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The Cosmology Large Angular Scale Surveyor (CLASS) is a telescope array thatobserves the cosmic microwave background (CMB) over 75% of the sky from theAtacama Desert, Chile, at frequency bands centered near 40, 90, 150, and 220GHz. CLASS measures the large angular scale ($1^\circ\lesssim\theta\leqslant90^\circ$) CMB polarization to constrain the tensor-to-scalar ratio at the$r\sim0.01$ level and the optical depth to last scattering to the samplevariance limit. This paper presents the optical characterization of the 40 GHztelescope during its first observation era, from 2016 September to 2018February. High signal-to-noise observations of the Moon establish the pointingand beam calibration. The telescope boresight pointing variation is$<0.023^\circ$ ($<1.6$% of the beam's full width at half maximum (FWHM)). Weestimate beam parameters per detector and in aggregate, as in the CMB surveymaps. The aggregate beam has an FWHM of $1.579^\circ\pm.001^\circ$ and a solidangle of $838 \pm 6\ \mu{\rm sr}$, consistent with physical optics simulations.The corresponding beam window function has a sub-percent error per multipole at$\ell < 200$. An extended $90^\circ$ beam map reveals no significant farsidelobes. The observed Moon polarization shows that the instrumentpolarization angles are consistent with the optical model and that thetemperature-to-polarization leakage fraction is $<10^{-4}$ (95% C.L.). We findthat 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 measurementsestablish degree-level precision for instrument polarization angles.

astrophysical journal/the astrophysical journal

Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: 40  GHz Telescope Pointing, Beam Profile, Window Function, and Polarization  Performance