WFPC2 Instrument Science Report 95-05:
WFPC2 Ghosts, Scatter, and PSF Field Dependence
by John Krist
August 9, 1995
Scattered light and ghosts in WFPC2 images are discussed, with emphasis on their effects on imaging of faint objects near bright ones. Scattering within about two arcseconds is primarily due to diffraction effects from zonal errors in the HST optics. At larger angles, the main source of light is scattering from the CCD surface.
Also, a number of ghosts are present in WFPC2. Some of these may be mistaken as stars, and others may interfere with the imaging of nearby material.
The field dependence of the WFPC2 PSF is also discussed. PSF structure can change appreciably across a single chip due to obscuration shifting, focus, coma, and astigmatism changes. These changes can affect PSF subtraction and profile fitting photometry.