The Infrared Array Camera (IRAC) on the Spitzer Space Telescope is absolutely calibrated by comparing photometry of a set of A stars near the north ecliptic pole to predictions based on ground-based observations and a stellar atmosphere model. The brightness of point sources is calibrated to an accuracy of 3%, relative to models for A-star stellar atmospheres, for observations performed and analyzed in the same manner as for the calibration stars. This includes corrections for the location of the star in the array and the location of the centroid within the peak pixel. Long-term stability of the IRAC photometry was measured by monitoring the brightness of A dwarfs and K giants (near the north ecliptic pole) observed several times per month; the photometry is stable to 1.5% (rms) over a year. Intermediate-timescale stability of the IRAC photometry was measured by monitoring at least one secondary calibrator (near the ecliptic plane) every 12 hr while IRAC was in nominal operations; the intermediate-term photometry is stable, with a 1% dispersion (rms). One of the secondary calibrators was found to have significantly time-variable (5%) mid-infrared emission, with a period (7.4 days) matching the optical light curve; it is possibly a Cepheid variable.
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