fluximage & merge_obs backgrounds for acis

[kglotfelty]

I forget the reasoning for only creating backgrounds for HRC-I, but would it be possible now with the blanksky and blansky_image script to create per-obi and combined blank sky background files for ACIS?

[DougBurke]

I think the main issue is that for the HRC case we have - if memory serves - only the channel range as a choice when creating the background estimate. For the ACIS case it's a bit-more involved [*], so I've never sat down to look at what options we could provide/allow the user to tweak.

[*] this is not to say that the HRC background reconstruction is perfect, modulo the choice of PI range, just that there's been less Maxim-style write ups about it.