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Note, though, that we are conflating at least three four issues here, and we need to think a bit before we assign root cause and create a plan for future delivery of calibration constants. I think we need to do at least two versions of the measurement of PSF with AGN.
I describe the three four issues and the two tests below.
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Four potential problems in xtal asymmetry maps
The three four issues are:
- Time evolution of xtal response maps.
- Poor map algorithm in the penultimate slice (i.e. slices 1 and 10 of 0 to 11).
- Coarse binning of the small/small response ratio.
- Biased ratio from FHE crosstalk when only one end fires FHE.
More details on these issues follow.
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Note that I also think it was a mistake to train CTBCORE on an MC dataset that had the same xtal maps used for MC generation and MC recon. Recon used longitudinal position accuracy that the actual CAL cannot possibly provide. I wouldn't be surprised to learn that this contributes significantly to the discrepancy between observed and MC CTBCORE, but that's another discussion.
(3) Coarse binning of the small/small response ratio.
The xtal response maps for the small-versus-small diode (i.e. the HE v. HE map) were created from fitting binned plus-over-minus signal ratios, but the binning was a bit coarse. The on-orbit maps are being created by fits to finer binning in the ratio. This is probably a minor effect, but it is a change from the ground maps.
(4) Biased ratio from FHE crosstalk when only one end fires FHE.
We know that when FHE fires, some small signal is coupled into the slow-shaped spectroscopy channel. As measured with the LCI calibration scripts, it has a small effect (<1%) on the energy read out from the GCFE, and we've decided to ignore it in the front-end linearity calibration constants. When FHE fires in both GCFEs (i.e. at both ends) of a xtal, the longitudinal position calculated in the xtal is unaffected, since the crosstalk is very similar in all channels.
Note, though, that it can have a larger effect on the calculated longitudinal position if only one end of the xtal has FHE firing. This can happen near one end of a xtal, where because of the light taper, the optical signal at the near end is larger than at the far end.
Tests to perform on an AGN skim or hard pulsar skim
I suggest that we need at least two test measurements of the observed PSF. Those tests are designed to separate issue (1) from (2)+(3), i.e. the time-evolution from the algorithm issues. Because we do not have maps with the crosstalk removed, we don't yet have a test for the effect of (4).
If it's important to improve the purity of the photon data sample in order to see changes in PSF with good sensitivity, I suggest we try a skim of hard (>few GeV) photons from bright pulsars and look only in the on-pulse phase intervals.
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