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. This bug was introduced in CIAO 4.3.
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.
This time-filtering is one of the standard reprocessing steps for grating data, so anyone that has reprocessed a grating dataset in CIAO 4.3 or 4.4 is affected.
In this example, the time filter is an flt1.fits file from the Archive. The filter might also be supplied as a GTI file created with the dmgti tool, an ASCII table, or a range specified on the command line.
unix% dmcopy "evt1a.fits[EVENTS][@flt1.fits]" evt2.fits opt=all
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.