What to Do When You Get an Exception Report for COS
If something may have gone wrong with your COS observations, you 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 for how to inspect COS data for potential problems.
- Check the exposure time of all exposures in your visit. If the COS shutter was not open, you will see an exposure time of zero seconds reported, and the corresponding data files from MAST will be empty.
- Review the target acquisition files to determine whether the target was centered well in the aperture.
- Review the x1d files, not just the x1dsums. You may conclude that the signal-to-noise ratio in the x1dsum is suitable for your purpose, but it is important to note whether any of the individual x1d files have dramatically fewer counts than expected or are entirely empty of counts.
- Evaluate the wavelength scale of the data. If the target was not centered well in the aperture, the wavelength scale could be unacceptably inaccurate even if the signal-to-noise ratio of the spectrum is acceptable.
- 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. Investigators sometimes decide that, even though there was a problem with the observation, the spectrum is suitable for their needs. Please file a HOPR even in this case, noting that you are not requesting a repeat. This allows us to flag the affected dataset in MAST for the benefit of potential future investigators.