ACIS: Multiple GTIs
When extracting a lightcurve using opt=ltc1|ltc2, dmextract uses the information from the input file's GTI. ACIS data contains a separate GTI for each active CCD (up to 6); however, they are typically nearly identical except for a small time offset due to the how the CCDs are read-out. dmextract uses the last GTI it finds in the file. Since the GTIs are nearly identical (especially compared to the time resolution of the lightcurve) the difference in GTIs is usually irrelevant.
However, there are some ACIS observations where the GTIs are very different for each CCD. These are mostly due to very bright sources imaged with or with out the gratings that cause telemetry to saturate and results in dropped exposures. Since exposures from individual chips are dropped separately, the GTIs can be very different for different CCDs. For example OBS_ID=3505 has very different GTIs for each CCD due to a large number of dropped exposures.
% dmlist 3505/secondary/acisf03505_001N003_flt1.fits.gz blocks
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Dataset: 3505/secondary/acisf03505_001N003_flt1.fits.gz
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Block Name Type Dimensions
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Block 1: PRIMARY Null
Block 2: FILTER Table 0 cols x 0 rows
Block 3: GTI7 Table 2 cols x 1 rows
Block 4: GTI4 Table 2 cols x 3566 rows
Block 5: GTI5 Table 2 cols x 3981 rows
Block 6: GTI6 Table 2 cols x 2826 rows
Block 7: GTI8 Table 2 cols x 2748 rows
Block 8: GTI9 Table 2 cols x 3998 rows
This may lead to an error in the lightcurve exposure values and thus in the count_rate if the user specifies an extraction region that is on a different CCD or one that spans multiple chips.
Workaround:
Users can specify a ccd_id filter to select a single CCD when they run dmextract. This will essentially select the correct GTI. Users who need to compute a lightcurve for multiple CCDs need to run dmextract separately for each CCD and then combine them together.