  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
$r\sim0.01$ 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
$<0.023^\circ$ ($<1.6$% 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 an FWHM of $1.579^\circ\pm.001^\circ$ and a solid
angle 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 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.
