Adrienne -- Some of us at the Penn State ACIS Team had to address
the problem of point source extraction across the field for the
Orion Nebula field which has >1000 sources. Below is the relevant
paragraph of our paper on what we did, and on linear/quadratic
approximations to the off-axis PSF that might be useful to others.
(Note that \theta is in arcmin and R_xtr is in arcsec not pixels.)
We didn't feel that a very precise fit is needed, as there is not
much error in whether one corrects an ACIS flux by 1/0.95 or 1/0.94.
Statistical and spectral uncertainties are much larger than this
PSF fitting uncertainty. Eric
For most sources, we chose to extract events from the 95\% encircled
energy radius as a function of off-axis angle, based on the PSF of a
1.49 keV monochromatic source\footnote{These 95\% encircled energy
radii were calculated using the CIAO program mkpsf and are consistent
with those obtained with the detailed raytraces using the SAOSAC model
for the Chandra mirror assembly (P. Zhao, private communication). It is
reasonably well-approximated by the quadratic function $R(95\%EE) =
2.05 -0.55 \theta + 0.18 \theta^2$. The 99\% encircled energy radii
used for bright sources is approximately $R(99\% EE) = 8 + 0.2 \theta$
and the 50\% radii used for nondetections in \S \ref{limits_sec} is
approximately $R(50\%EE) = 0.43 - 0.10 \theta + 0.050 \theta^2$.
\label{radius_xtr_footnote}}. For very bright sources with $>1000$
counts, the extraction radius was increased to around the 99\% curve,
as the benefit of increased source photons exceeds the increase in
background. The radius was reduced below the 95\% curve for nearly 200
sources principally due to source crowding. For each source, we
extract counts $C_{xtr}$ in the total $0.5-8$ keV band from within
radius $R_{xtr}$. These are the events used in all later spectral and
variability analysis. For all cases, we calculate the corresponding
PSF fraction $f_{xtr}$ using CIAO program mkpsf.
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