int wfs pipeline processing

New Astronomy Reviews 45 (2001) 105–110www.elsevier.nl / locate /newar

INT WFS pipeline processing*Mike Irwin , Jim Lewis

Institute of Astronomy, Madingley Road, Cambridge, UK

Abstract

We give a brief overview of the INT Wide Field Camera (WFC) together with the automated pipeline processingdeveloped specifically for the Wide Field Survey (WFS). The importance of accurate and complete FITS header informationis stressed. Data processing products output from the complete pipeline are discussed. 2001 Elsevier Science B.V. Allrights reserved.

Keywords: Wide field surveys; Automatic processing

1. An overview of the INT WFC data readout (16-bit pixels) is ¯70 Mbytes perexposure. Current readout time is 160 s in the usual

The INT WFC comprises of 4 thinned coated EEV readout mode, which in practice leads to a deadtime4 k32 k CCDs for the science array plus a thinned between exposures of around 3 min. Typical ob-2 k32 k Loral CCD for use as a dedicated frametransfer autoguider. A schematic of the cameralayout, with stars from the GSC overlaid, is shown inFig. 1 for the default rotator angle of 180 degrees.The inner circle of this figure denotes the 40 arcmindiameter boundary between the unvignetted andvignetted regions, while the dashed outer boundary isthe 52 arcmin diameter circle corresponding to 50%vignetting — the practical limit for obtaining viabledata. Note that only the top half of the autoguiderCCD is active, the bottom half is masked and used asa frame transfer buffer.

The EEV science devices have 13.5 micron pixelscorresponding to 0.33 arcsec /pixel at INT prime andeach covers an area on sky of 22.8311.4 arcmin.The total sky coverage per exposure for the array istherefore 0.29 square degrees and the total amount of

*Corresponding author.E-mail address: [email protected] (M. Irwin). Fig. 1. Overview of the layout of the INT WFC.

1387-6473/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S1387-6473( 00 )00138-X

106 M. Irwin, J. Lewis / New Astronomy Reviews 45 (2001) 105 –110

servations yield between 5 and 10 Gbytes of data per therefore be constructed from dusk and dawn skynight which is archived on single DDS3 DATs for flats.later transfer to Cambridge for pipeline processing. The data processing pipeline is heavily dependent

The Loral autoguider has 15 micron pixels, or 0.37 on the integrity of the FITS header information fromarcsec /pixel, although charge adjacency effects limit the telescope plus instrument and is also used as athe true Loral resolution to 30 microns. The au- repository of information derived and generated bytoguider sees through the same filter as the science the processing stages. We cannot emphasise enoughdevices and is rigidly mounted in the same dewar the vital importance of this information for auto-ensuring that the guiding on the science devices is mated pipeline processing.optimal. The autoguider system uses an APM-style The following stages briefly describe the indi-image centroiding algorithm and provides a real-time vidual tasks carried out during the pipeline process-information display of the guiding status, including: ing operations in the order they are applied:tracking errors, relative transparency and seeingestimates. 1. De-biassing and trimming: Stacked bias frames

A six position filter wheel, holds the 14 cm using the default clear and readout speeds showdiameter filters in the f /3 beam, just above the dewar some low level repeatable structure due to elec-window. Low level N flushing between the filters tronic transient effects, therefore full 2-D bias2

and the dewar window is used to keep the window removal is necessary. All of the requisite debias-moisture-free. Filter changing is automatic and only sing and trimming data and bias sections aretakes a few seconds. The shutter assembly, a uni- specified in the FITS headers.directional circular plate with a contoured aperture, 2. Non-linearity correction: Linearity tests usingensures even illumination and is timed by an on- sequences of dome flats revealed CCD[2 andboard microprocessor to a few ms. The whole CCD[4 to have significant, and similar, non-assembly is light tight making sky dawn/dusk linearities over the whole dynamic range. CCD[1flatfielding a practical proposition. and CCD[3 are essentially linear to ,1% over

the full range. A non-linearity correction is ap-plied via a Look Up Table (LUT) to all data.Since CCDs [2 and [4 share one ADC and [1

2. INT WFS pipeline processing and [3 another, we believe most of the non-linearity arises in the ADC and should therefore

The amount of data generated during the INT be stable with time.WFS make it impractical to process in an ad hoc 3. Bad pixel replacement: Bad pixels and dud partialmanner. We have, therefore, developed an IRAF- columns are flagged in a master file and are thenbased pipeline to handle the large amounts of data in interpolated over using neighbouring regions.a mainly automatic way. Although most of the 4. Flatfielding: For each observing run a sequenceprocessing is routine and automatic we have found of master sky flats was constructed. Because ofthat creating, updating and maintaining master li- readout overheads it is impossible to generatebrary bias frames, flat-field frames and defringing complete sky flats in, say, UBVRIZ each night,frames is difficult, if not impossible, to automate in a hence flats from several nights are combined tosurvey such as this. Since we have no direct control produce master flats. Some low level fringingover the observing protocol, a rather varied set of ,0.5% is still visible on the master i9, I and Z skyproblems/special cases generally means that we have flats but for the current processing we haveto interactively construct the master library reference ignored the slight inaccuracies introduced by thisframes for each survey run. Fortunately, given the problem.relatively slow readout speed, we have found that the 5. Gain correction: Although processed indepen-flat-field frames are stable for periods of around 1 dently, the mean flatfield sky level in eachweek, the typical survey time allocation slot, and can passband is used to place all the CCDs on the

