The WFPC2 cameras have significant geometric distortion which not only affects astrometry, but also affects photometry (because it induces an apparent variation in surface brightness across the field, and hence impacts the flat fields). ... The pixel scale can be estimated from the commanded offsets between the frames (relying on the FGS scale and distortion calibrations). ... The cubic distortion coefficients can be used to derive effective pixel areas as presented in Figure 5.16 . ...