Bugs: dmmerge
Caveats
Extra GTI blocks in merged event file (16 Apr 2007)
When merging ACIS event lists with dmmerge, the "data subspace" of the two event lists is combined. Due to a technical subtlety in the way that the EXPNO (ACIS frame exposure number) column is defined, this generates unwanted extra GTI extensions in the output file if the exposure numbers are not equal.
unix% dmmerge "acisf01587N002_evt2.fits,acisf07073N001_evt2.fits" NGC3379_merge.fits
unix% dmlist NGC3379_merge.fits blocks
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Dataset: NGC3379_merge.fits
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Block Name Type Dimensions
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Block 1: PRIMARY Null
Block 2: EVENTS Table 15 cols x 1392531 rows
Block 3: GTI7 Table 2 cols x 1 rows
Block 4: GTI2 Table 2 cols x 3 rows
Block 5: GTI5 Table 2 cols x 1 rows
Block 6: GTI6 Table 2 cols x 2 rows
Block 7: GTI3 Table 2 cols x 2 rows
Block 8: GTI8 Table 2 cols x 2 rows
Block 9: GTI7_CPT7 Table 2 cols x 1 rows
Block 10: GTI2_CPT8 Table 2 cols x 2 rows
Block 11: GTI5_CPT9 Table 2 cols x 1 rows
Block 12: GTI6_CPT10 Table 2 cols x 1 rows
Block 13: GTI3_CPT11 Table 2 cols x 2 rows
Block 14: GTI8_CPT12 Table 2 cols x 1 rows
This occurs because the filters could not be combined into a single, valid range for the chip. It is undesireable for further analysis, as the tools cannot use multiple GTI blocks for a single chip, resulting in incorrect exposure values.
Workaround:
The subspace-editing capabilities in CIAO 3.4 simplify the workaround for this issue. It is now possible to delete the EXPNO subspace before merging the files:
unix% dmmerge \
"acisf01587N002_evt2.fits[subspace -expno],acisf07073N001_evt2.fits[subspace -expno]" \
NGC3379_merge_new.fits
unix% dmlist NGC3379_merge_new.fits blocks
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Dataset: NGC3379_merge_new.fits
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Block Name Type Dimensions
--------------------------------------------------------------------------------
Block 1: PRIMARY Null
Block 2: EVENTS Table 15 cols x 1392531 rows
Block 3: GTI7 Table 2 cols x 2 rows
Block 4: GTI2 Table 2 cols x 5 rows
Block 5: GTI5 Table 2 cols x 2 rows
Block 6: GTI6 Table 2 cols x 3 rows
Block 7: GTI3 Table 2 cols x 4 rows
Block 8: GTI8 Table 2 cols x 3 rows
There are now just six GTI blocks in the output file.
Note that the addition of the subspace filter means that any user who intends to create lightcurves binned on exposure number from the merged output cannot use merge_all, since that information is eliminated from the subspace. (In general, lightcurves are binned on time.)