Some of the instrument maps cover the channel ranges of other maps, for example, band 10 covers channels 52 to 90, band 4 covers channels 52 to 69, and band 5 covers channels 70 to 90. Is the band 10 instrument map just the same as the average of the band 4 and band 5 instrument maps? If so, then why have a map for band 10?
The map for band 10 is essentially the same as the average of the maps for bands 4 and 5. Whether it is acceptable to use band 10 or not depends on how closely your source spectrum matches that of the average diffuse background. It is more accurate to apply the exposure correction over small bandpasses separately, and then weight by the numbers of counts in each bandpass. For example, you could construct an exposure map over band 10 for your source by constructing maps over bands 4 and 5, weighting by the observed counts in bands 4 and 5, and then averaging.
Spectral differences are especially important at the highest and lowest energies. For example, vignetting changes significantly over the channel range 132 to 201, and if your source spectrum is different from the average spectrum, then there could be important differences between the calculated exposure map and the map that should be applied.
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