Incorrect GTIs when time-filtering grating data with dmcopy opt=all
The CIAO team has identified a bug which affects all ACIS and HRC grating data analysis.
When a time filter is applied to a level=1.5 (evt1a.fits) or level=2 (evt2.fits) grating file and the dmcopy opt=all option is used, the GTI block in the output file is not updated to reflect the correct time range.
Time-related header keyword values - such as ontime, livetime, and exposure - will be incorrect because they are calculated from the time ranges in the GTI block.
Workaround:
Run dmcopy without "opt=all" when applying any time filters. Then run dmappend to copy the grating REGION block to the filtered output file, e.g.
unix% dmcopy "evt1a.fits[EVENTS][@flt1.fits]" evt2.fits unix% dmappend "evt1a.fits[region][subspace -time]" evt2.fits
The subspace filter is necessary so that the GTIs aren't reapplied to the output file, as explained in the dmappend caveat.