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WFPC2 History

This file provides a chronological history of the WFPC2, including:

Title Last Updated Updated by
1. Decontaminations 17 April 2008 M. McMaster
2. Darks and Superdarks 14 April 2008 M. McMaster
3. Focus Changes 5 Sep 2001 S. Baggett, M. Wiggs
4. Temperature Changes 5 Sep 2001 S. Baggett, M. Wiggs
5. Pipeline Changes 5 Sep 2001 S. Baggett, M. Wiggs
6. Safings 6 May 2003 I. Heyer
7. Plate Scale Changes 5 Sep 2001 S. Baggett, M. Wiggs
8. List of WFPC2 Observations 5 Sep 2001 S. Baggett, M. Wiggs
9. Miscellaneous Items
(incl. WFPC2-rel. STSDAS changes)
5 Sep 2001 S. Baggett, M. Wiggs
10. OTFC Keyword Exception Table 5 Sep 2001 S. Baggett, M. Wiggs
11. WFPC2 History in Table Format 5 Sep 2001 S. Baggett, M. Wiggs

It is designed to provide observers with information about the state of the WFPC2 when their observations were taken. The most important change occurred on APRIL 23, 1994, when the operating temperature was changed from -76° C to -88° C in order to minimize the CTE (Charge Transfer Efficiency) problem and the growth of hot pixels. Care should be taken not to mix reference files from one side of the temperature change with data from the other side.

Each item includes a short description of how this might affect the observer. More detailed information is available in:

You may contact (410-338-1082) for a paper copy of any of these documents.

This history file contains sections sorted by item type, then a table sorted by date.


The WFPC2 has been warmed approximately once a month (every 49 days since Cycle 11) to about +20° C for 6 - 12 hours to remove any buildup of contaminants. This appears to:

1) Restore essentially the full UV sensitivity of the instrument.
   Recent photometric monitoring results are kept updated in the
   WWW memo and associated figure.

2) Repair many of the "hot pixels" (see Figure 2 of the
   Holtzman et al. article).  

The calibration programs include regular photometric monitoring of both a standard star (usually GRW+70D5824; alternate is Feige 110) and a standard cluster (Omega Cen = NGC 5139; alternate is NGC 6752), usually done before and after decontamination procedures. Darks are taken at the rate of at least 5 per week (and 5 more before and after decons). More details on the calibration proposals are available in the WFPC2 Instrument Handbook.

From the observer's standpoint the decon dates may be useful in order to:

  Make small adjustments to the photometric zeropoints 
  in the UV. Enhancements to SYNPHOT have been made 
  to aid the user in taking these effects into account.
  Please refer to the ISR98-03: WFPC2 Longterm Photometric Stability and the 
  SYNPHOT section of the WFPC2 Photometry page.

The following link provides a list of the decontamination dates:



  • Dark Quality
  • Typically, OTFR will have used the best available pipeline dark for calibration; OTFR can be accessed via the HST Archive or STARVIEW. To assess the pedigree of the dark, see the WFPC2 Reference Files.

  • Improving upon the pipeline dark
  • Pipeline darks are normally available for each 5-10 day period. Prior to Aug. 01, 1996, the dark reference files were created from sets of 10 dark frames, 1800 sec. each, (taken over ~2 weeks time). After to Aug. 01, 1996, the darks have been generated by adding hotpixels (as identified from ~5 dark frames, 1800 sec. each, taken within 2 days) to a superdark generated from more than 100 darkframes. Please refer to the Pipeline Section in this document, or to our online Summary of Darks for more details, or to the HISTORY keywords in the headers of the dark reference files themselves.

    To improve upon the pipeline dark calibration, one option is to recalibrate manually using a custom dark reference file (generated from, e.g., the shorter 1000 sec. darks that are taken daily and archived only - not used in pipeline). Scripts and cookbook examples of dark generation are available.

    Another option is to use a cosmicray rejection routine that can identify the hot pixels (e.g., COSMICRAYS in IRAF), but care should be taken to set the parameters conservatively or you may also clip out the peaks of bright stars. If the data has been dithered, the Drizzle software may be used to remove both cosmic rays and hot pixels.

    NOTE: At T = -76° C (i.e., before April 23) the mean dark count was about 2 DN per 1000 seconds. At T = -88° C (i.e., since April 23) the dark count has been about 0.5 DN per 1000 seconds. For an analysis of the dark current versus time, see ISR 01-05.

