Point source fitting of IRAC images using a PRF

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Positional accuracy

Tests were performed on GLIMPSE AOR 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.