WFPC2 Technical Instrument Report 99-01:
WFPC2 Aperture Photometry Corrections as a Function of Chip Position
-Gonzaga, O'Dea, and Whitmore
The WFPC2 PSF shape is a function of position in the chip. As the PSF changes, the relative fraction of encircled light inside and outside a given aperture will also change. The effect is larger for smaller apertures as shown by ISR 97-01 (Suchkov and Casertano). This report looks at the effect of PSF changes across the field-of-view on photometric aperture correction as a function of position on the chips by comparing aperture photometry in apertures of different radii.