The BLAST gondola incorporates a 2 m primary mirror, a 50 cm diameter correcting secondary mirror, and a rigid support structure for the secondary. The secondary mirror has been designed to give diffraction limited performance over a 14′ x 7′ FOV at the Cassegrain focus at λ=250 μm. The estimated antenna efficiency is >=80%, and is determined by a combination of the 5 μm rms surface accuracy of the primary and the quality of the re-imaging optics.

Radiation from the telescope enters the cryostat through a 7.7 cm diameter vacuum window near the Cassegrain focus. The f/5 Cassegrain focus of the telescope is about 18 cm behind the back surface of the primary mirror. The cryostat window is made from 0.002 inch thick polypropylene, which has <0.1% loss. Blocking filters at the intermediate cold stages of 77 K and 20 K reduce the radiation loading on the LHe to 10 mW.