On Wed, 01 Nov 2000 10:52:46 -0500, Pat Broos <patb@pokum.astro.psu.edu> said:
>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
I am not sure that this would happen in practice unless the size of
the PSF is appreciably larger than the size of the dither pattern on
the detector (about 20 arc-secs). That is, you would have to go
off-axis to a point at which the PSF becomes larger than 20
arc-seconds. I do not know what off-axis angle this would imply.
The two main assumptions made by mkarf are:
1. The size of the dither pattern is smaller than the size of the
PSF.
2. The off-axis effective area does not vary appreciably over the
dither pattern.
Finally, until the PSF fraction is incorporated into mkarf, it is
assumed that the spatial extraction region (sky coordinates) includes
all of the PSF.
--John
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