WFPC2 Instrument Handbook for Cycle 10

Photometric Anomalies

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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. 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 physical cause of the long vs. short anomaly is not fully understood, and it does not appear to change with time. We have developed correction formulae which appear to reduce the impact of both anomalies to about 2-3% for faint sources. 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.