IRAC Instrument Handbook - IRSA - California Institute of Technology

C.1 Use of the Five Times Oversampled PRFs Outside of APEX
Point Source Fitting IRAC Images
with a PRF
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IRAC Instrument Handbook
As supplied in the documentation website, the PRFs are oversampled by a factor of five in Delta_x and
Delta_y. This allows for 5x5=25 independent realizations of a point source, corresponding to 25 different
pixel phase combinations (five each in x and y). To obtain any given point source realization (PSR), the
PRF needs to be sampled every fifth pixel in x and y at the appropriate phase, i.e.,
PSR1(i,j) = PRF(5i-4,5j-4)
PSR2(i,j) = PRF(5i-4,5j-3)
PSR3(i,j) = PRF(5i-4,5j-2)
PSR4(i,j) = PRF(5i-4,5j-1)
PSR5(i,j) = PRF(5i,5j)
PSR6(i,j) = PRF(5i-3,5j-4)
…………………………….
PSR13(i,j) = PRF(5i-2,5j-2)
PSR25(i,j) = PRF(5i,5j)
where i,j are integers running from 1 to n in the case of a PRF table which is 5n x 5n in size. In this case,
PSR13 corresponds to the source landing in the center of a pixel. Note that the PRF should not be block
averaged, as this will result in the loss of the pixel phase information.
These PRFs may be implemented directly by those willing to write their own code. In IDL, for example, a
point source realization may be generated using the /SAMPLE switch in REBIN, e.g,
psr = rebin(phasedPRF,n,n,/SAMPLE)
where phasedPRF is the 5n x 5n PRF shifted to the appropriate pixel phase in both dimensions. In IRAF,
use:
imcopy PRF.fits[1:5n-4:5,1:5n-4:5] PSR1.fits
Note that the PSRs are normalized to unity at infinity, not to the IRAC 10 pixel calibration aperture.
Fluxes obtained with these thus need to be multiplied by the appropriate infinite aperture correction.
These have been determined to be 0.943 in channel 1 and 0.929 in channel 2, based on measurements of
the PRF, and can be compared to "direct" measurements of 0.944 and 0.937. Estimates from the PRF are
unavailable in channels 3 and 4, but the corrections given in this Handbook are 0.772 and 0.737,
respectively (Table 4.7).
C.2 Modifications to the IRAC PRFs for Use with APEX