CalDB 4.6.8 Public Release Notes
Public Release Date: 01 JUL 2015
SDP Installation Date: 2015-07-08T12:12:50 (UTC)
CalDB 4.6.8 is an upgrade to the Chandra CalDB, which includes the following items:
- ACIS T_GAIN Epoch 61, with finalizations for Epoch 60
- ACIS T_GAIN Epochs 56-59 Revision A
For the CIAO 4.7 / CalDB 4.6.8 release notes see How CalDB 4.6.8 Affects Your Analysis.
II. SUMMARY OF CHANGES
The periodic ACIS T_GAIN correction upgrade for Feb-Apr 2015 (Epoch 61) has been released. Modifications have also been made to the previously released Epoch 60 T_GAIN corrections (Nov '14-Jan '15). The corrections are at nominal/expected levels of less than 2% of the energy value. This upgrade as usual affects observation data taken over the two most recent T_GAIN Epochs (60 and 61), and so would update PHAs for OBS_IDs taken since 01 Nov 2014. Users with such datasets are encouraged to revalidate their analysis results after reprocessing their data with the upgraded CalDB files. For specific details of the new calibrations in this release, see the technical details section below.
See the ACIS T_GAIN why page for more information.
The ACIS calibration team has released modifications to all of the T_GAIN correction data released during the year 2014. The details are discussed in the technical detail section below. As it is, the above files have been constructed with the modified corrections, and are being released with the CalDB 4.6.8 upgrade. The nature and degree of changes in the T_GAINs depends on the chip type (FI or BI) and the location(s) on the chip in question. For the FI chips, the deltas introduced by the files above are in general below the noise level in the PHA or ENERGY values in the data. In such cases were the deltas are larger, they are limited to a relative value of 5% of the correction, which is less than 2% of the altered PHA value. Hence we expect these to be inconsequential. For the BI chips, however, virtually all the DELTA_PHA values in the files have changed, and in some limited cases by very large degrees, particularly on ACIS-S1 (ACIS-5).
We therefore encourage users of FI chip data (with DATE-OBS occuring during the period of 2013-11-01 to 2014-11-01) simply to ignore these upgrades. Should they have occasion to reprocess their data with CIAO (chandra_repro) they will incorporate the updated T_GAIN values as a matter of course, and should notice virtually no or little affect. For those with ACIS-S1 data, such as would occur with gratings, it is possible that the order-sorting will be affected over the (negative order) spectral regions subtended by that chip. Hence they may want to reprocess their data and review their results.
See the ACIS T_GAIN why page for more information.
III. TECHNICAL DETAILS
The ACIS time-dependent gain corrections (T_GAIN) have recently been updated for current changes from the previous T_GAIN epoch, specifically Epoch 61, which was Feb-Apr 2015. With the addition of these new corrections, derived from ACIS External Cal Source (ECS) data taken during radiation zone passes, the CalDB files extending from Nov 2014 through Jan 2015 (i.e. Epoch 60) have been finalized, and new non-interpolating T_GAIN files are now implemented for Epoch 61.
The magnitudes (in eV) of the new gain corrections, versus photon energy, are given in Figs. 1-3 below. The corrections are of the usual order in magnitude, specifically less than 2.0% of the photon energy value. 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.
Long term trends for chips I3 and S3 are plotted below in Figure 4a and b, through Epoch 61, the latest release. The chip locations for these plots are given in the respective labels above the plots. Note that the locations are not the aimpoint positions, as with Figs. 1-3 above, but are near the centers of each chip. The full CTI correction has been applied to the data in both figures.
|Figs 4a and 4b: Long term gain trends for specific ECS line energies from Epoch 1 to Epoch 61 inclusive, and for the positions on the respective chips given in the figure labels. (Click on images to expand the plots.)|
The ACIS calibration team has identified a bug in their procedure for identifying ECS line centroids that are used to generate the T_GAIN correction values tabulated in the CalDB files. While initially this issue created only a modest number of clipped T_GAIN corrections, the problem is exacerbated with the decreasing ECS count rate and poorer count rate statistics. The problem becomes noticeable (in hind site) as far back as Feb 2014, and the CalDB T_GAIN files over Epochs 56-59 (Nov 2013 through Oct 2014) are affected, and require an upgrade to revision A ("_revA").
