Two-year cosmology large angular scale surveyor (CLASS) observations: 40 GHz telescope pointing, beam profile, window function, and polarization performance
Date
2020-03-10Author
Xu, Zhilei
Brewer, Michael K.
Rojas, Pedro Fluxá
Li, Yunyang
Osumi, Keisuke
Pradenas, Bastián
Ali, Aamir
Appel, John W.
Bennett, Charles L.
Bustos Placencia, Ricardo
Chan, Manwei
Chuss, David T.
Cleary, Joseph
Couto, Jullianna Denes
Dahal, Sumit
Datta, Rahul
Denis, Kevin L.
Dünner, Rolando
Eimer, Joseph R.
Essinger-Hileman, Thomas
Gothe, Dominik
Harrington, Kathleen
Iuliano, Jeffrey
Karakla, John
Marriage, Tobias A.
Miller, Nathan J.
Núñez, Carolina
Padilla, Ivan L.
Parker, Lucas
Petroff, Matthew A.
Reeves, Rodrigo
Rostem, Karwan
Nunes Valle, Deniz Augusto
Watts, Duncan J.
Weiland, Janet L.
Wollack, Edward J.
Publisher
Astrophysical JournalDescription
Metadata
Show full item recordAbstract
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
tensor-to-scalar 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 signal-to-noise 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 sub-percent 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 temperature-to-polarization
leakage fraction is <10−4 (95% C.L.). We find that the Moon-based 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.