When performing integrated photometry, users should
be aware that WFPC2 has significant distortion which causes
the pixel area to vary across the field. The application of
flat-fields "corrects" for this and normalizes all pixels to
have the same area. However, this renders integrated photometry
incorrect, because all of the light falls on the distorted
pixels and correcting for the pixel area changes the amount
of total light.
Consequently if you are measuring integrated brightnesses (as
opposed to surface brightnesses), you will want to put back
the true relative pixel areas. Maps of these can be found below.
Multiply by these maps to put the relative
areas back into the image. The maps are normalized to unity at
(400,400) in each chip.
This file, rootname f1k1552bu, is available at
http://www-int.stsci.edu/instruments/wfpc2/Wfpc2_phot/