March 6, 2006:
Further repair of the WF4 anomaly: On February 20th at 20:33 UT we again reduced the set points on the WFPC2 Replacement Heaters in an effort to further mitigate the bias anaomaly. This was quite successful. The WF4 bias levels are now in the range 250 - 300 DN, with most images (>95%) in the range 280 - 300 DN. This places most images quite close to the normal ~311 DN bias level.
Specific details are that the Replacement Heater upper set point was reduced from 12.2C to 11.3C, and the lower set point was reduced from 10.9C to 10.0C. This had the effect of reducing temperatures in the WFPC2 electronics bays and optical bench by about 1C.
We are still checking for adverse side effects due to this new temperature, but so far there appear to be none. The PSF size in the PC1 channel is identical to historical values. Some small motion of the CCDs in the focal plane was seen in response to the temperature change, but even the largest motions ~0.02 arcsecond, are well within the expected accuracies for the astrometry between the WFPC2 CCDs (0.1 arcsecond), and should be inconsequential.
There are no immediate plans for further temperature adjustments. It is entirely possible that the anomaly will re-appear over time, and we will re-assess the situation as needed.
January 14, 2006
Partial repair of the WF4 anomaly: Over the last few days we tested a strategy to mitigate the WF4 anomaly by reducing the set point of one of the heaters in WFPC2. Specifically, at 12:28 UT on 9 January 2006 we reduced the upper set point of the WFPC2 Replacement Heater from 14.9C to 12.2C.
The test was extremely successful. The WF4 bias levels are now in a much narrower range 150 - 270 DN (gain 7), and most importantly, we are no longer getting blank (zero bias) images. Prior to the test the bias level was ranging from 0 to 240 DN with 20% to 30% of the images being blank. We are still somewhat below the normal bias level ~311 DN, but overall the situation is greatly improved.
There was some concern that lowering the temperature might impact the WFPC2 optical bench and possibly change the optical alignment. We have tested this, and while there is a small change in the alignment, it is very small and generally well within the range of random fluctuations in the alignment we see during the year. Specifically, motions in the KSPOTs caused by the temperature change were around 0.1 pixel in all cameras. We have also made some initial assessments of the image quality using external star observations, and can see no change in the stellar images.
Our immediate plan is to leave the heater at its new, lower setting for the forseeable future. We are beginning plans for a second test in February where we will further reduce the heater set point. This should further stabilize the WF4 bias levels, and "buy" more time before the problem potentially reappears.
December 13, 2005
A detailed ISR (PDF) on the WF4 anaomly has been posted in the Instrument Science Report area. The anomaly only impacts the WF4 CCD; the other CCDs are fine.
It appears that the photometric effects can be quite significant. At gain 7 and for very low bias levels and faint targets, the photometry can be up to about 40% low in WF4; at gain 15 it can be up to about 70% low. In more typical situations the photometry will be 10% to 20% low. So far the photometric error appears to depend only on the gain, bias level, and counts in a pixel, but not on epoch. We believe the photometry is correctable and the report outlines preliminary corrections for gain 7. A new calibration proposal, ID 10772, has been created to monitor the photometric effects.
We have reviewed all un-executed WFPC2 prime observations on the HST schedule, and will attempt to move observations earlier, if possible. This will help minimize impact of the anomaly as it worsens.
Episodes of the anomaly appear to be strongly correlated with temperature spikes in some of the WFPC2 electronics bays, and we are planning tests of a possible mitigation strategy that involves lower the temperatures in these bays by a few degrees C. If all goes well, we may be able to greatly reduce or eliminate the anomaly in a few months time.
An Anomaly Review Board has also been convenied at GSFC, with a detailed report expected in a few months.
October 28, 2005
Update on WF4 CCD bias anomaly: we have located several standard star observations at gain 15 on 6/25/2005 that were impacted by the low-bias condition. It appears that the low-bias anomaly does have some photometric impact. Images with bias level around 200 DN appear to have count levels that are 15% low, and those with bias 100 DN or less are about 25% low. While images near the normal bias level ~300 DN display normal photometry. We will probably attempt to get additional on-orbit data to further study this effect.
Observers can check the bias level in their WF4 images with the IRAF command line:
imhead uxxxxxxxx.c0h[4] l+ | grep BIASEVEN
where uxxxxxxxx is the image name. Values around 310 DN are normal.
October 25, 2005
Original announcement of the WF4 anomaly.