Temperature Reductions to Mitigate the WF4 Anomaly

The WF4 anomaly is characterized by low or zero bias levels, faint horizontal streaks, and low photometric counts (reduced gain) on the WF4 detector. In an effort to mitigate these effects, the temperature of the WFPC2 electronics has been reduced three times since January 2006. Each temperature reduction has temporarily restored the WF4 bias and gain to near-normal (pre-anomaly) levels with no significant degradation of instrument performance. As the underlying electronics failure progresses, additional temperature adjustments will be necessary.