  The Cosmology Large Angular Scale Surveyor (CLASS) instrument will measure
the polarization of the cosmic microwave background at 40, 90, and 150 GHz from
Cerro Toco in the Atacama desert of northern Chile. In this paper, we describe
the optical design of the 40 GHz telescope system. The telescope is a
diffraction limited catadioptric design consisting of a front-end
Variable-delay Polarization Modulator (VPM), two ambient temperature mirrors,
two cryogenic dielectric lenses, thermal blocking filters, and an array of 36
smooth-wall scalar feedhorn antennas. The feed horns guide the signal to
antenna-coupled transition-edge sensor (TES) bolometers. Polarization diplexing
and bandpass definition are handled on the same microchip as the TES. The feed
horn beams are truncated with 10 dB edge taper by a 4 K Lyot-stop to limit
detector loading from stray light and control the edge illumination of the
front-end VPM. The field-of-view is 19deg x 14deg with a resolution for each
beam on the sky of 1.5deg FWHM.
