Why are there instrument maps over different channel ranges? What assumptions go into using exposure maps created from these instrument maps?
The exposure maps are meant to correct for two effects -- vignetting, and shadowing by the grid wires. Both of these effects depend on energy. Shadowing by wires increases with decreasing energy, and is especially important below channel 40. Changes in vignetting also can be important, especially at higher energies. Each instrument map is an average of these effects over the respective bandpass.
In using exposure maps constructed from these instrument maps, two assumptions are made: 1) that the background counts used to construct the instrument map uniformly illuminate the detector, and 2) that the spectrum of your source matches that of the average diffuse background (i.e., averaged over the entire sky). Because the instrument map is constructed of many scans over the whole sky, the first assumption is very good, especially after bright point sources and bright extended features have been excluded. (Each scan represents only 1/1800./15. of the total.) How well the spectrum of your source matches that of the average diffuse background determines how well the average of vignetting and wire shadowing over your source spectrum will match that in the instrument map.
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