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Note, though, that we are conflating at least three 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 issues and the two tests below.
Three potential problems in xtal asymmetry maps
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Sasha has demonstrated that xtal response maps evolve systematically in the direction of increasing asymmetry (= increasing taper = increasing light attenuation) with time as light yield declines. This is consistent with our understanding and ground measurement of radiation damage in CsI(Tl), and the magnitude of the decline in light yield (~1% per year) is consistent with our pre-launch calculation.
See https://confluence.slac.stanford.edu/display/CAL/Time + evolution + of + CAL +asymmetry+calibration
in asymmetry maps in the "(C&A 21 June please look at the following section second)" heading.
The The change in the slope of the xtal response with time is non-trivial in many xtals.
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(2) Poor map algorithm in the penultimate slice.
See https://confluence.slac.stanford.edu/display/CAL/Time + evolution + of + CAL +asymmetry+calibration
at asymmetry maps at the "(For C&A 21 June, look at the plot below first)" heading for a description of the problem.
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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.
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.
(a)
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Use the on-orbit xtal maps derived for the epoch of the start of the AGN skim (or epsilon before) to analyze the entire year of the skim.
Here we use the on-orbit xtal maps, i.e. maps created with the fixes to the mapping algorithm, to reconstruct the entire year of data. During that time, radiation damage to the xtals will have caused the actual response to have deviated from the maps by of order a few mm, on average, near the xtal ends.
(Of course, we could instead use maps from just after launch to increase the discrepancy. Perhaps we should do that in an additional test.)
I suspect we'll see better agreement with MC CTBCORE and PSF, with still some imperfection, and that neither quantity will evolve with time (which is what Marshall sees for PSF with ground maps).
(b)
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Use 4 sets (i.e. say, one new set of maps every 3 months) of xtal maps as appropriate in time to analyze the skim.
Here we use on-orbit xtal maps appropriate for each epoch. If the PSF is improved relative to test (a), then we will have demonstrated the need to map every 3 months or so. Otherwise, annual updates will likely be adequate.
Johan B created a skim of AGN photons that spans June 2009 - June 2010, but the pipeline processing of GCR data from which xtal maps are made stopped in November 2009. However, until the GCR data processing restores those data, we can't make maps that span the full epoch of Johan's skim.
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Original email
This page is generated from an email I sent on 5 August 2010. The original email is below.
On 5 Aug 2010 (Day 217), at 11:57 AM PDT, J. Eric Grove wrote:
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