Pipeline Correction of Images Impacted by the WF 4 Anomaly

The WF4 CCD anomaly is characterized by low or zero CCD bias levels, lowered count levels on the WF4 detector (i.e., low CCD gain), and faint horizontal background streaks. To correct the first two effects, a new processing step has been added to the WFPC2 calibration pipeline. It rescales each pixel using a gain correction that depends on the observed pixel value and the bias level of the image. Internal VISFLAT observations have been used to derive the corrections, which are tabulated into separate reference files for gain 7 and gain 15 data. After correction, the WF4 images show normal bias levels. Photometric tests using the standard star GRW+70D5824 indicate that the corrections are generally accurate to ~ 0.01 magnitude, with lower accuracy of ~ 0.02 magnitude in some infrequent cases. In most cases, the photometric properties of the corrected WF4 images are essentially indistinguisable from normal images taken in the other WFPC2 CCDs.