WFPC2 Instrument Handbook for Cycle 11

Long-term Photometric Stability

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The long-term photometric stability of WFPC2 has been evaluated by examining the photometric monitoring data collected over a period of more than four years. Our primary standard, GRW+70D5824, has been observed roughly every four weeks, before and after decontamination procedures, both in the far UV and in the standard photometric filters. Early observations were taken monthly in both the PC and WF3. Later observations (since Cycle 6) were on a rotating schedule, where observations are taken in a different chip each month. Overall, a baseline of over four years is available for the PC and WF3, and about two and a half years in WF2 and WF4. The data have been analyzed and reported by Baggett and Gonzaga (1998); here we summarize their main conclusions.

Overall, the WFPC2 photometric throughput, as measured via our primary standard, has remained remarkably stable throughout. Its long-term behavior in filters longward of F336W is characterized by small fluctuations (2% peak-to-peak) which appear to have no specific pattern, and there is no significant overall sensitivity trend. Aside from contamination corrections, which are only significant shortward of F555W, the same photometric zeropoints can be applied to non-UV data throughout the life of WFPC2.

In contrast, the UV photometric throughput of WFPC2 has changed measurably over the years. In most cases, the throughput has increased slowly, perhaps as a result of continuing evaporation of low-level contaminants. In F170W, the best-characterized UV filter on WFPC2, the clean throughput (immediately after a decontamination) has increased in the PC by about 12% from 1994 to 1998. Not all UV filter / detector combinations show this behavior; some combinations show a modest decline in throughput (e.g. 3% in F255W). Baggett and Gonzaga (1998) report the details of the secular throughput changes for the filters we monitor.

Finally, the contamination rates - the rate at which the camera throughput declines after a decontamination, due to the gradual buildup of contaminants on the cold CCD windows - have generally decreased since installation of WFPC2, possibly also because the environment has become cleaner with time. (This excludes brief periods of increased contamination just after servicing missions.) For example, the contamination rate in F170W in the PC has decreased from ~0.56%/day to ~0.45%/day. See Short-term Time Dependence of UV Response for additional discussion of the UV response variations.

Baggett and Gonzaga (1998) suggest a number of ways users can correct long-term changes in WFPC2 photometry. While these changes are generally small, users wishing to achieve high-precision photometry, especially in the UV, should follow their recommendations.