Instrument Science Report 2005-02:
Early Assessment of the WF4 Anomaly
J. Biretta and S. Gonzaga
December 13, 2005
A new anomaly has been discovered in the WF4 channel of WFPC2 which is characterized by sporadic images having low or zero bias levels. Early beginnings of the problem can be traced near the time of Service Mission SM3B in March 2002 where images begin to appear with progressively lower and lower bias levels. By mid-2004 some images appear with very low bias levels far below the normal 311 DN, and in February 2005 the first images with zero bias level are seen. As of this writing all images are significantly below normal bias levels, and nearly 30% have zero bias level. While the calibration pipeline will correct for low bias levels, close examination of these images reveals faint horizontal streaks (<1 DN RMS) and low photometry (up to 70% low for faint targets at very low bias and gain 15); these effects are correctable and preliminary corrections are given in the Appendices. Images with zero bias, however, suffer much more serious problems -- these are blank except for brightest pixels and the data are not recoverable. Both A-to-D gains 7 and 15 show similar anomalies, though the onset is slower at gain 15 due to its greater digitization range. Extrapolation of current trends suggests gain 7 will become completely unusable in mid-2006 while gain 15 will produce some useful data until late-2006. Fortunately, the other WFPC2 CCDs appear unaffected by the anomaly, and show no evidence for early onset of similar problems. Examination of bias levels during periods with frequent WFPC2 images shows 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. We suggest that lowering the Replacement Heater temperature set points by a few degrees C might effectively eliminate the WF4 anomaly.
The Complete Paper (PDF 1.7 MB) is available.