On the photometric consequences of charge-transfer inefficiency in WFPC2

Charge-transfer effects in photometry with Wide Field Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope an investigated by a comparison of WFPC2 observations with ground-based photometry for the Galactic globular clusters omega Centauri and NGC 2419. Simple numerical formulae describing the fraction of lost signal as functions of position on the detector, stellar brightness, and the diffuse sky brightness recorded in an image are presented, and the resulting corrections are compared with those previously derived by Whitmore & Heyer. Significant lost-charge effects are seen that are proportional to both the Y-coordinate (i.e., the number of shifts along the parallel register during readout) and the X-coordinate (number of shifts along the serial register). A "typical" star image (one containing similar to 10(4) photoelectrons) near the center of a "typical" intermediate-length exposure (one with a diffuse sky brightness of similar to 10 e(-) pixel(-1), obtained at a camera temperature of -88 degrees C) loses approximately 2% of its electrons to charge traps during readout; a star in the corner of the image most remote from the readout electronics loses twice that. The percentage of charge lost decreases as the star brightness or the diffuse sky brightness increases. Charge losses during the brief period when WFPC2 was operated at a temperature of -76 degrees C were approximately 85% greater but apart from that, no significant change in the charge-transfer losses with time during the first 3.5 years of WFPC2's mission is evident, except possibly a weak effect for the very faintest star images. These results are quite similar to those of Whitmore & Heyer, which were based on a much smaller data set, but there are some differences in detail. Even with the present set of corrections, additional sources of calibration uncertainty that I am unable identify or characterize with the available data probably limit the external accuracy of photometry from WFPC2 to of order 1%-2%.