWFS mode is processed in the Instrument Workstation and is logically part of the TCS. However, as it uses the IR detectors, all of whose data are passed to the archive, it is considered as a separate observing mode for VISTA pipeline purposes.


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In HOWFS mode a special beam-splitting filter is used to make a curvature sensor in which two images (above and below focus) of a reference star are formed and used to generate corrections to the forces in the M1 support unit, ensuring the mirror figure is maintained. This mode will typically be used of order twice a night (start and around midnight), or less often if the repeatability of the lookup table is good.

2.2.4 Calibrations
The HOWFS uses some of the science mode IR detectors, but has a special beam splitting filter whose unique signature needs to be removed from the HOWFS data before it can be analysed. However, this flat-fielding is carried out within the HOWFS image-analysis software (which is part of the Camera Software) and not by the pipeline, and is noted here for completeness.

2.3 Pipeline
The VISTA pipeline will produce photometrically and astrometrically calibrated pawprints, with instrumental artefacts removed. In order to achieve almost uniform coverage of a full contiguous area of sky, a six point offset pattern is used by default. A template that implements this pattern is defined and the pipeline will calibrate the resulting six pawprints individually. The further step of combining these into a contiguous map is left to the science user. For certain science programs the OS will allow distinct OBs for eventual "PI" processing; the main example of this would be observing offset sky frames to calibrate the sky in extended-object science frames. The QC pipeline is not required to associate such observations, but will perform routine reductions on such data. Other processes which are not calibration issues, but which may nevertheless relate to achievable data quality, are not discussed here. Such (excluded) processes include: · co-addition of individual integrations of a pawprint into a single exposure within the data-acquisition system; · combination of many pawprints to cover contiguous areas of sky; · co-addition of many pawprints to go deeper.

2.4 Operation
This section defines the observing modes, Section 3 contains an error discussion, Section 4 describes the calibration data required for instrumental signature removal, Section 5 describes the calibration data required for photometric calibration. Section 6 describes the calibration data to be derived from science data, including astrometric calibration. Section 7 discusses Quality Control measures based on regularly measured selected sets of calibrations for the purpose of instrument "health checks". Section 8 describes all templates and Section 9 the Technical Programs. Finally Section 10 details the Format of Data Frames. The philosophy throughout is that the VI