Instrument Science Report 98-03:
WFPC2 Long-Term Photometric Stability
S. Baggett and S. Gonzaga
October 21, 1998
We examine the WFPC2 stellar photometric monitoring data obtained over the last four years. The new analysis shows that, for bright targets, (1) the long-term photometric throughput continues to be quite stable: fluctuations are ~2% or less peak-to-peak over 4 years in filters longwards of and including F336W, (2) the UV throughput has gradually evolved, that is, the clean count rates (immediately after the DECONs) have increased over the 4 years in some filters, e.g., F160BW by ~12% in the PC, and F170W by ~9%, while decreasing slightly in others, e.g., F255W by ~3% and, (3) the contamination growth rates have slowed slightly for the UV filters, e.g., ~1% per day to 0.5% per day in F160BW on the PC. The small discontinuity around Feb. 1995 is discussed. These results are being used to update the WFPC2 time-dependent photometric calibration within SYNPHOT.