  The Next Generation Balloon-borne Large Aperture Submillimeter Telescope
(BLAST-TNG) is a submillimeter mapping experiment planned for a 28 day
long-duration balloon (LDB) flight from McMurdo Station, Antarctica during the
2018-2019 season. BLAST-TNG will detect submillimeter polarized interstellar
dust emission, tracing magnetic fields in galactic molecular clouds. BLAST-TNG
will be the first polarimeter with the sensitivity and resolution to probe the
$\sim$0.1 parsec-scale features that are critical to understanding the origin
of structures in the interstellar medium. With three detector arrays operating
at 250, 350, and 500 $\mu$m (1200, 857, and 600 GHz), BLAST-TNG will obtain
diffraction-limited resolution at each waveband of 30, 41, and 59 arcseconds
respectively.
  To achieve the submillimeter resolution necessary for its science goals, the
BLAST-TNG telescope features a 2.5 m aperture carbon fiber composite primary
mirror, one of the largest mirrors flown on a balloon platform. Successful
performance of such a large telescope on a balloon-borne platform requires
stiff, lightweight optical components and mounting structures. Through a
combination of optical metrology and finite element modeling of thermal and
mechanical stresses on both the telescope optics and mounting structures, we
expect diffraction-limited resolution at all our wavebands. We expect pointing
errors due to deformation of the telescope mount to be negligible. We have
developed a detailed thermal model of the sun shielding, gondola, and optical
components to optimize our observing strategy and increase the stability of the
telescope over the flight. We present preflight characterization of the
telescope and its platform.
