On Wed, 01 Nov 2000 10:52:46 -0500, Pat Broos <firstname.lastname@example.org> said:
>A useful upgrade to mkarf might be to have it figure out for itself which
>CCD's contribute to a given sky position and produce N ARFs.
This would require specifying 10 aspect histograms to mkarf, since
each CCD has its own distinct set of GTIs.
>* When generating an ARF for any single CCD, if the sky position you specify
>falls on a part of that CCD's exposure map that is changing rapidly on spatial
>scales comparable to your source extraction radius (e.g. 10's of pixels) then
>a paranoid observer might not be satisfied with sampling the "shoulder" of the
>"exposure map" at a single (dithered) point when the extraction region is a
>big circle. For example, one could theoretically have a fat off-axis point
>source whose sky position never actually touches CCDn, even though lots of its
>photons fell on CCDn. In that case the ARFn would be zero. Maybe it all
>comes out in the wash, but my intuition is to cover the extraction region
>(circle) with a bunch of ARF's and then average them (a crude form of
>integration). I'm currently using addarf to perform this average.
Until the PSF fraction library becomes available (and reliable), you
will have to extract counts on a region of the sky that contains the
PSF, in which case the PSF fraction will be 1 as is currently assumed.
Keep in mind that the use of PSF fraction in the ARF is meaningful
only under the circumstances that are in the appendix of the paper I
wrote upon the subject (which I need to upload to astro-ph). I
believe that the case that you are talking about invalidates this
assumption. In other words, even if we were to incorporate a PSF
fraction into the ARF, it would not help for the case that you
describe. As Jonathan suggested, you are probably better off
performing a ray trace.
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