Update: 12 December 2000
by J. Biretta
We have not seen any further incidences of the shutter anomaly since installation of the software patch on November 08. We have examined all (>1500) images taken since then, and all appear nominal. From all appearances, the patch has corrected the problem. We will continue to monitor the situation, and work is continuing to understand what caused the anomaly so we can better judge whether or not it is likely to re-appear as the mechanisms continue to age.
Update: 10 November 2000
by J. Biretta
The software patch was successfuly installed into the WFPC2 computer yesterday and a series of test exposures was run today. All exposures appear to have executed properly and produced nominal results.
The software patch attempts to correct the problem by turning-on the LED in the shutter position sensor 10 milliseconds earlier than before, hence allowing the phototransitor in the position sensor more time to respond before the shutter position is read by the microprocessor (the previous software allowed only 30 microseconds). These operational changes should be completely transparent to observers.
We expect to resume normal science observations on the evening of November 09.
Thanks go out to numerous people at GSFC, JPL, and STScI for their assistance over the last few weeks.
Update: 01 November 2000:
by J. Biretta
We have decided to suspend WFPC2 observations later today (01 Nov. 2000) until a software patch can be installed next week (06 - 10 Nov.). A few days ago we began seeing a new type of shutter anomaly which is precipitated by occurrences of the earlier anomaly previously described. There is some small probability that the new anomaly could damaged the shutter mechanism, so we have decided to suspend further observations until the software patch is installed. The patch will modify the way we operate the shutter position sensor, and hopefully will eliminate both types of anomaly.
Observers will be notified of lost or rescheduled observations through the usual notification process.
Update: 06 October 2000
We have begun to see anomalies in the WFPC2 shutter operation which result in occassional blank images. So far the error rate is very low, with the loss of three science images during the last 6 weeks (approx. 0.2% failure rate). We believe the mechanics of the shutter and the solenoids which operate it are healthy and fine. The main problem appears to be in an LED/phototransistor which senses the position of the "A" shutter blade. This device is used by the flight software to check the position of the blades prior to operating the shutter. Blank images are produced when the sensor gives an anomalous report, and the current flight software (as a safety measure) then disables the shutter until the status is cleared by external commands. We are currently looking at several work-arounds, including modifying the way the sensor is operated, software patches, and operating only the redundant "B" shutter. Unfortunately we expect it will take some weeks to fully study the problem, and design and test a suitable work-around.
Meanwhile, we plan to continue normal operations, and expect that occasional images will be lost until a long-term solution is implemented. While we plan to notify observers of future problems and lost images, it would nonetheless be prudent for observers to be especially watchful for anomalous or blank images. If blank images are found, they can be reported in the usual way by filing a Hubble Observation Problem Report (HOPR) and re-observation request at the STScI WWW site.
An immediate impact of the anomaly is that we decided to suspended WFPC2 observations between Oct. 02 - 06 while we conducted an initial investigation of the problem. WFPC2 observations were suspended at 11:47 GMT on 02 Oct. 2000 and were resumed at 19:02 GMT on 06 Oct. 2000. This was necessary while we determined that it was safe to continue operating the camera. Observers will soon be notified if their images were lost during this time.