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Last modified: 3 September 2013

URL: http://cxc.harvard.edu/caldb/caveats/acis_tgain_epoch_break.html

38 ACIS OBS_IDs with "Problem Reading TGAINFILE"


NOTICE: The problem below has been fixed completely with the release of CalDB 4.5.0, released on 27 June 2012. Chandra Repro 4 will reprocess all of the OBS_IDs below correctly as it proceeds now. Users who have interest in these OBS_IDs may first upgrade their CalDB to version 4.5.0 immediately, and then proceed with their own reprocessing with the script chandra_repro without concern about the CalDB performance. Their analysis may proceed normally.

There are 38 Observation Ids where manual reprocessing with chandra_repro fails to include the ACIS time-dependent gain (TGAIN) correction. Current archival reprocessing (Repro 4) is correcting for this error, but has not completed reprocessing on all of these Observation IDs. This page is only of interest to people analyzing any of the 38 Obs Ids listed in the ObsId list below. If you are using one of these datasets then below we show you:

A fix to the CalDB coming in version 4.5.0, anticipated for release in May 2012, will eliminate this issue.

As always, please contact the CXC HelpDesk with any questions or for further clarification.


Have you been affected?

If your data is not one of the 38 Obs Ids listed below then you do not need to worry about this problem.

The TGAINCOR header keyword in the evt2 file records whether the time-dependent gain correction was applied by acis_process_events, so the following check can be made (here we compare the archival data for ObsId 3339 to a version reprocessed with CIAO 4.4 and CALDB 4.4.8):

unix% dmkeypar acisf03339N0002_evt2.fits.gz tgaincor echo+
T
unix% dmkeypar acisf03339_repro_evt2.fits tgaincor echo+
F

The archival data - which was reprocessed as part of Repro 3 - does include the time-dependent gain correction, since the keyword is set to T (the FITS convention for True). The reprocessing via chandra_repro has not included the correction, and so the keyword is set to F (short for False).

So, if the dmkeypar call returns T then the data has had the time-dependent gain correction applied to it and you do not need to read the rest of this document. However, please note that if you intend to re-process the dataset then the following section needs to be followed.

The above check assumes that you did not explicitly turn off the time-dependent correction by setting apply_tgain=no in the acis_process_events call.


How to reprocess the data correctly

Follow the Reprocessing Data to Create a New Level=2 Event File thread, but explicitly set the tgainfile parameter, as shown below, before running acis_process_events. Please contact the CXC HelpDesk with any questions or for further clarification.

Notes:

  • if you are analysing Obs Id 2532 then - as discussed below - you also need to set the ctifile parameter;
  • the chandra_repro script can not be used to process these observations since it calls punlearn acis_process_events, and so uses the CALDB setting for the tgainfile parameter.

Using ObsId 3339 as an example, we find from the ObsId list that the correct tgainfile for the observation is acisD2002-08-01t_gainN0006.fits, so we set

unix% pset acis_process_events \
    tgainfile=${CALDB}/data/chandra/acis/t_gain/acisD2002-08-01t_gainN0006.fits

before running acis_process_events.

If you are analysing ObsId 2532 then you also need to set the ctifile parameter, using the following command (this should not be done for the other 37 Obs Ids):

# Obs Id 2532 only
unix% pset acis_process_events \
    ctifile=${CALDB}/data/chandra/acis/cti/acisD2000-01-29ctiN0007.fits

If you do not override the tgainfile parameter setting then you will see the following warning from acis_process_events:

# acis_process_events (CIAO 4.4): WARNING: problem reading tgainfile, tgain adjustment will not be applied. Changing apply_tgain=yes to apply_tgain=no.

Obs Id list

The following table lists those ACIS Obs Ids whose observations span T_GAIN epochal time boundaries. The T_GAIN column gives the tgain file name that should be explictly set for the tgainfile parameter of acis_process_events, as shown in the How to reprocess the data correctly section above.

