Last modified: 24 October 2023


Caveat: Zero-exposure Event File

This data caveat currently affects ObsIds 1413, 1452, 1425, 1414, 1424, 1426, 1409, 862, and 963. It is based on Issue 18 of the Chandra Observations Processing Status; check the issue report for updates to the list of open ObsIDs.

In some observations, the images of aspect fiducial lights (projections of fiducial lights onto the aspect camera detector) were not acquired for part or all of the observation. The reported aspect quality for these observations is bad, which causes empty Good Time Intervals (GTIs). If there is zero good time defined for an observation, all the events are screened out of the final level=2 event file.

The lack of FID lights is not a simple matter of absolute coordinates being offset. During many observations, the Science Instrument Module (SIM) drifts by up to an arcsec; without FIDs, this does not get corrected. On account of this, spurious nebulosity, etc., may be revealed when analyzing these datasets. Additionally, combining data from a multi-obi observation affected by this problem may produce double sources in the final event file.

If you are attempting analysis that reveals one of these problems, email the CXC Helpdesk for guidance. To create a new event file for basic analysis procedures, use the provided workaround.

Workaround for both imaging and grating datasets:

The fact that a) there were no guide-stars to the OBC aspect solution was used and b) that the observation violated the pointing-angle to the moon is contained in the mission timeline (mtl1.fits) file.

Use the dmgti tool to create a new GTI file (flt1.fits) which does not include those limits from the MTL. For example:

unix% dmgti acisf00963_001N003_mtl1.fits new_963_flt1.fits \
      lkupfile=acisf00963_001N003_mtl1.fits"[limits][exclude #row=2,21]" \

Then reprocess the level=1 data to create a new level=2 event file using the new flt1.fits file in place of the one from SDP. For help reprocessing your data, follow the Create a New Level=2 Event file thread.

Finally, one may create new GTIs to remove background flares by completing the Eliminate High Background Times section of the Filtering Data thread.