  • Hot Pixel Information
  • There are lists of hotpixels available via WWW. Note that there are several flavors: those for use with the STSDAS task "warmpix", plain hot pixel lists, and HDF hotpixel lists.These may also be retrieved in the form of compressed ASCII files via anonymous ftp:

    ftp (username) anonymous (password)  
    cd /instrument_news/WFPC2/Wfpc2_warmpix
    mget filename (include wildcards to select files for the period of interest)

    NOTE: As an additional aid in hot pixel identification, as of July 1997, more frequent dark frames are being taken. The new Supplemental Darks program (7621, 7712, 7713) is designed to provide more frequent, short (1000s) darks to be used primarily for the identification of hot pixels. Shorter darks are used so that observations can fit into almost any occultation period, making automatic scheduling feasible. The program is designed to complement the Standard Darks program (7620), whose longer individual observations are better suited to produce high-quality pipeline darks and superdarks. This program has become possible after the Second Servicing Mission thanks to the larger storage capacity of the Solid State Recorder. Currently, about 15 darks are taken each week as part of this program. The darks will not be used in standard STScI products, but are available to observers via the Hubble Data Archive.

  • Alternate Dark Reference Files for 1994 data
  • We also have in the archive superdarks generated from 100 individual dark frames taken from April 23 to October 01, 1994 (NOTE: standard pipeline darks are also superdarks as of Aug. 1996; see previous section for discussion). These 1994 superdarks were not used in the pipeline but may be useful for recalibrating your data in cases where the total exposure time is relatively long (e.g., > 30 minutes) and the background noise is a limiting factor in your science (e.g., faint object detection).

    1994    November  (for data taken after April 23 decon)

    NOTE 1: the WFPC2 IDT has delivered alternate superdarks, superbiases, and deltadarks. The IDT Reference Files memo describes how these files were generated, the names for retrieval from starview, and how to use them.

    NOTE 2: The Dec. 1995 Hubble Deep Field images were processed by the HDF team using the standard HST pipeline (the STSDAS task calwp2) but with different reference files (similar to the IDT superdarks, superbiases, and deltadarks) and a slightly different treatment of the darktime. These alternate reference files are also available from the archive; the HDF Pipeline Reduction page provides some details as well as the filenames of these products. The history records in the reference file headers provide more details as well.


    In general, the focus changes have been very minor and are unlikely to have an effect on your science; a short summary table follows. For more details, please refer to the online HST Focus Pages.

    2000  Jun. 15    Secondary mirror moved +3.6 microns. (Day 167 19:19)
    2000  Jan. 09    Secondary mirror moved +4.2 microns.
    1999  Sep. 15    Secondary mirror moved +3 microns.
    1998  Jun. 28    Secondary mirror moved -15.2 microns back at end of campaign.
    1998  Jun. 04    Secondary mirror moved +16.6 microns for second NIC3
    1998  Feb. 01    Secondary mirror moved -18.6, at end of NIC3 campaign.
    1998  Jan. 12    Secondary mirror moved +21, for NIC3 campaign.
    1997  Mar. 18    Secondary mirror moved towards primary by 2.4 microns
    1996  Oct. 30    Secondary moved outwards by 5 microns (day 304,
                     1700 UT). The average focus position is now ~+2.5 from
                     nominal, compared to -2.5 prior to the move.
    1996  Mar. 14    Secondary moved +6 microns to correct for
                     desorption (day 074/1848Z).
    1995  Aug. 28    Secondary moved by ~6.5 microns to compensate for
                     OTA desorption.  Before the move, WFPC2 ~ 3.5 microns
                     inside optimal focus; move put WFPC2 about
                     3 microns outside "best focus."  Most observers
                     should be unaffected by move.
    1995  Feb. 23    Possible anomaly; if your data was taken between
        - Mar. 03    these dates, and the scientific results might be
                     sensitive to the PSF shape, please contact us.
                     See Note 1, below.
    1995  Jan. 15    5 micron change to adjust for desorption.
    1994  Jun. 29    5 micron change to adjust for desorption.
    1994  Mar. 04    Minor change.
    1993  Dec. 28
    Limiting factors: Breathing worst case ~ 5-10 microns during a single 
                      orbit. Desorption ~ 1 micron/month during cycle 4.

    NOTE 1: Faint Object Camera observations taken on Day 58 (Feb. 27) appear blurred. A possible cause of the blurring is a defocus and/or decenter of the secondary mirror, potentially related to the unusual thermal environment of the telescope - which had been pointed very close to the anti-sun direction for several days (ie., most of the telescope structure is in the shadow caused by the wider instrument compartment at the "bottom" of the telescope). WFPC2 observations taken shortly before and during the blurred FOC observations have been inspected for degradation of the PSF. These observations contain very few well-exposed stars in the PC, and therefore our assessment of the PSF properties in WFPC2 is necessarily limited. However, it appears that any degradation in the light distribution is very minor, and the FWHM of the observed stars is close to normal - the measured increase of less than 10% (to 1.8-1.9 PC pixels in F814W) is within the errors of the measurement. The PSF monitoring carried out on Mar. 04 shows nominal results.

