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Last modified: 27 September 2010


ACIS Time-dependent Gain


The tool acis_process_events includes a time-dependent gain (TGAIN) adjustment. This adjustment is necessary because the "effective gains" of the detectors are drifting with time as the result of an increasing CTI.

There is TGAIN calibration data for the entire ACIS detector (chips I0-3 and S0-5) at the -120 C focal plane temperature.

There is also TGAIN calibration data for the ACIS-S1 and S3 back-illuminated chips at the -110 C focal plane temperature.

If the data have gone through Reprocessing III, a TGAIN correction has been applied. Repro III started with DS 7.6.7. There may be a newer TGAIN available, requiring the user to reprocess the data.

The observation date and focal plane temperature are recorded in the header of the event file:

unix% dmkeypar acis_evt2.fits DATE-OBS echo+

unix% dmkeypar acis_evt2.fits FP_TEMP echo+

What Analyses are Affected?

Users interested in CCD spectroscopy should apply the gain adjustment. Grating spectroscopy benefits as well, in the form of improved order sorting. It is not necessary to apply this adjustment if you are only doing timing or imaging analysis, but it will not have a negative effect, either.

TGAIN header keywords

Information on the TGAIN correction which has been applied is stored in the TGAINCOR and TGAINFIL header keyword values:

unix% dmlist acis_1838_new_evt1.fits header | grep TGAIN
0074 TGAINCOR             T                              String
0075 TGAINFIL             acisD2000-08-01t_gainN0006.fits String

TGAINCOR is set to "T" for true. If either of these keywords is missing or set to a different value ("F" or "none"), the gain adjustment was not applied.

Also, a PHA_RO column should exist in the output file and contain non-zero values:

unix% dmlist acis_1838_new_evt1.fits"[cols PHA_RO]" data rows="1:5"
Data for Table Block EVENTS
ROW    pha_ro
     1        202
     2        183
     3        107
     4       2399
     5       3660

It is recommended that you keep this column in your data file (i.e. do not use a DM filter to remove it). These are the original (RO = readout) PHA values which are needed if you later want to rerun with new calibration.

Improvements in Calibration

Technical Details

Technical details can be found in the Corrections for Time-dependence of ACIS Gain calibration page.

Last modified: 27 September 2010