The charge-transfer efficiency and calibration of WFPC2

A new determination of WFPC2 photometric corrections is presented, using HSTphot reduction of the WFPC2 omega Centauri and NGC 2419 observations from 1994 January through 2000 March 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 the charge-transfer efficiency (CTE) and zero-point solution. The CTE loss time dependence is determined to be very significant in the Y-direction, causing time-independent CTE solutions to be valid only for a small range of times. On average, the present solution produces corrections similar to those of Whitmore and coworkers, although with an improved functional form that produces less scatter in the residuals and determined with roughly a year of additional data. In addition to the CTE loss characterization, zero-point corrections are also determined as functions of chip, gain, filter, and temperature. Of interest, there are chip-to-chip differences of order 0.01-0.02 mag relative to the calibrations of Holtzman and coworkers, and the present study provides empirical zero-point determinations for the nonstandard filters such as the frequently used F450W, F606W, and F702W.