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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. See the plot below, which is the average over all crystals, where Sasha finds evidence for a bias of up to a few mm in the segments near the ends of the average xtal near the 1 GeV FHE threshold. Note that the bias is in the direction we expect: it is positive for segment 10, where the signal is typically ~30% bigger than the signal at the negative end (and vice versa in segment 1).
In these plots, Sasha has converted the measured asymmetry to longitudinal offset using the average xtal asymmetry slope (as usual). The segments (slices, whatever) are labeled 0 to 11, where 0 is closest to the negative face and 11 is closest to the positive face.
Tests to perform on an AGN skim or hard pulsar skim
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