Obs ID Epoch break Start Date (UTC) End Date (UTC) Exposure (ks) DATA MODE T_GAIN Filename PI Name
649 1-2 2000-04-30T16:19:51 2000-05-01T07:11:53 52.13 FAINT acisD2000-01-29t_gainN0006.fits K Mukai
79 2-3 2000-07-31T15:30:29 2000-08-01T05:43:05 48.96 FAINT acisD2000-05-01t_gainN0006.fits S Murray
547 3-4 2000-10-31T23:52:08 2000-11-01T14:16:41 50.16 VFAINT acisD2000-08-01t_gainN0006.fits VanSpeybroeck
2459 5-6 2001-04-30T23:06:11 2001-05-01T04:46:29 18.90 CC33_FAINT acisD2001-02-01t_gainN0006.fits Kouveliotou
1982 6-7 2001-07-31 17:00:51 2001-08-01T01:57:07 29.95 FAINT acisD2001-05-01t_gainN0006.fits Seward
3436 8-9 2002-01-31 22:18:50 2002-02-01T01:49:46 9.97 FAINT acisD2001-11-01t_gainN0006.fits Fox
3138 9-10 2002-04-30 13:22:55 2002-05-01T03:00:22 47.73 FAINT acisD2002-02-01t_gainN0006.fits Kishimoto
2532 10-11 2002-07-31 20:44:55 2002-08-01T03:39:21 22.75 FAINT acisD2002-05-01t_gainN0006.fits
CTI file: acisD2000-01-29ctiN0007.fits
Huenemoerder
3339 11-12 2002-10-31 21:56:29 2002-11-01T00:08:11 4.96 FAINT acisD2002-08-01t_gainN0006.fits Nandra
4107 12-13 2003-01-31 20:50:07 2003-02-01T00:04:50 10.13 VFAINT acisD2002-11-01t_gainN0006.fits Wilson
3802 14-15 2003-07-31 18:12:10 2003-08-01T01:30:42 24.8 VFAINT acisD2003-05-01t_gainN0006.fits Tomsick
3512 15-16a 2003-10-18 19:46:56 2003-10-19T01:53:23 20.24 VFAINT acisD2003-08-01t_gainN0006.fits Garmire
5011 16a-16b 2003-11-17 19:33:53 2003-11-18T01:09:56 18.77 VFAINT acisD2003-10-19t_gainN0006.fits VanSpeybroeck
4546 16b-16c 2003-12-31 15:04:37 2004-01-01T02:20:14 37.02 VFAINT acisD2003-11-18t_gainN0006.fits Kellogg
4518 16c-17 2004-01-31 21:19:08 2004-02-01T02:05:30 15.18 CC33_GRADED acisD2004-01-01t_gainN0005.fits Strohmayer
5142 17-18 2004-04-30 21:53:39 2004-05-01T00:45:24 7.76 VFAINT acisD2004-02-01t_gainN0006.fits Calibration
4676 19-20 2004-10-31 08:35:59 2004-11-01T02:02:39 61.31 VFAINT acisD2004-08-01t_gainN0006.fits Safi-Harb
6186 20-21 2005-01-31 23:31:14 2005-02-01T14:23:50 52.23 VFAINT acisD2004-11-01t_gainN0006.fits Forman
4537 21-22 2005-04-30 15:03:07 2005-05-01T00:29:29 32.17 VFAINT acisD2005-02-01t_gainN0006.fits Greiner
6771 24-25 2006-01-31 23:11:34 2006-02-01T02:17:35 9.13 FAINT acisD2005-11-01t_gainN0006.fits Garmire
5932 25-26 2006-04-30 13:04:27 2006-05-01T00:47:14 40.16 FAINT acisD2006-02-01t_gainN0006.fits Jenkins
7910 28-29 2007-01-31 20:50:08 2007-02-01T02:56:56 20.03 VFAINT acisD2006-11-01t_gainN0006.fits Fabian
7943 29-30 2007-04-30 23:15:56 2007-05-01T11:46:52 43.15 VFAINT acisD2007-02-01t_gainN0006.fits Murray
8211 30-31 2007-07-31 23:11:00 2007-08-01T13:04:43 47.97 FAINT acisD2007-05-01t_gainN0006.fits Chu
7911 32-33 2008-01-31 18:24:08 2008-02-01T00:24:59 20.18 VFAINT acisD2007-11-01t_gainN0006.fits Fabian
9087 33-34 2008-04-30 23:08:33 2008-05-01T08:33:52 32.76 VFAINT acisD2008-02-01t_gainN0006.fits Grupe
9892 34-35 2008-07-31 07:49:43 2008-08-01T02:39:25 66.67 VFAINT acisD2008-05-01t_gainN0006.fits Revnivtsev
10118 36-37 2009-01-31 21:43:38 2009-02-01T01:07:54 9.98 VFAINT acisD2008-11-01t_gainN0006.fits Pooley
10464 37-38 2009-04-30 16:09:06 2009-05-01T02:38:44 36.15 VFAINT acisD2009-02-01t_gainN0006.fits Russell
10808 38-39 2009-07-31 19:19:31 2009-08-01T04:10:43 30.18 VFAINT acisD2009-05-01t_gainN0006.fits Park
10459 39-40 2009-10-31 17:54:23 2009-11-01T03:23:51 32.04 VFAINT acisD2009-08-01t_gainN0006.fits Stanford
11736 41-42 2010-04-30 13:10:39 2010-05-01T05:50:41 58.83 VFAINT acisD2010-02-01t_gainN0006.fits Boehringer
11333 42-43 2010-07-31 22:08:30 2010-08-01T00:32:37 6.74 VFAINT acisD2010-05-01t_gainN0006_revA.fits Gallo
13158 43-44 2010-10-31 16:46:35 2010-11-01T20:00:28 95.34 VFAINT acisD2010-08-01t_gainN0006.fits Park
12399 44-45 2011-01-31 16:21:30 2011-02-01T02:41:16 35.05 FAINT acisD2010-11-01t_tgainN0006.fits Balman
12317 45-46 2011-04-30 05:23:07 2011-05-01T03:58:09 79.53 VFAINT acisD2011-02-01t_gainN0006.fits Branduardi-Raymont
12891 46-47 2011-07-31 19:18:12 2011-08-01T06:59:16 40.04 VFAINT acisD2011-05-01t_gainN0006.fits Jonker
13535 47-48 2011-10-31 23:23:09 2011-11-01T00:24:42 2.07 VFAINT acisD2011-08-01t_gainN0006.fits Gastaldello

What happened

The ACIS TGAIN calibration is done in such a way that the calibration files cover 3 month periods ("epochs"), so there is the possibility that observations will take place that span the end of one epoch and the start of the next one. The 38 Obs Ids listed in the table above are all the ACIS observations that have occured during such "epoch breaks".

Unfortunately, since the December 2008 release of CIAO 4.1 and CALDB 4.1, the CALDB query to find such files will fail, resulting in the acis_process_events warning

WARNING: problem reading tgainfile, tgain adjustment will not be applied. Changing apply_tgain=yes to apply_tgain=no.

and the data will be processed without the tgain correction. As noted at the top of the page, this problem has been fixed in CalDB 4.5.0.


Last modified: 3 September 2013
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