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| STScI Analysis Newsletter (STAN)
| 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 

 - J. Biretta 11/7/2005

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