    However, the _shape_ of the PSF in the WFPC2 observations may be consistent with a defocusing of the secondary mirror of order of 10 micron, the value suggested by the FOC data. Such a defocusing could affect photometry based on small apertures or PSF fitting at the 5-10% level; scientific programs that depend on accurate knowledge of the PSF itself, such as observations of faint objects in the vicinity of bright point sources, would also need to take the possible defocusing into account. On the other hand, morphology and surface photometry of extended sources would be largely unaffected. Information about this event is detailed in the March 1995 STAN.

    More general information about the OTA and focus changes may be obtained from the Observatory Group pages.


    The operating temperature was changed on >>> APRIL 23, 1994 <<< to reduce the CTE problem and the number of hot pixels (see the Holtzman et. al., paper for details). This date represents a major change in the calibration of the WFPC2, since many instrument characteristics were affected by this temperature change (e.g., sensitivity, dark current, hot pixels, bias level, etc.).

    1994    December 26     -76° C
            April 23        -88° C


    2001 Sep.
    OPUS pipeline software has been updated to fix a bug in the computation of statistics of raw (d0*) data as well as populate the FILTROT keyword (the rotation angle for partially rotated filters such as the ramps or polarizers).

    2001 July 18
    Alternate WFPC2 flatfields, with reduced noise, are available for manual recalibrations. Note that these are NOT in the pipeline as of Sep 2001.

    2001 May 16
    OTFR (On-The-Fly Reprocessing) is publically released. OTFR can be accessed via the HST Archive or STARVIEW. NOTE: uchcoord no longer needs to be run on OTFR data; for details as well as a comparison of OTFC and OTFR, see OTFR page.

    2000 Fall
    A minor bug affecting only short INTFLATs has been fixed in the OPUS pipeline (pre-calwp2) software. Archival exposures will be fixed automatically by OTFR (On-The-Fly Reprocessing), planned for release in early 2001. The problem was that INTFLATs with non-integer exposure times have incorrect integer exposure times in the headers. More details and a full listing are given in this problem report.

    2000 Spring
    Minor bug in calwp2 affects bias level calculation in AREA mode images only. Typically, the errors in the bias values are ~0.01 DN, though for a very small number of images (~20, those with large numbers of saturated pixels), the error can be ~1 DN or larger. Note however, that there are very few affected exposures: out of a total of nearly 90,000 WFPC2 images in the archive, only ~150 are AREA modes. All Cycle 7 and Cycle 8 GOs using AREA mode have been notified and CALWP2 will of course be updated as soon as possible. In the meantime, for assistance with any AREA mode images, please contact

    1999 December
    OTFC (On-The-Fly Calibration) is publically released.

    1999 Apr. - Jul.
    Single chip data was processed as PC, regardless of the actual chip that was read out: generic conversion (the step preceding calwp2) was erroneously setting detector to 1 for all images with less then 4 groups. A software fix was installed on July 08, 1999. This mode is rarely used; no GO images were affected, only WFPC2 calibration data were corrupted (17 images total).

    1999 Apr.
    A new version of the OPUS software was installed in the pipeline Apr. 05, 1999, as part of switching the OPUS platform from VMS/VAX to ALPHA/VMS. The data processing steps remain the same however, there have been some minor keyword changes. The changes should be transparent to the majority of WFPC2 users, further details may be found in the software update summary.

    1998 Dec.
    Update to SYNPHOT contamination tables. Note however, that the pipeline does not automatically correct for contamination (e.g., the PHOTFLAM keyword). The correction must be applied manually, using either the information in Tables 2 and 4 of ISR98-03: WFPC2 Longterm Photometric Stability or via SYNPHOT (see section 4 of ISR 98-03).

    1998 Sep.
    Excessive noise in the F160BW flat; for alternate flatfield, see Aug. 1998 STAN or WFPC2 Reference Files memo.

    1998 Jan.
    STARVIEW and bestref now provide most recent GRAPHTAB and COMPTAB names, for use by calwp2 in computing the PHOTFLAM and other photometry-related keywords in the calibrated header files. (See also Note 2.)

    1997 Aug. 15
    A rare problem in the population of the PA_V3 keyword has been found, however, only 50 observations are affected. For more details, including a list of affected images and a fix for those images, please refer to our online PA_V3 Memo.

    1997 May 16
    SYNPHOT tables updated. With a few exceptions, changes to non-UV filter modes were relatively minor: generally ~1-2% or less. F785LP, F850LP, and F1042M were more: ~1-4%, ~1-6%, and ~2-15% for them, respectively, depending on the chip. The UV filters required somewhat larger changes to bring SYNPHOT into agreement with the observations, ranging from ~2% (e.g., F255W and F300W) to ~5% (F170W, F218W) to 10% or more (F160BW, F375N). Any filters not updated at this time will be checked with future observations (planned for summer of 1997) and updated if necessary. An ISR is expected out end of June. A table summarizing the photflam changes and new zero-points, as well as the README file (guide to WFPC2 SYNPHOT tables) and ASCII versions of the tables, can all be found in the Photometry and SYNPHOT Page. See also the May 1997 STAN and ISR97-10: WFPC2 Synphot Update and Note 2 below.

