Hello chandra-users,
There is a bug in the merge_all script which will produce incorrect results if
one (or more) of the observations was done with an offset. Refer to the
merge_all bug page for details (included below) and a workaround:
http://cxc.harvard.edu/ciao/bugs/merge_all.html#offsets
The merge_all script is in the process of being rewritten. The new version
will address this problem.
If you have any questions or problems with the workaround, please contact the
Helpdesk for assistance.
http://cxc.harvard.edu/helpdesk/
Elizabeth Galle
for the CIAO team
----This bug affects a small percentage of observations. For instance, roughly 5% of observations done with Chandra in 2008 used a pointing offset.
When merge_all runs dmmerge to combine the event files, it creates a custom lookup table containing merging rules for header keys. Instead of omitting the DETNAM, ROLL_NOM, and SIM_Z header values that differ by more than the tolerance in dmmerge, it always uses the value from the first file.
The dz values in the aspect solution are relative to the SIM_Z in the event header. If the observation was done with an offset, the aspect solution files for the second observation is not corrected for the difference in SIM_Z between the files. The result is that the combined exposure map and fluxed image will be wrong.
Use the Chandra Target Information page to see if your observation used an offset: http://cxc.harvard.edu/targets/
Again, a workaround is available in the merge_all bug page:
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