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We describe the measurement of the beam profiles and window functions for theAtacama Cosmology Telescope (ACT), which operated from 2007 to 2010 withkilo-pixel bolometer arrays centered at 148, 218, and 277 GHz. Maps of Saturnare used to measure the beam shape in each array and for each season ofobservations. Radial profiles are transformed to Fourier space in a way thatpreserves the spatial correlations in the beam uncertainty, to derive windowfunctions relevant for angular power spectrum analysis. Several corrections areapplied to the resulting beam transforms, including an empirical correctionmeasured from the final CMB survey maps to account for the effects of mildpointing variation and alignment errors. Observations of Uranus made regularlythroughout each observing season are used to measure the effects of atmosphericopacity and to monitor deviations in telescope focus over the season. Using theWMAP-based calibration of the ACT maps to the CMB blackbody, we obtain precisemeasurements of the brightness temperatures of the Uranus and Saturn disks ateffective frequencies of 149 and 219 GHz. For Uranus we obtain thermodynamicbrightness temperatures T_U^{149} = 106.7 \pm 2.2 K and T_U^{219} = 100.1 \pm3.1 K. For Saturn, we model the effects of the ring opacity and emission usinga simple model and obtain resulting (unobscured) disk temperatures of T_S^{149}= 137.3 \pm 3.2 K and T_S^{219} = 137.3 \pm 4.7 K.

The Atacama Cosmology Telescope: Beam Measurements and the Microwave Brightness Temperatures of Uranus and Saturn