VIRCAM Catalogues

The main changes from WFCAM are correct use of the table WCS and the location of the telescope-specific information in the primary header. The Hall flux and radius have been replaced by the half-light flux and radius since this is a more astrophyically useful quantity. Petrosian and Kron fluxes are still also currently computed within circular apertures, a future upgrade will convert these to elliptical apertures. Parameter #55 will also be upgraded to include the average confidence level withing the default analysis aperture.

Preamble
The derived object catalogues are stored in multi-extension FITS files as FITS binary tables, one for each image extension with the primary header unit containing the telescope andobservation-specific information. The catalogue extension headers contain a copy of therelevant detector-specific information. Each detected object has an attached set of descriptors, forming the columns of the binary table and summarising derived position, shape and intensity information. During further processing stages ancilliary information such as the sky properties, seeing and so on are derived from the catalogues and stored in the FITS headers attached to each catalogue extension.

The catalogue format was derived from similar APM/SuperCOSMOS/INT WFC/CIRSI analysis which produced 32 4-byte parameters per detected object. This was first enhanced for WFCAM to an 80 4-byte parameter set to include extra parameters for flux estimation and error estimates, and this has now been further refined for the VIRCAM catalogues.
The following tables cover the VIRCAM standard and further processing pipeline (TBD) output catalogues, where for simplicity all derived parameters are stored as floating point numbers even though some of them are more naturally integers.

 Columns Content Description

Derived catalogue FITS header contents

Examples of extra FITS header items derived from the catalogues, or during catalogue creation, or specifying user-selectable input parameters, one set for each extension.

SKYLEVEL=            91.82     / Median sky brightness (counts/pixel)

An automatic 2D background-following algorithm is used to track and "remove" slowly varying background features such as image gradients etc.. The default scale size for background tracking (NBSIZE) is currently set to 64 pixels, coupled with a smidge of non-linear filtering this gives a background tracking scale of order 100 pixels. (A bilinear interpolator is used to generate pixel resolution background maps internally).

SKYNOISE=             6.06     / Pixel noise at sky level (counts)

Robust MAD estimator for noise scaled to equivalent Gaussian rms value ie. = MAD x 1.48 after removing large scale sky background variations. MAD = Median of the Absolute Deviations about the median

THRESHOL=             9.09     / Isophotal analysis threshold (counts)

User-selectable parameter, the default is to set this to 1.5 skynoise as a compromise between detecting close to the limit of the data and not being swamped by spurious sources. It is possible to push the data limit fainter but at the expense of a large increase in spurious sources. 

MINPIX  =                4     / Minimum size for images (pixels)

User-selectable parameter, in conjunction with the threshold above this determines how deep and how small ``real'' images can be. This default precludes many of the few pixel-hit cosmic rays from being considered since ``real'' images must have 4 contiguous simply-connected pixels in the union of the detection filter and data domains. For more details on image detection and parametersiation see the papers in http://www.ast.cam.ac.uk/vdfs/publications.html .

CROWDED =                1     / Crowded field analysis flag (0 none, 1 active)

User-selectable parameter, detection algorithm tried to disentangle overlapping images or images supperposed on the ``slowly'' varying background of other large images (default) otherwise just straighforward isophotal detection.

RCORE   =             3.50     / Core radius for default profile fit (pixels)

User-selectable parameter, aperture flux designed to match median seeing of survey data. It is straighforward to show that if rcore = FWHM then for typical profiles encountered the rcore flux estimate has between 80-90% of the accuracy of an idealised perfectly known PSF model method.

OPTFILT =             2.0      / FWHM of Gaussian detection filter (pixels)

User-selectable parameter, should be chosen to match the average FWHM of stellar images in the data. This defines the Gaussian matched detection filter to use.

SEEING  =            2.915     / Average FWHM (pixels)

An average realistic FWHM estimated directly from the stellar images on the frame. Mutiply by pixel scale size to convert to arcsec (e.g. *0.334 VIRCAM).

ELLIPTIC=            0.054     / Average stellar ellipticity (1-b/a)

A direct estimate of the average stellar ellipticity, useful for spotting trailed frames usw.. Should not average much above 0.15 for "normal" frames.

CLASSIFD=                    T / Classified

Has image morphological classifier been run ? if so an object classification flag and a stellarness index is included in the binary table columns.

SATURATE=          41593.8     / Average saturation level in frame

An estimate directly from saturated images on the frame at what level image saturation occurs, including sky. This varies from detector to detector depending on the relative gains applied to bring them to a uniform flatfield reponse etc..

Photometric Information

APCORPK =            2.812     / Stellar aperture correction - peak height    
APCOR1  =            0.635     / Stellar aperture correction - core1 flux      
APCOR2  =            0.114     / Stellar aperture correction - core2 flux      
APCOR3  =            0.028     / Stellar aperture correction - core3 flux      
APCOR4  =            0.000     / Stellar aperture correction - core4 flux      
APCOR5  =            0.000     / Stellar aperture correction - core5 flux      
APCOR6  =            0.000     / Stellar aperture correction - core6 flux      
APCOR7  =            0.000     / Stellar aperture correction - core7 flux

Aperture corrections in magnitudes needed to correct the assorted aperture-like measures produced in the catalogues onto the equivalent of a total flux stellar system. These constitute the components of a curve-of-growth analysis contained within the catalogues with radii defined in the previous table; t o be used in the sense that

corrected photometry = 2.5 * log10(flux) + apcor

They also work well as a first order seeing correction for faint galaxies. Larger aperture corrections are not included.

