A recent paper by Giacconi et al '01 (astro-ph 0007240) gives a
3rd-order polynomial approximation to the FWHM of the PSF for
ACIS-I as a function of off-axis angle.
They reference asc.harvard.edu/udocs/docs/docs.html, which is
the Proposer's Observatory Guide, but I can't find any statement
there about a polynomial approximation.
Does anyone know where this approximation might have come from?
Also, does anyone know of any similar approximations for encircled
energy radii off-axis for ACIS-I? (e.g., 50, 80 or 90%)
I have been using mkpsf in ciao to determine encircled energies
off axis, but am intersted to know if analytical approximations
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