BLAST-TNG uses an on-axis Cassegrain telescope, with a 2.5 m-diameter primary mirror, and a 57 cm-diameter secondary mirror. The primary and secondary mirrors are currently being developed through a partnership with Vanguard Space Technologies, under a NASA Small Business Innovation Research (SBIR) grant. The primary will be the largest mirror ever flown on a balloon, and the largest submillimeter carbon fiber mirror in operation. The telescope will have a resolution range of 25″ to 50″ with a ~22′ diameter field of view.
The Cassegrain telescope focuses the beam into a cryogenic receiver, where a series of mirrors re-image the beam onto the three detector arrays. These “cold optics” are cryogenically-cooled to 4 K, and are arranged in a modified Offner relay configuration. Polarization modulation is provided by a wide-band metal-mesh half wave plate, which is stepped periodically during observations to allow for the removal of systematic polarization errors.