Instrument Science Report 98-02:
The Long vs. Short Anomaly in WFPC2 Images
S. Casertano and M. Mutchler
September 24, 1998
We present an improved characterization of the so-called "long vs. short" anomaly, a non-linear behavior of the WFPC2 chips which results in a decrease of the measured count rate with decreasing exposure time. We demonstrate that: 1) this non-linearity depends strictly on total counts in a stellar image, and is independent of exposure time; 2) the effect depends on position on the chip only through the well-known CTE effect, and is independent of position after the CTE correction is applied; 3) F555W and F814W appear similarly affected; and 4) there is a marginal decrease in non-linearity at high background levels, but the effect is statistically insignificant in the overall description of the anomaly. We present a simple formula that can be used to correct the observed counts in a variety of conditions; this formula is recommended for use with most point-source photometry obtained with WFPC2.