Wide Field and Planetary Camera 2 Instrument Handbook for Cycle 14
6.8 Photometric Anomalies
There are two photometric anomalies resulting from nonlinearities of the WFPC2 detectors. The first is due to the imperfect charge transfer efficiency (CTE) of the detectors, which causes sources at high row and column numbers to appear fainter because the charge is transferred over a bigger fraction of the chip. This anomaly is increasing with time, especially for faint sources, presumably as a consequence of on-orbit radiation damage. We have developed correction formulae which appear to reduce the impact of this anomaly to about 1-3% for faint sources. The second, called "long vs. short", causes sources to have a lower count rate - and thus appear fainter - in short exposures than in longer exposures, and appears independent of the position on the chip. The most likely explanation is that this effect is due to an overestimate of the sky measurement in the short exposure due to the presence of scattered light around bright stars. For further discussion, see Photometric Anomalies: CTE and Long vs. Short.
We also note the F1042M filter has an anomalous PSF which can impact aperture photometry. See PSF Anomaly in F1042M Filter.
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