The Atacama Cosmology Telescope: Beam Measurements and the Microwave
  Brightness Temperatures of Uranus and Saturn
  We describe the measurement of the beam profiles and window functions for the
Atacama Cosmology Telescope (ACT), which operated from 2007 to 2010 with
kilo-pixel bolometer arrays centered at 148, 218, and 277 GHz. Maps of Saturn
are used to measure the beam shape in each array and for each season of
observations. Radial profiles are transformed to Fourier space in a way that
preserves the spatial correlations in the beam uncertainty, to derive window
functions relevant for angular power spectrum analysis. Several corrections are
applied to the resulting beam transforms, including an empirical correction
measured from the final CMB survey maps to account for the effects of mild
pointing variation and alignment errors. Observations of Uranus made regularly
throughout each observing season are used to measure the effects of atmospheric
opacity and to monitor deviations in telescope focus over the season. Using the
WMAP-based calibration of the ACT maps to the CMB blackbody, we obtain precise
measurements of the brightness temperatures of the Uranus and Saturn disks at
effective frequencies of 149 and 219 GHz. For Uranus we obtain thermodynamic
brightness temperatures T_U^{149} = 106.7 \pm 2.2 K and T_U^{219} = 100.1 \pm
3.1 K. For Saturn, we model the effects of the ring opacity and emission using
a 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.
