Re-appearance of WF4 Bias Anomaly

11/08/2006

The bias level in the WF4 CCD has begun to trend downwards again, mimicking its behavior in late 2004 and early 2005. It is possible that will see a few blank images in the WF4 CCD over the next few months. We are tentatively planning another temperature adjustment in January or February 2007 to counteract this trend. Observers are reminded that WF4 images with low bias values (below about 300 DN) will have low photometric counts, but the data should be correctable.