The long-term photometric stability of WFPC2 has been evaluated by examining the photometric monitoring data collected over the lifetime of the instrument. 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. Observations starting in Cycle 6 were on a rotating schedule, where observations are taken in a different chip each month. Since Cycle 11 (starting Fall 2002) these observations are taken every 49 days either before or after a decontamination procedure, and starting in Cycle 14 they will be taken every 55-60 days due to the more limited target visibility resulting from 2-gyro mode.
Figure 1.1 shows the photometric monitoring data for the standard star GRW+70D5824 (a white dwarf classified DA3; B-V = -0.09) in the WF3 and PC1 for the set of filters which are routinely monitored. Only data after April 24, 1994, when the CCD operating temperatures were lowered from -76°C to -88°C, are shown.
Figure 1.1: Photometric Monitoring Data for WFPC2.
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