    1997 Jan. 03
    Calwp2 update ( fix problem with processing of images containing less than 4 groups. Since Dec 1994 (, observations that read out less than 4 chips AND whose first chip was not PC have been incorrectly calibrated as if all chips were PC. The problem has been identified, current GOs notified, the code fixed and installed. The problem did not affect 4-group images.

    1996 Oct.
    New flatfields for polarizers installed in pipeline. Filters include polarizers (i.e., polq, polqn33, polqn18, or polqp15) used with one of the following:

    f300w, f336w, f390n, f410m, f439w, f547m, f555w, f606w, f656n, f675w
    Filenames are given in the WFPC2 Reffiles Memo.

    1996 Aug. 15
    Fix for WFPC2 images taken as a coordinated parallel with FOS (only). From Dec 95 to Aug 22,1996, WFPC2 images taken in parallel with the FOS were inadvertantly assigned FOS instead of WFPC pointing information. Note: these images have been fixed via reprocessing and rearchived as of May 1998; for a historical record of the problem, see online WFPC2 parallels fix memo.

    1996 Aug. 09
    New version of calwp2 (; no change to data). Software now prints message to trailer file, header, and screen, if bias jump of greater than 0.09DN is found (jumps greater than 0.5DN have these messages as well as a WARNING printed to the PDQ files). For more details, please refer to the ISR 97-04: Properties of WFPC2 Bias Frames.

    1996 ~Aug. 01
    Darks are now generated in a new fashion; most importantly, more pixels are being flagged in the data quality files. Please refer to the short Summary of WFPC2 darks for details; an Instrument Science Report is being prepared. A Summary of WFPC2 Darks is also available.

    1996 July
    SYNPHOT tables updated for FQCH4N* and FQUVN*. Affects only filter curves for 6193A and 8929A methane quad filters and the UV quad filters. Adjustments consisted of 3-15% decreases in filter throughputs. More details are available in the June 1996 STAN as well as the WFPC2 SYNPHOT Readme File.

    1996 Jun./July
    Improved Cycle 4 flatfields installed in pipeline. Errors at edges now < 0.5% rms. Filters include all those redward of f300w:

    f300w, f336w, f343n, f375n, f380w, f390n, f410m, f437n, f439w,
    f450w, f467m, f469n, f487n, f502n, f547m, f555w, f569w, f588n,
    f606w, f622w, f631n, f656n, f658n, f673n, f675w, f702w, f785lp,
    f791w, f814w, f850lp, f953n, f1042m
    Filenames are given in the WFPC2 Reffiles Memo.

    1996 Apr. 18
    WFPC2 headers from dithered data updated (dithered using scans, dither-line, or dither-box). Previously, due to the relatively small number of scans being performed, the pipeline provided only minimal support for WFPC2 dwell scans: only the initial dwell point position was used for the target pointing, so each exposure had the same CRVAL data, resulting in slightly incorrect metric results on all but the initial scan image. Note: the fix was only necessary for scan data; manually dithered images, e.g. using the POS-TARG option, had headers which properly reflected the pointing.

    1995 Dec.
    New flats for f300w, f375n, f450w, f555w, f606w, and f814w were installed in the pipeline. These are similar to the Cycle 4 flatfields but have been improved at the CCD edges: errors are now estimated to be < 1% rms. Filenames are given in the WFPC2 Reffiles Memo or will be found by Starview's 'bestref' screen.

    1995 Nov.
    Minor update to calwp2 (V. now prints out reference file names to go along with PEDIGREE/DESCRIP. Does not affect data; produces new informational messages during pipeline processing.

    1995 Aug. 10
    New maskfile (r0h) used in pipeline; columns above traps now flagged with "256" in c1h files. See ISR 95-03 for details.

    1995 July
    WFPC2 related SYNPHOT tables updated. See Note 2, and the July 1995 STAN.

    1995 March 24
    There are 184 WFPC2 images, dating from Apr. '94 - Mar. '95, which have corrupted FILT* header keywords. Only those images taken with rotated filters (e.g., fqch4n, polqn33, polqp15, fquvn33, fr418n33, etc.) were affected. There were 2 sorts of problems: 1) when rotated filters were taken with a second (non-rotated) filter, the second name was erased from the header, and 2), when a rotated filter was used in isolation, OPUS would erroneously add some second filtername to the header. For more information, see the Rotated Filter Problem memo.

    1995 March
    Observations taken 4/15/94 to 3/27/95, using WF3-FIX (only) may have incorrect pointing information in their image headers. In particular, errors on the order of ~2" may be present. Note that this only impacts 2 values in the header (although it can cause derived positions to be in error); there are no problems with the image data or with the telescope pointing. Note: these images have been fixed via reprocessing and rearchived as of May 1998; for a historical record of the problem, see online WF3-FIX Memo.

