WFPC 2 Standard Star CTE

Observations of the WFPC2 standard star GRW+70D5824 were made at five positions along the diagonal of each detector to directly evaluate CTE effects on standard star data. Two filters commonly used for standard star monitoring, F170W and F555W, were tested. The data were subsequently corrected with the Dolphin CTE equations, and the results were studied for any residual variations in photometry with detector position. Two (and sometimes three) exposures were made at each detector position. In the course of our work we discovered a significant 'first exposure effect' where the first exposure produced significantly fewer counts than the subsequent ones. For both filters, the count deficit in the first exposures increased approximately linearly with Y position, and reached ~5% at Y=800. For the F555W filter, the Dolphin equations were reasonably accurate, and the first exposures tend to be under corrected by about 0.5%, and the second exposures were over-corrected by about 1.5%, both evaluated at the chip centers. For F170W the residual errors are larger, with the first exposures being ~3% over-corrected, and the second exposures being 6% to 7% over-corrected, both at the chip centers. These results for F170W suggest systematic errors of 3% to 7% could be present in the photometric calibration of UV filters for data taken late in the WFPC2 mission.