CalDB 4.8.0 Public Release Notes
Public Release Date: 11 SEP 2018
SDP Installation Date: 2018-09-10T18:22:55 (UTC)
CalDB 4.8.0 is an upgrade to the Chandra CalDB, which includes the following items:
For the CIAO 4.10 / CalDB 4.8.0 release notes see How CalDB 4.8.0 Affects Your Analysis.
II. SUMMARY OF CHANGES
The ACIS Calibration team has released the semiannual time-dependent gain (T_GAIN) updates from analysis of ECS data for the period Feb 01 - Jul 31 2018 (epochs 73+74), and hence the above files have been generated as a result, for immediate release. The secular time variations in the corrections calculated from these new data are similar in magnitude and direction to previous upgrades, wherein maximal changes are a few percent or less of the deltaE/E value. In particular the variations at the ACIS-I aimpoint are significant, whereas those at the ACIS-S aimpoint are very small for this period and the previous epochal period (epochs 71+72, Aug 01 2017 - Jan 31 2018) which is now finalized. NOTE: Users with observations taken between (DATE-OBS=) 2017-11-01T12:00:00 and 2018-05-02T12:00:00 are advised to reprocess with their data with finalized versions of the applicable T_GAIN files.
Users with observations taken since 2018-05-02T12:00:00 will have access to an updated provisional T_GAIN file (one of the version N0002 filenames above); They will not however have a finalized file until after Epoch 76 (Nov 01 2018 - Jan 31 2019) is completed.
ACIS Level 1 pipeline tool acis_process_events.
The ACIS Calibration Team has begun analysis peculiar to their own purposes of External Calibration Source (ECS) data taken with warmer than usual FP_TEMP values; part of their procedure is to reprocess these ERs removing the CTI correction, but having the "apply_tgain" parameter set to "yes", so that a T_GAIN file should still be selected and applied. Ordinarily these ERs are not in GRADED data mode, so that the N0006 (full CTI correction) T_GAIN files are selected for these. With the CTI correcction OFF, CTI_APP = NNNNNNNNNN, and the corresponding N0005 file would be selected. For some of the warmer FP_TEMPs, however, it has been found that the default FP_TEMP boundary condition range was set to (148.2-158.2)K, which for some 24 ERs, a-p-e returns "NO FILES FOUND" for the T_GAIN, and turns off the apply_tgain parameter, leaving the data uncorrected. See Table 1 below.
There were however four additional cases where ERs were taken with normal FP_TEMPs, but still no T_GAIN could be found. It was determined that these cases were at periods crossing the epochal boundaries of the T_GAIN files. The effective date ranges for these cases had been fixed for the usual N0006 (CTI-corrected) T_GAINs in CalDB 4.5.0 (June 2012). See also the CalDB caveats page 38 ACIS OBS_IDs with "Problem Reading TGAINFILE However the corresponding N0005 files had not been corrected for this situation. Hence, turning off the CTI correction causes the T_GAIN CalDB search to fail for them. The solution is to correct the Calibration Validity End Dates CVED* (and Times) CVET* in those N0005 file headers to be the same as those for the corresponding N0006 files.
It has been decided for the sake of consistency that all of the -120C N0005 T_GAIN files from 29 Jan 2000 through 2011 May 01 should have their FP_TEMP ranges reset to (148.2 - 188.2)K, the standard setting used for all current T_GAINs for the OBS_MODE=SECONDARY case, which is the setting for the ACIS ECS ERs. Such a change will not affect the reprocessing of ordinary observations. It will, however, allow the procedure used by the ACIS Calibration team to be performed on any observation (ER or OR), with a non-CTI T_GAIN always being available, regardless of the FP_TEMP, the DATE-OBS, or the DATE-END of the observation.
As always, see the technical details section below for more information on specfic observations.
ACIS Level 1 pipeline tool acis_process_events, with GRADED mode observations taken during the respective effective date ranges of the specific files above. Also with non-GRADED datamode observations provided that the parameter apply_cti is set to "no" while the apply_tgain parameter is set to "yes". Note that in this second case, the CIAO script chandra_repro cannot be used, as it is not possible to turn off the "apply_cti" parameter, because chandra_repro will reset it to "yes" when intializing with your dataset.
