Each image is a Multi-Extension FITS file (MEF) containing the four CCD frames. Each CCD is photometrically and astrometrically calibrated as described in the previous section. Photometry can be extracted from the images using the following expression
m = ZP - k ( X - 1 ) - 2.5 log (f / t) + c
where m is the magnitude, ZP is the zero point as extracted from the image headers, k is the extinction coefficient, X is the airmass, f is the flux in counts and y is the exposure time. The c correction is based on the median dark sky recorded in science frames compared to the median for all the CCDs and as such is an ancilliary correction to the gain correction derived from the flatfield (usually twilight flats). This correction is in the header keyword
PERCORR and is typically smaller than 1%.
Astrometric information is written in the FITS headers using a Zenithal Polynomial (ZPN) projection which allows for a description of the non uniform scale over the field of view without performing pixel resampling (necessary e.g. in the case of describing the distortions using a more usual Tangential projection). However the change of scale also creates photometric effects, the magnitude of which can be modelled as (Irwin et al. 2007, in prep)
d = ( 1 + 3 P_3 R^2 / P_1 ) ( 1 + P_3 R^2 / P_1 )
where R is the distance from the field centre and for the WFC the coefficients are P_1=1 and P_3=220 (corresponding to the PV2_1 and PV2_3 WCS keywords) . This value has to be subtracted from the magnitude calculated in equation above and it is at the level of 2% at the outer parts of the field.
The following table lists the relevant astrometric and photometric header keywords to use if you want to convert your own measured fluxes to magnitudes.