polynomial appx. to off-axis ACIS-I PSF?

From: Adrienne Cool (cool@quark.sfsu.edu)
Date: Thu Aug 02 2001 - 17:10:09 EDT

Hi folks,

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
exist.

Thanks much,

Adrienne

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