+----------------------------------------------------------+ | STScI Analysis Newsletter (STAN) | WFPC2 | 09 Nov 2005 +----------------------------------------------------------+ WFPC2 WF4 CCD Bias Level Anomaly We have identified a serious anomaly in images from the WF4 CCD in WFPC2. The WF4 CCD bias level appears to have become unstable, and we are seeing sporadic images with either low or zero bias level. The severity and frequency of the problem is rapidly increasing, and it is possible that WF4 will soon become unusable if no work-around is found. Both the CCD gain settings of 7 and 14 are affected. The other three CCDs (PC1, WF2, and WF3) appear to be unaffected and continue to operate properly. The impacts from "low" and "zero" bias are somewhat different, but in both cases the effects are obvious in the images. Images with low bias will tend to have horizontal (x-direction) streaks and stripes with an amplitude of ~0.5 DN RMS in WF4. We believe these data should be mostly recoverable with some effort. We are currently working on an image repair algorithm, and preliminary tests are very encouraging. Photometry is also impacted, in the sense that count levels can be up to ~25% low in the low bias images; we are currently working to calibrate the photometric effects. "Zero bias" is a much more serious problem and is evidenced by images which are blank in WF4, except for showing occasional cosmic rays, bright targets, and negative pixels from dark subtraction. These images with zero bias are probably unusable for most purposes. The frequency of the anomaly is rapidly increasing. The first significant instances of low bias appear to have been in late 2004 when a few images were impacted. However, within the last few weeks over half the images are beginning to show the low bias problem. The more serious "zero bias" problem appears to have first occurred in Feb. 2005, but it is also increasing and now impacts 10% to 20% of WFPC2 images. At present there are still many images which appear fine and unaffected, but the situation is quickly evolving. The science impact will depend on the target size. It will be minimal for observations of small targets as they are by default placed on either the PC1 or WF3 CCDs which continue to operate properly. However, observers requiring the full field of view (survey projects, large targets, etc.) will potentially lose one-third of their imaging area. Our understanding of this anomaly is still evolving, and most of the information is tentative. Additional details will be posted on the WFPC2 website as they become available: http://www.stsci.edu/instruments/wfpc2/wfpc2_advisory.html - J. Biretta 11/7/2005 +----------------------------------------------------------+ If you have received this email in error and wish to be removed from the distribution list, please contact Dixie Shipley at shipley@stsci.edu +----------------------------------------------------------+ | Need help? http://www.stsci.edu/hst/acs/help.html | or send your questions to the Help Desk: help@stsci.edu +----------------------------------------------------------+ | To subscribe or unsubscribe to this STAN, send a message | to majordomo@stsci.edu with a blank subject line and | the following in the body: [un]subscribe acs_wfpc2_news +----------------------------------------------------------+ | The Space Telescope Science Institute is operated by the | Association of Universities for Research in Astronomy, | Inc., under NASA contract NAS 5-26555. +----------------------------------------------------------+
