Information on the WF4 CCD Anomaly

May 2, 2007:

At this point in time, we have effective strategies to manage the WF4 CCD anomaly, and we believe the WF4 CCD will continue to operate producing good quality images throughout Cycle 16. WF4 will continue to have low photometry and elevated background noise (streaks), but corrections for these are available. WF4 has produced no blank images since we began actively managing its temperature in January 2006, and we expect no further blank images.

Work is underway to install automatic corrections for the WF4 photometry in the calibration pipeline, and we believe the corrected data will be only slightly less accurate than the other CCDs (1% or 2% increased uncertainty). The background horizonal streaks (~1 DN) are correctable in most situations using available IRAF scripts which observers can apply to their data, and at some point those corrections may also become automatic in the calibration pipeline.

The temperature reduction activity performed on March 27, 2007 to mitigate the most severe symptoms was completely successful, and returned WF4 to near-normal bias levels and photometry. However, we are only treating the symptoms, and we expect that the root hardware failure will continue to progress slowly over time. We will continue to carefully monitor the WF4 performance, and make additional temperature adjustments in approx. September 2007 and March 2008 to keep the WF4 CCD operational. Details of the recent temperature reduction activities can be found here.

Past advisories on the WF4 anomaly are given below.