A liquid‐He automatic cooling system for a balloon‐borne infrared telescope

A cooled telescope for astronomical infrared observations at balloon altitudes in the range 2–5 μm was successfully flown on June 12–13, 1995 from the NASA/NSBF base at Palestine, Texas. A cylindrical cryostat (42 cm in diameter, 120 cm high), housing the optics and the detector, was cooled by using liquid He as the refrigerating fluid. During the 11 h flight, the temperatures of the detector, optics, and internal telescope subparts were regulated between 45 and 80 K with very good stability. The cryogenics were controlled by a pressure control equipment designed to operate at both ground level and at 40 km altitude. Further thermal stabilization of the focal plane detector array was provided by a dedicated servocontrol to obtain a stability of ±0.02 K over time scales of a few minutes. The main features of the telescope, as well as on‐ground and in‐flight performances of the cryogenic system are described.