4.12 Photometric Anomalies: CTE and
Long vs. Short

There are two photometric anomalies which have now been extensively characterized. The first effect is due to the imperfect charge transfer efficiency (CTE) of the detectors, which causes sources at high row and column numbers to appear fainter than otherwise 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. In this section, we provide correction formula which reduce the impact of this anomaly to about 1-3% in typical cases. The second effect, 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. This nonlinearity is very small (i.e. a few percent) or non-existent for uncrowded fields, with less than ~1000 stars per WFC chip. However, for crowded fields with ~10,000 stars per chip, apparent nonlinearities of tens of percent are possible in extreme cases (e.g., when comparing 10 sec. with 1000 sec. exposures). The most likely explanation is that this effect is the result of an overestimate of the sky measurement in the short exposure due to the presence of scattered light around bright stars. Because the magnitude of the "long vs. short" effect is highly dependent on the parameters of the photometric analysis, no standard correction formula have been provided. Both the CTE and "long vs. short" effect are more fully described below.