Charge-transfer efficiency of WFPC2

Observations of omega Centauri have been used to characterize the charge-transfer efficiency (CTE) of the Wide Field and Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope. A set of formulae has been developed to correct aperture photometry for CTE loss with dependencies on the X- and Y-positions, the background counts, the brightness of the star, and the time of the observation. The observations indicate that for very faint stars on a very faint background, the CTE loss from the top to the bottom of a chip has increased from about 3% shortly after the cooldown of WFPC2 (1994 April 23) to roughly 40% in 1999 February. In general, typical WFPC2 exposures are much longer than these short-calibration images, resulting in higher background which significantly reduces the CTE loss and minimizes the CTE problem for most science exposures.