The following PostScript plots show the effect of the safing on UV Throughput:
F170W/PC
F170W/WF2
F170W/WF3
F170W/WF4
F160BW/PC
- Stefano Casertano (from STAN - WFPC2 #43, Sept. 2000)
On August 5, 2000, WFPC2 suffered an unplanned warm-up as a consequence of a telescope safing. The data indicate that the WFPC2 detectors remained for about a day at a temperature of -40 C, compared with the normal operating temperature of -88 C. At the time of the safing, more than three weeks had elapsed since the previous WFPC2 decontamination, thus the UV throughput had been reduced as usual from the nominal (post-decontamination) values.
Photometric verification data taken on Monday, August 7, indicate that the warm-up event acted exactly as a decontamination on the UV throughput of the camera, in other words, the throughput on August 7 was indistinguishable from the throughout the day after a decontamination. Therefore we recommend that all data taken between August 7 and August 10 --the day of the scheduled decontamination, which executed as planned-- be corrected for the UV throughput assuming a decontamination event on August 6, at MJD 51762.9049. In most cases, the contamination accumulated over a four-day interval is negligible, and thus contamination corrections can be safely ignored for these data.
There is also evidence for a moderate annealing of warm pixels during this event. Therefore some (about 30%) of the hot pixels newly formed over the previous month will no longer be hot between August 6 and August 10. The hot pixel lists used by the task warmpix include this information, and thus the annealed hot pixels should be properly identified. However, if your results depend critically on hot pixel identification, we recommend verifying hot pixels via the special darks taken on a daily basis for this purpose, as part of calibration proposal 8826. Because of resource limitations, we cannot directly support the analysis of these darks.
More information on WFPC2 decontamination dates can be found in the WFPC2 History page.