On the Photometric Consequences of Charge‐Transfer Inefficiency in WFPC2

Charge-transfer effects in photometry with Wide Field Planetary Camera 2aboard the Hubble Space Telescope are investigated by a comparison of WFPC2observations with groundbased photometry for the Galactic globular clustersomega Centauri and NGC 2419. Simple numerical formulae describing the fractionof lost signal as functions of position on the detector, stellar brightness,and the diffuse sky brightness recorded in an image are presented, and theresulting corrections are compared to those previously derived by Whitmore &Heyer (1997, Instrument Science Report WFPC2 97-08). Significant lost-chargeeffects are seen that are proportional to both the Y coordinate (i.e., thenumber of shifts along the parallel register during readout) and the Xcoordinate (number of shifts along the serial register). The percentage ofcharge lost decreases as the star brightness or the diffuse sky brightnessincreases. Charge losses during the brief period when WFPC2 was operated at atemperature of -76 degrees C were approximately 85% greater, but apart fromthat no significant change in the charge transfer losses with time during thefirst 3.5 years of WFPC2's mission is evident, except possibly a weak effectfor the very faintest star images. These results are quite similar to those ofWhitmore & Heyer, which were based on a much smaller data set, but there aresome differences in detail. Even with the present set of corrections,additional sources of calibration uncertainty which I am unable identify orcharacterize with the available data probably limit the external accuracy ofphotometry from WFPC2 to of order 1-2%.Comment: 5 tables, 4 figure