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M. S. Wiggs, S. Casertano, B. C. Whitmore, M. Mutchler, J. A. Biretta (STScI), WFPC2 Photometry Team
We summarize the current knowledge of two photometric anomalies which primarily affect faint stellar targets observed with the Wide-Field / Planetary Camera 2 (WFPC2) onboard HST. The CTE (Charge Transfer Efficiency) problem causes targets to appear too faint, and primarily depends on target position on the CCD, the background level in the image, and the target counts. For typical cases (i.e. long exposures in broad filters) the effect is much smaller than the combined photon and read noise. However, in the worst case scenario of a narrow band filter and a faint (10 DN) target, the effect can be as large as ~40%. There is evidence that the CTE effect is slowly increasing with long-term radiation exposure of the CCDs. A second effect, called the long vs. short exposure effect for historical reasons, is primarily a function of target counts. The effect ranges from 2% for star images containing > 300DN, to as much as ~40% for images containing only 10 DN. Algorithms are available to correct both effects to <2% accuracy for bright targets, and ~5% accuracy for faint ones.
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