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 ^ { \circ } \lesssim \theta \leqslant 90 ^ { \circ } ) CMB polarization to constrain the tensor-to-scalar ratio at the r \sim 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 September 2016 to February 2018 .
High signal-to-noise observations of the Moon establish the pointing and beam calibration .
The telescope boresight pointing variation is < 0.023 ^ { \circ } ( < 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 a FWHM of 1.579 ^ { \circ } \pm . 001 ^ { \circ } and a solid angle of 838 \pm 6 \mu { sr } , consistent with physical optics simulations .
The corresponding beam window function has sub-percent error per multipole at \ell < 200 .
An extended 90 ^ { \circ } 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 .