Point source fitting of IRAC images using a PRF

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Use of the 5x sampled IRAC PRFs outside of APEX

As supplied, 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..

Where i,j are integers running from 1-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 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 in the IRAC Data handbook. Estimates from the PRF are unavailable in channels 3 and 4, but the corrections given in the Data Handbook are 0.772 and 0.737, respectively.