The Dependence of WFPC2 Charge Transfer Efficiency on Background Illumination

We have examined the charge transfer efficiency (CTE) of WFPC2's CCDs near the end of mission as a function of background illumination. Internal lamps were used to flash the CCDs before or after external exposures of ! Cen to produce average background signals of 0–160 e. These signals span the natural sky backgrounds observed in ~99% of archived WFPC2 science images. Most of the stellar flux lost to poor CTE was recovered when the background signal was comparable to the average flux within the photometric aperture. Higher backgrounds contributed only more photon noise to the measurements. CTE losses from stars with aperture fluxes > 10 e are relatively small and insensitive to the background signal. For background signals > 10 e, WF4 showed better CTE than WF2 and WF3 at a statistically significant level. We also examined the efficacy of the latest formula for correcting CTE effects on WFPC2 aperture photometry obtained with the HSTphot and DAOPHOT software packages. The correction performs best on WF2 and WF3 photometry obtained with HSTphot; the residual error is ! 0.15 mmag/row (i.e., ! 0.06 mag at the centers of the CCDs) for almost all combinations of star and background signals. The correction does not perform as well on PC1 and WF4 photometry with background signals < 50 e. The DAOPHOT magnitudes of WF2 and WF3 sources are overcorrected by ~0.1 mmag/row relative to their HSTphot counterparts. However, the dispersion of the CTI-corrected DAOPHOT magnitude residuals for moderately bright stars imaged in all cameras is ~ 1/3 smaller than their HSTphot counterparts. These discrepancies probably reflect small differences between DAOPHOT’s and HSTphot’s aperture summing and sky-subtraction algorithms. Operated by the Association of Universities for Research in Astronomy, Inc., for the National Aeronautics and Space Administration.