    1994 Dec.
    Minor update to 'calwp2' (V. now checks the PEDIGREE and DESCRIP keywords within reference files. Does not affect data; produces new informational messages during pipeline processing.

    1994 May 04
    Switch to separate values for even and odd columns. (Version of 'calwp2'; note that new biasfiles are required with the new 'calwp2'. See Note 1.)

    1994 Apr. 14
    Fix installed for parallel WFPC2 data headers. Note that some archival data may still be incorrect. (See Note 3.)

    1994 March 08-09
    Prelaunch flats were used until this date. These were generally good to a few percent over most of the field, with a maximum deviation of 10 %. See Holtzman et al., paper for details.

    1994 March 08
    Overscan region used to determine the blev correction changed from [3:14,*] to [9:14,10:790]. (Version of 'calwp2'. See Note 1.)

    NOTE 1. Change to Overscan Region used for BLEVCORR

    Until March 08, all 12 columns of the overscan region were used in the pipeline to determine the bias level. On March 08, a change was made to use only 6 columns, since the first few columns are too high or even saturated, having been affected by the exposure level. This results in the removal of too much bias level; a quick check of the DEZERO group keyword will indicate the amount removed, normal levels are ~300 to 400 DN.

    There is also a very slight difference in bias level for the even and odd columns, typically about 0.1 DN difference although in WF4 it can be as high as 0.4 DN. Until May 04, 1994, a single value was used in the pipeline; separate values have been used in the pipeline since that time (these values are stored in the BIASEVEN and BIASODD group keywords in the science data headers). Note: the trailer (trl) file includes the calwp2 version that was used at the time of processing.

    NOTE 2. New SYNPHOT Tables Available for recomputing PHOTFLAMs

    Observers using the header photometry keywords (i.e., PHOTFLAM) from data processed in the pipeline before May 1997 should recompute these keywords, using either SYNPHOT directly or calwp2 (after updated SYNPHOT tables are retrieved and installed at their local site), or the SYNPHOT PHOTFLAMs and zeropoints table.

    To identify epoch of SYNPHOT tables used for your observations: check the GRAPHTAB and COMPTAB keywords in the science data headers (c0h). The graphtab/comptab names used in the pipeline are based on the time of installation; for the May 1997 update, the GRAPHTAB and COMPTAB names are h5* or later (h=year since 1980, 5 is month, d is day. If the GRAPHTAB and COMPTAB names are h5* or later, the photometry keywords are already uptodate. If they predate May 1997, then the keywords should be recomputed. For more details about changes, installing files, and recomputing the keywords, refer to the Guide to WFPC2 Synphot Tables.

    NOTE 3. Incorrect Pointing Information in some WFPC1 and WFPC-2 Parallel Data Headers;

    Approximately 150 WFPC-1 and WFPC2 observations, taken in parallel with the FGS between Oct. 11, 1993 and Apr. 19, 1994, may have incorrect pointing information in their headers. Not all the data prior to the Apr 19 fix is affected, since workaround tools were developed after Mar 3 to allow PODPS operations personnel to fix the data but some observations were missed. The problem was related to the FGS observations, for which PODPS received the wrong roll orientation data. Some of these roll errors were significant and would affect the validity of the CD matrix. The images will be reprocessed and rearchived (expected completed by May 1996); if a list of affected observations is desired, please contact the STScI help desk at

    6. SAFINGS

    Safings are usually not a major problem for WFPC2, which is placed in HOLD mode and does not warm up for most spacecraft safing situations. Even if the instrument should warm up and require a subsequent decontamination procedure, this is not an issue (as it was with WFPC 1) since there is no UV flood to preserve (although as with all decons, the UV calibration will change, see Section 1).

