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Events to be overlaid are taken from periodic triggers in the data files. These are grouped into collections according to some distribution, where Mark Strickman and Eric Grove advocate the following:
Divide large sample of real periodic triggers into McIlwain L bins (size of bins left as an exercise for the reader). Why McIlwain L? Because background is generally more linear in McIlwain L than in rigidity or magnetic latitude. Perform a pedestal subtraction on all the xtal channels using cal peds. This will of course result in both positive and negative signals.
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