Below I have some quadratic fits for ACIS-I & ACIS-S at several different
energies for 50 & 80 percent encircled energies. I made these for some
simulations which they worked fairly well for.
These are rough but seemed to fit the data (from psflib) pretty well. So use
with normal caveats.
If I remember correctly X is in arcmin and the result is in pixels. I have a
plot I made for the HEAD meeting if you want an idea how "good" the fits were.
80% ACIS-S
energy CX^2 + BX + A
0.2770 0.0981 -0.2655 1.1952
1.4967 0.0994 -0.2768 1.2821
4.5100 0.0952 -0.1809 1.3138
6.4000 0.0956 -0.1764 1.6154
8.6000 0.0923 -0.1298 2.3854
ACIS-I
energy CX^2 + BX + A
0.2770 0.0732 0.1730 -0.1675
1.4967 0.0729 0.1760 -0.0830
4.5100 0.0705 0.2416 0.0254
6.4000 0.0680 0.3054 0.2205
8.6000 0.0669 0.3122 1.1315
50%
ACIS-I
energy CX^2 + BX + A
0.2770 0.0686 -0.2549 0.4933
1.4967 0.0684 -0.2513 0.5199
4.5100 0.0666 -0.1922 0.4626
6.4000 0.0658 -0.1724 0.5018
8.6000 0.0638 -0.1165 0.4968
ACIS-S (worst)
energy CX^2 + BX + A
0.2770 0.0661 -0.2091 0.7335
1.4967 0.0655 -0.1952 0.7421
4.5100 0.0644 -0.1628 0.7487
6.4000 0.0631 -0.1238 0.7448
8.6000 0.0615 -0.0798 0.7596
On Thursday 02 August 2001 05:10 pm, Adrienne Cool wrote:
> 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
--
John Grimes - Chandra Mission Planning
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