WFPC2 ADVISORIES:

Last update: 11 Aug 2006.

New WFPC2 Web Site: We anticipate release of a new WFPC2 web site in the near future. The new site will have a format more similar to those of the other instruments, and should be easier to maintain. Nearly all pages have been transformed to the new site, though some of the menus and navagation aids will be different.

Any bookmarks that people have set to various WFPC2 pages will, instead, direct them to the top page of the new site. You will need to navigate through the new site to find your bookmarked page again, but hopefully this will be easy enough. (JB/Aug 11, 2006)

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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. (JB / March 6, 2006)

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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. (JB / January 14, 2006)

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A detailed report on the WF4 anaomly has been posted in the Instrument Science Report area, or can be found here. 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. (JB / December 13, 2005)

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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. (JB / October 28, 2005)

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IMPORTANT: WFPC2 WF4 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. 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 immediately obvious. Images with low bias will tend to have horizontal (x-direction) streaks and stripes with an amplitude of about 0.5 DN in WF4. We believe these data should be mostly recoverable with some effort, though at a loss in the detectability of faint targets. "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. Both the CCD gain settings of 7 and 14 are affected.

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.

We believe the science impact for most observers will be minimal. Targets are by default placed on either PC1 or WF3 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. (JB / October 25, 2005)

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We are currently (Sept 2005) beginning to plan close-out activities for WFPC2, with the expectation of its eventual de-orbit in two to three years. The community and observers are welcomed to suggest areas where additional on-orbit calibration data are needed, or where improved calibration would be useful. While resources are limited, input is nonetheless useful in setting priorities and perhaps discovering areas we may have overlooked. Please direct suggestions to John Biretta.

Due to recent staffing reductions, we plan to reduce the frequency at which we manufacture WFPC2 dark calibration reference files and the associated warm and hotpixel tables. In the recent past we have generated new files once a month, but we are now doing it less often -- probably every two to three months in the near term. This means that observers with new data will sometimes need to wait 2 - 3 months to obtain optimal calibration of the dark current and hotpixels. Calibrated data will still be available immediately after observations, but better calibration will be available at the later date. The change will mostly impact observers with faint targets or with narrowband filters. (Sept 20, 2005)

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A summary of the WFPC2 Calibration Plan for Cycle 14 can be found here. (Aug 10, 2005)

The Cycle 15 update for the WFPC2 Instrument Handbook for WFPC2 is available.

A WFPC2 Memo has been issued regarding "WFPC2 Filter Wavelengths in the HST Archive." Due to discrepancies between the "Central Wavelength" and other filter quantities, it has been decided to replace the use of "Central Wavelength" with the use of "Pivot Wavelength" instead. For details please follow the link.

The 2002 HST Calibration Workshop Proceedings site is open. The site provides keyword and full-text searches to the PDF papers.

A new paper on "An Improved Distortion Solution for HST's WFPC2" by Jay Anderson and Ivan King is available as a preprint (PostScript, 4.4MB). The paper should appear in the Jan 2003 version of the PASP. There is also a text file containing a Fortran routine that implements the distortion solution.

We have just re-organized the Documents section of this WFPC2 website, to make documents and calibration memos easier to find. Documents such as manuals are found in the Documents section, while documents such as calibration memos are found in the new General Resources section. Please look at the description under each link for details.

A new edition of the WFPC2 Data Analysis Tutorial, a step-by-step guide to reducing WFPC2 data (Version 3.0 - July 2002) is available in PostScript (3.8 MBytes) and PDF (1.0 MBytes)!

A new edition of the Data Handbook for WFPC2 is now available!

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