No specific CIAO threads are affected.
III. TECHNICAL DETAILS
The ACIS T_GAIN epoch 73+74, combining two quarterly epochs, and covering the period of Feb 01 through Jul 31, 2018 has recently been completed, and the ACIS External Calibration Source (ECS) radiation zone data has been reduced as per some modified procedures, to generate the time-dependent gain correction tables for this epoch. The procedural modifications were made to address the decreasing statistics available from the ECS due to its natural decay in count rates. The two epochs are being combined to produce a single new T_GAIN correction update.
The original method of derivation for the ACIS T_GAINs is given in the memorandum "Corrections for time-dependence of ACIS gain", posted on ACIS Calibration web page ACIS TGAIN.
The magnitudes (in eV) of the new gain corrections, versus photon energy, are given in Figs. 1-3 below. The secular variations are as follows: Specifically from 5% at ~0.l keV to 1% at ~10 keV for ACIS-I the aim point; for the ACIS-S aim point, the variations are less than 1%. Fig. 1 below gives the corrections for the ACIS-I aimpoint on chip ACIS-3, for the CTI-corrected case, which is the only one applicable to FI chips. Figure 2 gives the corrections for the ACIS-S aimpoint on ACIS-7, for the case where the BI chips are CTI-corrected. Finally Fig. 3 gives the corrections for ACIS-7 for NON-CTI-corrected BI chips. These would be relevant to GRADED DATAMODE observations with ACIS-S, for example.
Fig. 1: Time dependent gain corrections versus photon energy for the ACIS-I aimpoint on ACIS-I3 (CHIPX=970, CHIPY=975) for the new and recent comparable T_GAIN files in CalDB. The parallel CTI correction for the FI chips has been applied to all data.
Fig. 2: Time-dependent gain corrections versus energy for the ACIS-S aimpoint on ACIS-7 (CHIPX=210, CHIPY=520) for the new and recent comparable CalDB T_GAIN files. For these data, the series and parallel CTI correction for the BI chips has been applied.
Long term trends for chips I3 and S3 are plotted below in Figures 4a and b, through Epoch 73+74, which includes the latest release. The chip locations for these plots are given in the respective labels above the plots. The chip locations selected are near the aimpoints, as with Figs 1 and 2 above. The full CTI correction has been applied to the data in both figures.
Table 1 below includes the 28 specific ECS ERs for which the ACIS Calibration Team had a problem with finding an applicable T_GAIN file with the CTI correction turned off in a-p-e. Included are the FP_TEMP values from the Level 2 events file, as well as the DATE-OBS and DATE-END values. Also included are indicators for whether or not the FP_TEMP value violates the original FP_TEMP boundary condition (148.2-158.2)K, and whether the date range extends over an epochal boundary.
|DATE-OBS (UTC)||DATE-END (UTC)||Epoch
|Required N0005 T_GAIN file|
Table 2 below includes details on the specific header changes made to the 47 N0005 T_GAIN files, including the specific ECS ERs that are affected by those changes. Also included are certain ORs from the CalDB Caveats page given above. For all of the listed ORs, the problem was not in the FP_TEMP value, but in the fact that the observation extended over the T_GAIN epoch time boundary, and needed an extension in the (N0006) T_GAIN file to allow the correct T_GAIN data to be applied. The change put in for the CVED* and CVET* values in CalDB 4.5.0 has now been applied to the corresponding N0005 files.
|FILENAME||New CBD2* Value||Old CVED* Value||New CVED* Value||ER(s)||OR|
Note that for the cases where the ER and OR value is set to a bold N/A there is no obsevation to be found that will violate the original FP_TEMP boundary condition, and for these cases it has not been necessary to alter the original CVED* or CVET* values in the headers. Hence, for these files, the FP_TEMP boundary condition change is the only one done, and this was just for consistency's sake.