M. Irwin, J. Lewis / New Astronomy Reviews 45 (2001) 105 –110 107

same zeropoint system, i.e. sky in deep frames is On photometric nights the derived CCD zero-the same for all CCDs to better than 0.5%, the points are consistent to within a few per cent,residual non-linearity level. This step grossly assuming an average extinction curve for the site.simplifies photometric calibration and mosaicing Current readout time limitations preclude observ-of images. ing sufficient standards to determine the extinc-

6. Defringing: Both I and Z-band data frames suffer tion for each night.from significant sky fringes at 63% and 66% ofsky respectively. R-band images also show weakfringing at 60.5% of sky due to the red tail of the 3. Data processing productsHarris glass filter. We find that the SDSS r9 filter,because of the well-defined bandpass of 5500– The observing logs are automatically generated at

˚7000 A, shows no sky fringing. The SDSS i9 filter the telescope from the FITS headers of the data andalso has less fringing, 62% of sky, than the provide the reference template for the SYBASE dataI-band filter, again due to the sharp cutoff at 8500 archive system. SYBASE provides a simple userA. Fringe patterns in the i9, I and particularly interface to the WFS archive allowing complexZ-band, can show significant variations on a querying and data retrieval from the archive. All ofnightly basis, making them difficult to remove the processed WFS data products are stored online,completely. We have developed algorithms for currently using a large disk array to facilitate accessautomatically removing the sky fringes using to the archive. Future planned upgrades includefixed fringe pattern masks, that generally reduce moving the ever expanding WFS archive to a DVDthe fringing level by a factor of 10 or more. An tower for longer term storage and access. All of theexample of the I-band fringing and its removal for master library bias frames, flatfield frames, andthe central region of CCD[1 is shown in Fig. 2. defringing frames plus non-linearity corrections are

7. Astrometry: A zeroth order astrometric World available via the www and have proven to be aCoordinate System (WCS), accurate to 5–10 valuable resource for other WFC users.arcsec is written to the FITS headers using thenominal telescope pointing, rotator angle and 3.1. Object cataloguesCCD characteristics. Following this, an astromet-ric solution based on the derived positions of Each pipeline processed CCD frame is analysedGSC objects visible in the data frames is used to using a standalone automated object detection andautomatically define a more accurate astrometric parameterisation algorithm based on the ideas pre-solution. Currently the external accuracy of this sented in Irwin (1985, 1996). Generated parametersastrometry is limited by the GSC accuracy to include position, intensity and shape information. Atabout 0.5–1 arcsec. Further improvements of this the limiting magnitudes reached by the WFS inare underway using deeper Schmidt plate-based 10–20 min integrations (m |24), the majority of theR

astrometry. Preliminary results suggest the final detected images are faint background galaxies.internal astrometric error will be well within 0.1 Therefore, to maximise the general usefulness of thearcsec for the whole mosaic. The celestial coordi- catalogues three different types of flux estimates arenate representation used is based on a polynomial made:extension of the Zenithal equidistant projection (i) isophotal — the integrated flux above a speci-(Greisen and Calabretta, 1995). fied isophote, i.e. the detection isophote of 1.5sn

8. Photometry: A series of selected Landolt standard above sky, where s is the pixel-to-pixel noise atn

fields (Landolt, 1992) are observed each survey sky;night to monitor and calibrate the photometry to (ii) total — takes the detection isophote for eachthe Johnson–Kron–Cousins photometric system. image and expands it using an elliptical aperture toColour equations for the INT WFC plus standard perform a curve-of-growth analysis (see Hall andfilter set are presented elsewhere in this volume. Mackay, 1984; Irwin, 1985; for more details);


Fig. 2. Fringing in the I-band on CCD[1 before and after automated removal.

M. Irwin, J. Lewis / New Astronomy Reviews 45 (2001) 105 –110 109

Fig. 3. Sky brightness and seeing estimates for a week of INT WFS data. The different colour dots represent different filters. Better seeing isfound with ‘redder filters.’

(iii) core — this is a ‘poor man’s’ PSF fit using a survey data since they determine how deep thetop hat function typically with radius equal to the survey goes. If we define the magnitude limit of aseeing (FWHM) including sub-pixel integration and dataset to be that magnitude at which PSF-dominatedsimultaneous fitting for blends. It is straightforward faint images have a signal-to-noise of, say, 5:1, thento show that for realistic image profiles this flux in the absence of confusion limits and using anestimate achieves |0.85 of the signal-to-noise, of the optimal detection filter, the attainable flux limit is

21‘perfect’ PSF fit, but comes at a fraction of the effort. ~seeing . Contrast that with the peak signal-to-22It also works for stars or galaxies. noise which varies as ~seeing . Likewise for sky 1

magnitude brighter than normal, exposures have to3.2. Data quality control be increased by a factor of 2.5 to reach the same

limiting magnitude.The software that generates the object catalogues

is used to automatically monitor the seeing, skybrightness and noise levels on all the science frames.Examples of the output of this data quality control Referencesinformation are shown in Fig. 3. These are thefundamental quantities that govern the usefulness of Greisen, E.W., Calabretta, M., 1995. In: Shaw, R.A., Payne, H.E.,


´Hayes, J.J.E. (Eds.), Astronomical Data Analysis Software and Irwin, M.J., 1996. In: Rodrıguez Espinosa, J.M., Herrero, A.,´Systems IV. ASP Conference Series, Vol. 77, p. 233. Sanchez, F. (Eds.), Instrumentation for Large Telescopes, VII.

Hall, P., Mackay, C., 1984. MNRAS 210, 979. Canary Islands Winter School, p. 35.Irwin, M.J., 1985. MNRAS 275, 514. Landolt, A.U., 1992. AJ 104, 340.

int wfs pipeline processing

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