WFPC2 WF4 Temperature Reduction #3

Details WFPC2 WF4 Temperature Reduction #3 WFPC2 WF4 Temperature Reduction #3

Jul 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006hst..prop11030d&link_type=abstract

HST Proposal ID #11030. Cycle 15

Computer Science

Performance

Scientific paper

In the fall of 2005, a serious anomaly was found in images from the WF4 CCD in WFPC2. The WF4 CCD bias level appeared to have become unstable, resulting in sporadic images with either low or zero bias level. The severity and frequency of the problem was rapidly increasing, making it possible that WF4 would soon become unusable if no work-around were found. Examination of bias levels during periods with frequent WFPC2 images showed low and zero bias episodes every 4 to 6 hours. This periodicity is driven by cycling of the WFPC2 Replacement Heater, with the bias anomalies occurring at the temperature peaks. The other three CCDs PC1, WF2, and WF3 appear to be unaffected and continue to operate properly. Lowering the Replacement Heater temperature set points by a few degrees C effectively eliminates the WF4 anomaly. On 9 January 2006, the upper set point of the WFPC2 Replacement Heater was reduced from 14.9C to 12.2C. On 20 February 2006, the upper set point was reduced from 12.2C to 11.3C, and the lower set point was reduced from 10.9C to 10.0C. These changes restored the WF4 CCD bias level; however, the bias level has begun to trend downwards again, mimicking its behavior in late 2004 and early 2005. A third temperature reduction is planned for March 2007. We will reduce the upper set point of the heater from 11.3C to 10.4C and the lower set point from 10.0C to 9.1C. The observations described in this proposal will test the performance of WFPC2 before and after this temperature reduction. Additional temperature reductions may be needed in the future, depending on the performance of WF4. Orbits: internal 26, external 1

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