The Atacama Cosmology Telescope: Measurement and Analysis of 1D Beams
  for DR4
  We describe the measurement and treatment of the telescope beams for the
Atacama Cosmology Telescope's fourth data release, DR4. Observations of Uranus
are used to measure the central portion (<12') of the beams to roughly -40 dB
of the peak. Such planet maps in intensity are used to construct azimuthally
averaged beam profiles, which are fit with a physically motivated model before
being transformed into Fourier space. We investigate and quantify a number of
percent-level corrections to the beams, all of which are important for
precision cosmology. Uranus maps in polarization are used to measure the
temperature-to-polarization leakage in the main part of the beams, which is <1%
(2.5%) at 150 GHz (98 GHz). The beams also have polarized sidelobes, which are
measured with observations of Saturn and deprojected from the ACT time-ordered
data. Notable changes relative to past ACT beam analyses include an improved
subtraction of the atmospheric effects from Uranus calibration maps,
incorporation of a scattering term in the beam profile model, and refinements
to the beam model uncertainties and the main temperature-to-polarization
leakage terms in the ACT power spectrum analysis.
