The Charge‐Transfer Efficiency and Calibration of WFPC2

A new determination of WFPC2 photometric corrections is presented, usingHSTphot reduction of the WFPC2 Omega Centauri and NGC 2419 observations fromJanuary 1994 through March 2000 and a comparison with ground-based photometry.No evidence is seen for any position-independent photometric offsets (the"long-short anomaly"); all systematic errors appear to be corrected with theCTE and zero point solution. The CTE loss time dependence is determined to bevery significant in the Y direction, causing time-independent CTE solutions(Stetson 1998; Saha, Lambert, & Prosser 2000) to be valid only for a smallrange of times. On average, the present solution produces corrections similarto Whitmore, Heyer, & Casertano (1999), although with an improved functionalform that produces less scatter in the residuals and determined with roughly ayear of additional data. In addition to the CTE loss characterization, zeropoint corrections are also determined as functions of chip, gain, filter, andtemperature. Of interest, there are chip-to-chip differences of order 0.01-0.02magnitudes relative to the Holtzman et al. (1995) calibrations, and the presentstudy provides empirical zero point determinations for the non-standard filterssuch as the frequently-used F450W, F606W, and F702W.Comment: 30 pages, 10 figures Accepted for publication in October 2000 PAS