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 degrees less than or similar to theta <= 90 degrees) CMB polarization to constrain the tensor-to-scalar ratio at the r similar to 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.degrees 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.degrees 579 +/- 0.degrees 001 and a solid angle of 838 +/- 6 mu sr, consistent with physical optics simulations. The corresponding beam window function has a sub-percent error per multipole at l < 200. An extended 90 degrees 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 degree-level precision for instrument polarization angles.