The remaining information necessary for photometric calibration may also be included in the following keywords:-

PERCORR =               0.000  / Sky calibration correction (mags)

this is a correction based on the median dark sky recorded in science frames compared to the median for all the detectors and as such is an ancillary correction to the gain correction derived from the flatfield (usually twilight flats) data. This correction is to be used in the same sense as before in that

corrected photometry = 2.5*log10(flux) + apcor + percorr

Note that this parameters is always set to zero for VIRCAM data and is included for compatibility with optical catalogues.

MAGZPT  =              22.105  / Photometric ZP (mags) for default extinction

Derived detector zero-point in the sense of what magnitude object gives a total (corrected) flux of 1 count/s. These ZPs are appropriate for generating magnitudes in the natural detector+filter system based on Vega, see http://www.ast.cam.ac.uk/vdfs/docs/reports/ for more details on colour equations etc.. The ZPs have been derived from a robust average of all photometric standards observed on any particular set of frames, corrected for airmass but assuming the default extinction values listed later. For other airmass or other values of the extinction use

ZP -> ZP - [sec(z)-1]*extinct + extinct default - extinct

You can then make use of any of the assorted flux estimators to produce magnitudes via

Mag = ZP - 2.5*log10(flux/exptime) - apcor - percorr

Note that for the so-called total and isophotal flux options it is not possible to have a single-valued aperture correction.

The 5 sigma magnitude limit for a pointlike source measured in the default aperture of radius rcore  can be calculated as

MagLim = ZP - 2.5*log10(5 * sqrt(pi*rcore^2) * skynoise / exptime) - apcor -percorr

MAGZRR  =               0.031  / Photometric ZP error (mags)

error in the zero point derived from all the detectors in the pointing. If a good solution and photometric this error will be at the level of a few percent. Values of 0.05 and above indicate correspondingly more unstable conditions or some other issue.

EXTINCT =               0.050  / Default extinction in passband

for VIRCAM these are currently set to a constant clear night level, which within the current measuring error is the same for all passbands. Note that the frame-by-frame derived ZP from 2MASS automatically corrects for extinction variations, assuming they are uniform across the field of view.

To compute approximate errors in the fluxes you can also use the following:-

error^2 = flux/gain + npixels * skynoise^2

where npixels is either the effective area ie. pi * rcore^2 for the ``core'' measures or the no. of pixels above the detection isophote ie. areal profile1; gain is the final overall detector system gain. (see http://www.ast.cam.ac.uk/vdfs/docs/reports/sv for values); flux is whichever measure you are using but note that for ``total'' this formula is not accurate since ``total'' fluxes are derived using a rather convoluted curve-of-growth technique; average skynoise can be obtained from the catalogue fits header.

NUMZPT  =                 623  / Number of standards used

the actual number of standards used in the photometric calibration for the pointing

NIGHTZPT=              22.851  / Average photometric ZP (mags) for night

a robust estimate of the average ZP in a particualar passband for the night

NIGHTZRR=               0.263  / Photometric ZP sigma for night (mags)

a robust estimate of the scatter in this ZP to give an indication of how photometric or otherwise the night was (0.03 or better is good; worse than 0.05 is usually indicative that the majority of the night was non-photometric; the example above says you should have gone to bed early).

Astrometric Information

For a description of the World Coordinate System (WCS) see Calabretta & Greisen 2002 A&A 395 1077 and Greisen & Calabretta 2002 A&A 395 1061.

TCTYP3  = 'RA---ZPN'           / Zenithal polynomial projection 
TCTYP5  = 'DEC--ZPN'           / Zenithal polynomial projection 

TCRPX3  =              2471.40 / Reference pixel X on axis 1
TCRPX5  =              6850.32 / Reference pixel Y on axis 2

TCRVL3  =         112.45449231 / [deg] Right ascension at the reference pixel
TCRVL5  =         -30.07196251 / [deg] Declination at the reference pixel     

TC3_3   =      -4.27375641E-09 / Transformation matrix element                
TC3_5   =       9.47570893E-05 / Transformation matrix element                
TC5_3   =      -9.48048395E-05 / Transformation matrix element                 
TC5_5   =       2.03270884E-09 / Transformation matrix element                

TV5_1   =                  1.0 / Pol.coeff. for pixel -> celestial coord       
TV5_3   =                 42.0 / Pol.coeff. for pixel -> celestial coord       
TV5_5   =             -10000.0 / Pol.coeff. for pixel -> celestial coord

NUMBRMS =                   87 / Number of standards used                     
STDCRMS =                0.089 / Astrometric fit error (arcsec)

In order to apply the distortion correction to the measured flux you need to follow the steps described in the next code snippet:

"""Converts x, y coordinates to standard coordinates
and works out the flux distortion factor."""
xi = hdr['TC3_3']*(x - hdr['TCRPX3']) + hdr['TC3_5']*(y-hdr['TCRPX5'])
xn = hdr['TC5_3']*(x - hdr['TCRPX3']) + hdr['TC5_5']*(y-hdr['TCRPX5'])
xi = xi*pi/180.; xn = xn*pi/180.
r = sqrt(xi*xi+xn*xn)
distortcorr=1.0+3.0*hdr['TV5_3']*r**2/hdr['TV5_1']+5.0*hdr['TV5_5']*r**4/hdr['TV5_1']
distortcorr=distortcorr*(1.0+hdr['TV5_3']*r**2/hdr['TV5_1']+hdr['TV5_5']*r**4/hdr['TV5_1'])

flux = flux / distortcorr