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What to Do When You Get an Exception Report for ACS

If something may have gone wrong with your ACS observations, you will get an email containing an HST exception report. It asks you to download your data from MAST, determine whether the S/N and/or flux of your data reached the expected levels, and determine from a quick-look inspection whether your data appear anomalous in any other way. Here we provide a checklist with more detailed steps on inspecting ACS data for potential problems.

  1. Review the observing logs (JIF files) for problems. The "Problem Flags and  Warnings" section of the headers of extensions 1+ will be populated if problems exist. There may be problems with guide star acquisitions (GSFAIL), the Take Data Flag (TDF-DOWN), or telescope slewing (SLEWING), among others. The GUIDEACT header keyword in the headers of extensions 1+ lists the actual guiding mode for each exposure. FINE LOCK/GYRO or GYRO may indicate that the target drifted in the field of view. 
  2. If your data are from WFC, check the exposure times of the FLT or FLC images. Compare the header keyword EXPTIME to the commanded exposure time in your Phase II. If EXPTIME is shorter than commanded or zero, the shutter was closed for all or part of your exposure. 

    If your data are from SBC, EXPTIME may match the commanded exposure time, but the data will show very few to no counts if the shutter is closed. In addition, you can be sure the shutter did not open if GSFAIL = NOLOCK and/or GUIDEACT = GYRO.
  3. Inspect any PSFs or sources that appear in your images for signs of drifting, especially if the observing log or exception report indicates that there was slewing of the telescope or drifting of the target during the observation.

If you wish to repeat the observations, please submit a Hubble Observation Problem Report (HOPR) within three months (90 days) of the date that the data were delivered, following the instructions provided in the email. As always, please contact the Help Desk if you have any concerns. 

Last Updated: 06/02/2023


HST Help Desk 

Please contact the HST Help Desk with any questions.