IRAC Instrument Handbook - IRSA - California Institute of Technology

Point Source Fitting IRAC Images
with a PRF
163
IRAC Instrument Handbook
A much more effective solution is to use aperture photometry for such sources. The SWIRE survey
performed detailed analyses to determine an "ideal" extraction aperture such that it minimized noise. This
aperture was 1.9 arcseconds in radius, or roughly twice the FWHM. Most other survey groups have found
similar results, and this mirrors well-known ideas about aperture photometry of small sources. When such
an aperture is used, even though some objects may be larger than this the number where the flux differs
by a factor of 2 falls to only 3%. This improvement over the PSF-fitting reflects the fact that the
summation over an aperture larger than the PSF FWHM will always capture a better representation of the
true flux of an extended object, even if that is more extended than the aperture itself. A more ideal
solution is to use Kron-like apertures (which are dynamically sized based on moments derived from the
image) which are either derived from the data themselves or from image priors in some other band.
We may thus conclude that for the extragalactic background, which is present in nearly all IRAC data, at
least half the objects are resolved by IRAC in a meaningful fashion. Ideally, measurements should
dynamically use shape information determined from the data themselves, or from priors derived from
other, higher resolution datasets. Barring the use of shape parameters, use of aperture photometry in
circular apertures somewhat larger than the PSF provides a more accurate result than PRF fitting.
C.3.6 Positional Accuracy
Tests were performed on GLIMPSE AORKEY 9225728, which contained approximately 10000 point
sources in channels 1 and 2. Comparisons were made with respect to SExtractor Gaussian-windowed
centroids XWIN_WORLD, YWIN_WORLD using both the pipeline mosaics, and mosaics made with the
original pointing. Using the 100x oversampled PRFs recentered as previously described we found that the
source positions agreed with SExtractor to within ~0.1". Systematic shifts with respect to 2MASS are
~0.2" in the pipeline (superboresight) pointing, and ~0.4" in the original pointing. Recentering the PRF
has no effect on photometry. The shifted and unshifted PRFs gave nearly identical photometric results in
channels 1–4.
C.3.7 A How-To-Guide for IRAC Point Source Photometry with APEX
It is recommended that APEX in point source fitting mode should be used only directly on the BCD data
using the Hoffmann PRFs modified for use with APEX as described above. Trying to fit point sources on
the mosaic is not recommended as the mosaicking process both blurs the undersampled point sources, and
loses the pixel phase information. We also do not recommend using the prf_estimate tool to derive a PRF
from IRAC data, as it does not deal correctly with the undersampling of the PRF.
We list below the steps towards producing a point source list using APEX in multiframe mode (i.e., on
the stack of individual [C]BCDs).
1. Source fitting versus aperture fluxes: ask yourself if point source photometry is appropriate for
your sources of interest. If in doubt after reading about photometry of moderately resolved
sources above, use aperture photometry with APEX or Sextractor.