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Point source fitting of IRAC images using a PRF

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Modifications to the IRAC PRFs for use with APEX

The IRAC PRFs are centered relative to the optic axis, so are slightly off center in array coordinates due to array distortion. APEX assumes that the PRF is centered on its array, so to use the PRFs with APEX requires them to be recentered. APEX also requires odd-valued axes.

APEX performs PRF fitting by varying the position and flux of a source using a modified simplex technique (see the APEX reference manual). However, for IRAC data, particularly in channels 1 and 2, where the PRF is undersampled, the default x5 sampling of the PRF is insufficient to obtain a sufficiently accurate position for fitting.

Therefore the following transformations were applied to the PRFs:

i) The PRFs were magnified (using linear interpolation) by a factor of 20 (so the resultant PRF sampling is x100).

ii) The last row and column were removed to give odd-valued axes.

iii) The PRF was recentered on a first-moment centroid measured using the array values within a 250 (resampled) pixel border.

iv) The PRF was zeroed out in a 50 (resampled) pixel border (to avoid wrapping problems).

v) Information describing the PRFs and their modifications was added to the headers.
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This file was last modified on Tue Aug 26 13:46:13 2008.

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