Photometry
approximate photometry at the 10% level can be
obtained by using the following default CCD zero-points:
U = 22.4, B = 24.5, V = 24.4, R = 24.4, I = 23.7, Z'= 22.2
ie. that magnitude star which gives 1 count/s total. Note that these
zero-points are in counts and the Z' system denotes natural WFC filter
+CCD response approximately normalised to the Vega system.
The renormalised CCD gain is approximately 3.12
so to convert to photons/s magnitude zero points add 1.24 magnitudes.
Full colour equations and accurate photometric zero points and
extinction correction for each survey night are in the process of
begin constructed (see the section on
photometric calibration for more information).