    Date Details
    Apr 17
    HST entered inertial hold at 2006.107:03:10:00.5 after failing a bright earth/moon avoidance test. At the time of the event, ACS and WFPC2 were taking darks. The cause was determined to be related to a command scheduling logic problem between the FGS ITS (Internal Test Source) test activity and the standard PCS termination activity which must occur at the start of earth and moon occultation events. The problem prevented the scheduling of the PCS termination commanding which in turn allowed the FGS high voltage to remain turned on during the occultation event that followed the FGS ITS activities. The associated onboard Earth-Moon Bright Object Protection check failure condition properly initiated the immediate spacecraft transition to Inertial Hold. The current failure is strictly related to the FGS ITS test scheduling and has no implications for recovery of the science activities.
    Jul 06
    HST entered zero gyro safemode at 2004.188.16:28:51. Analysis indicates that this was NOT a result of problems with any of the gyros. The3 gyros are still working fine. The problem came from the loading of a table into the Flight Software that has data concerning the orientation and conversion factors for the gyros. The table was being reloaded to make very small adjustments in some of the parameters to improve the accuracy of the telescope slews. These parameters are also used by the Flight Software to automatically do sanity checks of the gyro data. The table got loaded with erroneous data, which resulted in the Flight Software test failing.
    Apr 29
    HST entered zero-gyro safe mode at 2003.119:14:21:42. Gyro three has stalled again. Attempts to restart gyro 3 have failed. Three successive attempts were made, but the gyro did not restart on any of them. While there will certainly be some follow up analysis of the engineering data from the stall and the restart attempts, gyro 3 is now considered failed. Gyro 1, one of the spares, was then started and it came on and has been operating properly. This event occurred during a scheduled WFPC2 decon, and resulted in a 32.7-hour decon (instead of a 6-hour one). After completion of the photometric monitor no adverse results were found.
    Apr 13
    HST entered Zero Gyro Software Sunpoint at 103/22:21:08 due to failure of System Momentum Test caused by an increase in Gyro 3 current GRG2_3MC, which reached a maximium value of 347.2 milliamps. Two Gyro Configuration Test subsequently failed at 22:24:46. Gyro 3 was successfully powered up on first attempt by Ops Request 16881-1 at 103/23:51:28. This is very similar to the occurrence on 11/28/2002.
    Feb 05
    HST experienced an electrical anomaly (short) on 2003.036. After examining data before and after the event no anomalous WFPC2 behaviour was detected.
    Jan 24
    HST went into Inertial Hold safemode at 11:59:18 UT (~08:00 local). The cause of the event was a missed command load; basically, HST ran out of things to do. There were no instrument or vehicle problems. This event occurred during a scheduled WFPC2 decon suite. The decon suite was repeated, and no negative effects were found.
    Nov 28
    HST went into zero gyro safe mode caused by a glitch in gyro 3. Gyro 3 has had several of these in the past year, but this is the first that has led to a safemode entry. HST was recovered from safemode 11/30 morning and resumed observations with no problems.
    Apr 28
    HST entered Zero Gyro Sun Point (ZGSP) mode when the gyro #5 motor currents exceeded the motor current safemode test limit. Science operations recommenced Apr 30.
    Mar 05
    HST entered software/sunpoint safemode at 06:38:06 UT (magnetic torquing bars were found to be out of limits). WFPC2 warmed to -40C; no decon was performed.
    Jan 03
    HST autonomously entered Software Sun Point (SWSP) Safemode at 003/102417Z; WFPC2 DID NOT WARM UP.
    The safemode was a result of failing the Solar Array Drive Electronics (SADE) Torque Test. Analyses indicate a SADE-2R failure early in a solar array slew. The anomaly had similar characteristics of a bit-flip event on day 090 of 1997. Upon entry to SWSP, SADE-1R was powered on and the arrays were successfully slewed to 90 degrees. Safemode recovery was successfully completed with the interception of the Health and Safety SMS at 004/0000Z and a test of SADE-2R was successfully performed at 004/0200Z. All SADE-2R telemetry was nominal as both positive and negative seven-degree slews were executed; flight software was configured for SADE-2R usage. Recovery of all science instruments will be accomplished by 004/1321Z and science observations are scheduled to begin execution at 004/1400Z.
    Nov 01
    WFPC2 was intentionally safed (2000:306:22:35) as a precaution after additional shutter anomalies occurred with an apparent new type of behavior. The Anomaly Review Board recommendation for a software patch was implemented Nov. 08, 2000. WFPC2 was brought out of safe and into a nominal decon on Nov. 08, 2000 00:00 UT. For more information on the status of the shutter blade anomaly investigation, please see the shutter anomaly report.
    Oct 02
    After a series of shutter anomalies occurred, WFPC2 was intentionally safed (2000:276:11:47) while the problem was investigated. The conclusion was that the shutter was fine; instead, there may be a problem with a sensing circuit. An Anomaly Review Board has been formed to investigate further. WFPC2 was recovered from safe Oct. 05, 2000 at 21:16 UT; a normal decontamination procedure followed. For more information on the shutter blade anomaly, please see the shutter anomaly report.
    Aug 05
    HST entered PSEA (Pointing Safemode Electronics Assembly) safe mode at 17:06UT, due to a procedural error that occurred during a dump of the 486 memory. HST remained power positive and the aperture door open. Operations personnel transitioned to a Health and Safety SMS at 219/1000Z that remained in effect until 220/0000Z when a normal science load was resumed. WFPC2 was warm (-40C) for 2 days, which may change the contamination level. Standard star observations in the UV were taken Aug 7 (day 220), after WFPC2 was cooled down again; these will provide a check of the contamination level before WFPC2 enters the standard decontamination procedure on August 10. Details will be made available in the WWW standard star monitoring memo.
    Nov 13
    HST entered Zero Gyro safemode at 13:29 UT due to a gyro failure. Data acquistion was suspended until after the servicing mission in Dec 1999.
    1999 May 07 HST enters inertial safemode at 1058Z, the Earth/Moon bright object detection test failed. WFPC2 did not warm up.
    Feb 24
    HST enters inertial hold safemode due to bad ephemeris load. WFPC2 did not warm up.
    Apr 09
    Safed due to gyro 4 failure (zero gyro hold, aperture door shut).
    Mar 31
    Safe (software sunpoint mode) due to solar array torque test failure during a routine SA maneuver for slew to target.
    Dec 17
    Warmed up to ~-50. HST in hardware sunpoint safemode caused by flight software bug. Decon executed Dec 18.
    Dec 14
    Did not warm up. Software sunpoint safe, due to reaction wheel problem, occurred during WFPC2 decon. The procedure, however, was unaffected: WFPC2 was cooled back down to -88° C with realtime commanding.
    Oct 30
    Did not warm up.
    Jul 13
    Did not warm up. Brief summary can be found in July 1996 STAN.
    Jan 08
    Did not warm up.
    Jul 05
    Did not warm up.
    Apr 11
    Did not warm up.


