Twoyear Cosmology Large Angular Scale Surveyor (CLASS) Observations: 40 GHz Telescope Pointing, Beam Profile, Window Function, and Polarization Performance
Abstract
The Cosmology Large Angular Scale Surveyor (CLASS) is a telescope array that observes the cosmic microwave background (CMB) over 75% of the sky from the Atacama Desert, Chile, at frequency bands centered near 40, 90, 150, and 220 GHz. CLASS measures the large angular scale (1° ≲ θ ≤ 90°) CMB polarization to constrain the tensortoscalar ratio at the r ∼ 0.01 level and the optical depth to last scattering to the sample variance limit. This paper presents the optical characterization of the 40 GHz telescope during its first observation era, from 2016 September to 2018 February. High signaltonoise observations of the Moon establish the pointing and beam calibration. The telescope boresight pointing variation is <0°023 (<1.6% of the beam's full width at half maximum (FWHM)). We estimate beam parameters per detector and in aggregate, as in the CMB survey maps. The aggregate beam has an FWHM of 1°579 ± 0°001 and a solid angle of 838 ± 6 μsr, consistent with physical optics simulations. The corresponding beam window function has a subpercent error per multipole at ℓ < 200. An extended 90° beam map reveals no significant far sidelobes. The observed Moon polarization shows that the instrument polarization angles are consistent with the optical model and that the temperaturetopolarization leakage fraction is <10^{4} (95% C.L.). We find that the Moonbased results are consistent with measurements of M42, RCW 38, and Tau A from CLASS's CMB survey data. In particular, Tau A measurements establish degreelevel precision for instrument polarization angles.
 Publication:

The Astrophysical Journal
 Pub Date:
 March 2020
 DOI:
 10.3847/15384357/ab76c2
 arXiv:
 arXiv:1911.04499
 Bibcode:
 2020ApJ...891..134X
 Keywords:

 Astronomical instrumentation;
 Cosmic microwave background radiation;
 Early universe;
 Observational cosmology;
 Polarimeters;
 The Moon;
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 322;
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 Astrophysics  Instrumentation and Methods for Astrophysics
 EPrint:
 32 pages, 24 figures, published in ApJ