Correcting distortions in the infrared array camera during the cryogenic mission of the Spitzer Space Telescope

Abstract

We describe our ongoing efforts to model the field distortions of the Infrared Array Camera (IRAC) during the cryogenic portion of the Spitzer Space Telescope’s operations. We have compared over two million measured source positions in ~35,000 IRAC images with their positions in Gaia Data Release 1. Fitting 3rd and 5th order polynomials to the measured offsets, we find systematic uncertainties in IRAC-measured positions that are in the 50-60 milliarcsecond range for the 3.6 micron array, and 120-150 milliarcsecond range for the 4.5 micron array. A 5th-order fit does not appear to significantly improve the results over a 3rd order fit. However, this may be due at least partly to the failure of our current centroiding technique to account for variations in the Point Response Functions across each detector. We anticipate making several improvements in our continuing analysis, including (i) the refitting of the positions and position angles of each IRAC image using the Gaia catalog, (ii) making use of a less position-sensitive centroiding algorithm, (iii) correcting where possible for the proper motions of detected sources, and (iv) significantly increasing the number of source position measurements. Once finalized, the resulting distortion corrections will be incorporated into the headers of the archived images.