    The IRAF/STSDAS task "uchcoord" may be used to examine the plate scale of c0h files and/or update the CD matrices with a new plate scale; please see the online help for more details on this task.

    The accuracy of the platescales, as derived from the fit, is 0.00002"/pixel for PC1 and about 0.00004 for the WF chips.

                     PC1       WF2      WF3      WF4
    Nov. 1993        0.046     0.101    0.101    0.101
    Apr. 11, 1994    0.0452    0.0991   0.0991   0.0993
    Mar. 27, 1995    0.04553   0.09958  0.09956  0.09962
    Apr. 14, 1996    0.045518  0.099548 0.099527 0.099587

    NOTE 1: most images taken prior to April 11, 1994 will yield positional measurements ~3" in error on average, although it may be as much as 6". A software tool is being developed to aid in correcting the image headers for the pointing offset (and rotation) but until it is available, if you have any questions concerning the pointing of images taken in early 1994, please contact The rotation amounts to 0.8 deg for WF2 (smaller rotations for the other chips (0.28 deg in PC1, 0.46 deg in WF3, and 0.06 deg in WF4)). Since the STSDAS task METRIC uses WF2 as the reference chip, a 0.8 degree discrepancy will be present between data obtained before and after April 11, 1994, in addition to a shift.

    NOTE 2: Images taken April 11-19, 1994 have positional information in the headers which does not correctly reflect the aperture updates. Affected GOs will be notified personally and an effort will be made to have the images reprocessed and re-archived. In the meantime, if you are working with images from the April 11-19, 1994 timeperiod, and have any questions concerning the pointing of images, please contact


    This table lists the distribution of WFPC2 modes in the HST Archive. Last update: February 2000.


  • May 1993: Thermal Vacuum Test (TV) conducted. A Progress Report based on these tests was prepared by the WFPC2 Investigation Definition Team; copies are available by contacting, or accessible via Documents section of the WFPC2 Home Page.

  • Mid-Mar. 1994: SMOV ends.SMOV (Science Mission Orbital Verification) ends, STScI calibration and monitoring begins. (The SMOV Report is the Holtzman et al., PASP paper.)

  • Apr. 14, 1996: FGS-FGS alignments updated. WFPC2 users will notice a few pixel change in pointing between data obtained before April 14 and data obtained between April 14 and May 6 1996. After May 6, an additional adjustment to the WFPC2 V2-V3 coordinates will be made to improve pointing further still. Currently, there is a 0.5-0.6 arcsec error in the absolute location of the WFPC apertures and this will be finally corrected on May 6th. Please note therefore that the CRVALs in the science headers contain this additional 0.5 arcsec error on top of any relative guide star/target position error (~0.4"). The FGS-FGS (Fine Guidance Sensor) alignments have been evolving since 1990 and have been contributing ~0.3-0.4 arcsec pointing error, depending on the location of the guide stars in the pixel. To correct for the accumulated drift over the last 5 years (~0.1"/year drift), the relative positions of the FGSs have been updated. As a result, the V2-V3 reference frame has shifted by about 2 arcsec and rotated by 0.15 degrees (but not about the origin). Target-to-aperture-center miss distances will be reduced and less dependent on guide star positions in the pickle.

  • May 1996: WFPC-2 CTE and Long vs. Short Exposure Calibration Problems. Interim reports concerning this small but significant effect can be found in STAN for May 1996 as well as ongoing CTE Analysis.

  • July 1996: New release of STSDAS 1.3.5, includes new CRREJ. A new version of STSDAS was released in July. The hst_calib.wfpc package contains an improved version of crrej, including the following new features:
    - scale noise can be set separately for PC and WF
    - handles images with different exposure times
    - will create and/or update masks with cosmic rays flagged
    For more information about the changes and use of crrej, please refer to the online IRAF/STSDAS help. The previous version of crrej is still available as the STSDAS "mkdark". STSDAS 1.3.5 will be available from the STScI anonymous ftp site: ( in the directory /software/stsdas/v1.3. Information on STSDAS 1.3.5 will also be posted to "What's New" on the STSDAS Home Page, and to the adass.iraf.announce newsgroup.

