Enhancement of the Spitzer Infrared Array Camera Distortion Correction for Parallax Measurements

The Spitzer Space Telescope Infrared Array (IRAC) offers a rare opportunity to measure distances and determine physical properties of the faintest and coldest brown dwarfs. The current distortion correction is a 3 order polynomial represented by TAN-SIP parameters within the headers. The current correction, good to 100 mas, was derived from deep imaging, using marginally resolved galaxies in some cases, and has remained stable throughout both the cryogenic and warm mission. Using recent Spitzer calibration observations mapped to HST/ACS calibration observations of 47 Tuc with an absolute accuracy good to 1 mas, we are working towards a possible 5 order polynomial correction that theoretically could allow measurements to within 20 mas. Extensive testing, using observations of 47 Tuc, NGC 6791 and NGC 2264, are underway, after which the new parameters will be used to update all the 3.6 and 4.5um data taken within warm and cryogenic missions. We anticipate if achievable, this new accuracy could be combined with other ongoing enhancements (Ingalls et al, 9143-52) that will permit measurements of parallaxes out to about 50 pc, increasing the volume surveyed by a factor of 100, and enabling new capabilities such as luminosity measurements of the population of young brown dwarfs in the beta Pictoris moving group.