Passive thermal control of balloon-borne telescopes

NASA's Balloon Program Office (BPO) typically operates zero-pressure balloons at pressures near 3 torr and super-pressure balloons around 8 torr (around 125,000 ft and 110,000 ft altitude, respectively). These pressures are problematic for managing the temperature of a balloon-borne payload: the pressures are too low to support substantial convective cooling, but high enough to increase the conductivity of MLI (Multi-Layer Insulation). Nevertheless, thermal control of balloon payloads is often critical. Temperature gradients across a mirror or telescope can degrade their optical performance, and thermal emission from telescope mirrors can be a significant background source. We show that a combination of sunshades, earthshades and an enclosure around the OTA (Optical Tube Assembly) can reduce the mirror temperatures by 50 – 90 K, reduce gradients across the mirror to less than 0.5 K and reduce day/night temperature excursions to less than 3 K. We describe the relative importance of convection vs. radiation in the super-pressure and zero-pressure environments, and we compare simulation results and physical test results measured in a environmental chamber.