  • Mar 1997: SMOV2 for WFPC2 Successfully Completed. Please see the STAN for March, 1997 for preliminary summary of the details. Detailed results for the WFPC2 checkout following SMOV2 can be found in ISR 97-09.

  • June 1997: A New Version of Drizzle. A new version of the WFPC2 STSDAS code "drizzle", to combine dithered WFPC2 data, has been developed. See the STAN for June 1997 for more details.

  • Sept. 1997: HST Calibration Workshop Held at STScI in Baltimore, MD. The Proceedings for this workshop are now available online.

  • Dec. 1997: Polarization Calibration Update. Instrument Science Report ("WFPC2 Polarization Calibration") WFPC2 ISR 97-11 has been released which details the current state of the WFPC2 polarization calibration. The calibration described therein appears accurate to about 1.5%. The report also gives detailed examples showing how to calibrate GO data using the on-line WWW polarization tools. The ISR and upgraded WWW tools are available at the WFPC2 Polarization Calibration Page.

  • Jan. 1998: Time Dependence Found in the Charge Transfer Efficiency (CTE) on the WFPC2. An analysis of WFPC2 observations of the globular cluster Omega Cen (NGC 5139) indicates that the CTE problem on the WFPC2 has increased with time. Refer to the STAN for Jan. 1998 for more preliminary details. [NOTE: additional details now also in STAN for April 1998.] More details (including figures and modifications to the formulae in ISR 97-08) can be found in the CTE vs. time memo (PostScript, 1.2MBytes).

  • Apr. 1998: New STSDAS package, "dither", and new STSDAS task "stfov", released as part of STSDAS 2.0.1. The dither package contains the drizzle task, which was used for imaging the Hubble Deep Field, as well as a number of other tasks useful for aligning and combining dithered and rotated images. A description of the dither package, as well as an explanation of many of the concepts involved in imaging dithered data can be found in an article by Fruchter, Hook, Busko and Mutchler in the HST Calibration Workshop. The new "stfov" is a task (under graphics.stplot package) to overplot the science instrument apertures of the HST on a gray scale image. The input image can be any image with WCS (= World Coordinate System) parameters in the header or a Digital Sky Survey (DSS) image; the input aperture can be any one of the HST Apertures (i.e., not limited to WFPC2 apertures). Please refer to the STSDAS help for more details and examples. Full release notes for STSDAS 2.0.1 are available at the STSDAS Home Page.

  • July 2002: WFPC2 Tutorial Issued. A step-by-step WFPC2 Tutorial PostScript (3.8 MBytes) and PDF (1.0 MBytes), for basic reduction of WFPC2 data is available. Topics include general issues such as retrieving the data from the archives, displaying the image, instructions for calibration and recalibration of the data, a section on photometry, and a listing of other sources of related information. This tutorial provides a more detailed, hands-on approach than the Data Handbook which was designed to provide a higher level summary of a much wider set of topics.

  • June 1998: New Drizzle Tutorial/Tasks Released. A "beta" release of new tasks for the removal of cosmic rays from dithered data is now available. Associated with these tasks is an updated and enlarged version of the tutorial on removing cosmic rays from dithered data first published by Max Mutchler and Andy Fruchter in the 1997 Calibration Workshop. The tasks and the tutorial can be found in the "DitherII" section of Andy's Dithering Page.

  • Sep. 1998: Update on Long vs. Short Exposure Anomaly in WFPC2 Images. An improved characterization of the so-called "long vs. short" anomaly is presented in ISR98-02: WFPC2 The Long vs. Short Anomaly in WFPC2 Images. The anomaly is a non-linear behavior of the WFPC2 chips which results in a decrease of the measured count rate with decreasing exposure time. In summary: 1) the non-linearity depends strictly on total counts in a stellar image, and is independent of exposure time; 2) the effect depends on position on the chip only through the well-known CTE effect, and is independent of position after the CTE correction is applied; 3) F555W and F814W appear similarly affected; and 4) there is a marginal decrease in non-linearity at high background levels, but the effect is statistically insignificant in the overall description of the anomaly. A simple formula is presented that can be used to correct the observed counts in a variety of conditions; this formula is recommended for use with most point-source photometry obtained with WFPC2.
  • Note: this section 9 (Miscellaneous) has not been updated after Sep. 1998, if updates would be of use to you, please let us know via an Email to

    10. OTFC Keyword Exception Table

    The Keyword Exception Table is a listing of all observations in the HST Archive with incorrect keyword values, such as wrong filter number, shutter value, etc. When OTFC processes a dataset that is on this list, OTFC will automatically use the correct keyword value. This table is intended as a quick way for users to check if WFPC2 datasets have been added/removed from the list.

    11. WFPC2 History In Table Format

    This table is a record of WFPC2 history in chronological order. See above for full details.

    Last updated: Feb. 10, 2000.