IRAC Instrument Handbook - IRSA - California Institute of Technology

Point Source Fitting IRAC Images
with a PRF
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IRAC Instrument Handbook
where p is the radial pixe l phase, defined as the distance of the centroid of the stellar image from the
1
center of its peak pixel. This corrects to an average pixel phase of p = ≈ 0.
4 pix.
2π
The average PRF-fitted fluxes compared to aperture photometry are shown in Figure C.1. The weighted
average differences between PRF fluxes and (corrected) aperture fluxes are shown as long blue dashes.
There are offsets in all four channels between the aperture and fitted fluxes. In IRAC channels 3 and 4,
the offset is due to the fact that in these channels, the PSFs are wide and there is significant flux in the
12–20 pixel background annulus subtracted out in the IRAC calibration. APEX does not know about this
in its PRF normalization, so the PRF fluxes are too high. We examined the "core" PRFs and estimated
this factor. The estimated effect of the annulus on the PRF fluxes is shown in Fig. C.1 as black, short
dashes. These are within 1% of the IRAC channel 3 and 4 estimates from the calibration stars. For IRAC
channels 1 and 2, these annulus terms appear to be small, so we assume zero correction for the present
time. The annulus correction factors (divide PRF fluxes by these) are 1.022 for IRAC channel 3, and
1.014 for IRAC channel 4 (Table C.1).
C.3.2 Subpixel Response in Channels 1 and 2
The offset for IRAC channel 1 in Figure C.1 is due to a completely different effect, namely the pixel
phase effect described above. Aperture sums on the channel 1 IRAC PRFs match reasonably well the
pixel phase relation in Eqn. C.1 if we sum a 10 pixel radius aperture.
APEX performs normalization on the ''center-of-pixel'' (pixel phase [0,0]) PRF, and applies this
normalization factor to all sub-pixel positions. This results in an offset of the photometry relative to the
1
mean pixel phase of p = . We need to ''back out'' APEX's center normalization. Setting p=0 in Eqn.
2π
C.1 gives us the required factor: divide the PRF fluxes by 1.021. Similarly, using the pixel phase slope of
0.0301 in IRAC channel 2 leads to a correction factor of 1.012.
With these corrections, the PRF fitting on single CBCDs matches aperture results with any systematics
less than a percent in all IRAC channels (Fig. C.2). The remaining scatter is most likely due to residual
pixel phase effect not removed by the one-dimensional correction applied to the aperture photometry. The
true pixel phase effect has two dimensional structure which is included in the PRF (see also Mighell et al.
2008, [20]).
(C.1)