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Last modified: 26 October 2016

URL: http://cxc.harvard.edu/ciao/bugs/dmmerge.html

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
 
--------------------------------------------------------------------------------
Dataset: NGC3379_merge.fits
--------------------------------------------------------------------------------
 
     Block Name                          Type         Dimensions
--------------------------------------------------------------------------------
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
 
--------------------------------------------------------------------------------
Dataset: NGC3379_merge_new.fits
--------------------------------------------------------------------------------
 
     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.)


Last modified: 26 October 2016
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