Improving the photometric precision of IRAC Channel 1

Planning is underway for a possible post-cryogenic mission with the Spitzer Space Telescope. Only Channels 1 and 2 (3.6 and 4.5 μm) of the Infrared Array Camera (IRAC) will be operational; they will have unmatched sensitivity from 3 to 5 microns until the James Webb Space Telescope is launched. At SPIE Orlando, Mighell described his NASA-funded MATPHOT algorithm for precision stellar photometry and astrometry and presented MATPHOT-based simulations that suggested Channel 1 stellar photometry may be significantly improved by modeling the nonuniform RQE within each pixel, which, when not taken into account in aperture photometry, causes the derived flux to vary according to where the centroid falls within a single pixel (the pixel-phase effect). We analyze archival observations of calibration stars and compare the precision of stellar aperture photometry, with the recommended 1-dimensional and a new 2-dimensional pixel-phase aperture-flux correction, and MATPHOT-based PSF-fitting photometry which accounts for the observed loss of stellar flux due to the nonuniform intrapixel quantum efficiency. We show how the precision of aperture photometry of bright isolated stars corrected with the new 2-dimensional aperture-flux correction function can yield photometry that is almost as precise as that produced by PSF-fitting procedures. This timely research effort is intended to enhance the science return not only of observations already in Spitzer data archive but also those that would be made during the Spitzer Warm Mission