The actual affect on the PHAs in level 2 events data is actually not significant over most of the areas of ACIS-I3 and the other FI chips; however in the worst cases a shift in the PHAs/Energy values of ~0.5% will occur. For ACIS-S3, the "revA" updates may be ignored completely, regardless of CTI-correction.
The non-CTI_corrected ACIS-S1 shows large worst-case corrections (up to 50%) in the T_GAIN shifts for epochs 57-59 with the "revA" upgrades. Given the errors that already exist in the S1 gain functions, even this situation is not critical for most users. ACIS-S imaging observations during 2014-02-01 through 2014-10-31 will not frequently include S1 as an active chip due to its off-axis position. In fact we expect that only GRADED GRATING/ACIS-S observations may have their order-sorting on S1 affected noticeably. Therefore users of GRADED GRATINGS/ACIS-S observations taken over the 01 Nov 2014 through 31 Oct 2014 might consider reprocessing with the new "_revA" T_GAINs and checking their order-sorting and analysis results.
Most locations on the FI chips are not signficantly affected by the "revA" updates. Many will be identical with the original files. We give plots below of the T_GAINs from the original versus the "revA" files, pairing Epochs 56 and 57 in Figs 5 and 6, and Epochs 58 and 59 in Figs 7 and 8. The epoch 56 original T_GAINs are plotted in red, while the corresponding "revA" T_GAINs are plotted in green. For epoch 57, the original T_GAINs are ploted in blue-dash, and the "revA" data are plotted as black-dots. This same pattern is maintained in Figures 5 through 24 below.
For the aimpoints cases (Figs 5 and 7), it is clear that the "revA" updates have very little effect, and hence may be ignored. For much of ACIS-I3, the "revA" files in fact introduce zero correction. However, for the worst case examples (near the center of I3, Figs 6 and 8), one can see that the T_GAIN corrections are more significantly affected. However we point out that in all cases, the overall correction is less than 2% of the net PHA or Energy values (i.e. less than 200 eV correction out of 10 keV photon energy), so that even a 50% relative change in the T_GAIN would result in less than 1% change in the photon energy.
The effect on the other FI chips due to the "revA" upgrades are similar to those illustrated on I3.
For ACIS-S3 with CTI-correction applied:
Similar to what we plotted for ACIS-I3 above, is what we've plotted for the CTI-corrected ACIS-S3 below. See Figures 9-12.
Although every value of the T_GAIN corrections for S3 does change between the original and the revA files, the effects on the S3 CTI-corrected case are actually the most mild of all, even in the worst-case examples. Hence for ACIS-S3 the "revA" updates may essentially be ignored.
For ACIS-S1 with CTI-correction applied:
Following the same illustration scheme as with I3 and S3 above3, we introduce the CENTER of ACIS-S1 as a replacement for the "aimpoints" as the standard position on the chip, and then plot the worst-case, which occurs over the CHIPX=129-256 columns. However, the results are very similar to what occurs with ACIS-S3 with CTI correction on: no significant change with the "revA" files.
For ACIS-S3 with no CTI-correction:
Now to consider the affects of the "revA" updates on ACIS-S3 when CTI-correction is NOT applied, such as with GRADED datamode observations. With the aimpoint positions, there are somewhat larger affects from the "revA" updates, however the changes are quite insignificant even in the T_GAINs themselves.
In the worst case locations (the CHIPX=65-96 columns) show still slight changes for Epochs 56 and 57. However we begin to see significant changes in T_GAIN beyond about 5 keV. None of these would give changes in the PHA or Energy values greater than 0.5% of the value. Hence, for ACIS-S3 these "revA" updates may be ignored by users, regardless of CTI correction.
For ACIS-S1 with no CTI-correction:
For this case, S1 shows very little change in the T_GAINs mid-chip, but does show some significant variations in the worst-case location, specifically the columns with CHIPX=65-96, for Epochs 57-59. Since these results apply only to GRADED datamode observations, which are most frequently GRATING observations when S1 is included, we might recommend users of these observations reproces their data with the "revA" T_GAIN files. The affected DATE-OBS values would be between 2014-02-01 and 2014-11-01. However, for GRATINGs observations with ACIS, the order-sorting is the only relevant processing step.