The Atacama Cosmology Telescope: Measurement and Analysis of 1D Beams for DR4

TL;DR

This paper details the precise measurement and modeling of the Atacama Cosmology Telescope's beams for DR4, including polarization leakage and sidelobe characterization, crucial for accurate cosmological data analysis.

Contribution

It introduces improved beam measurement techniques, a refined physical beam model, and enhanced calibration methods for the ACT DR4 data release.

Findings

Beam measurements achieve -40 dB accuracy in the central region.

Temperature-to-polarization leakage is quantified as <1% at 150 GHz.

Polarized sidelobes are characterized and deprojected from data.

Abstract

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.