Operating Characteristics of the Stratoscope II Balloon-Borne Telescope

The operation of a precision telescope system suspended in near space by a large balloon presents hardware problems and conditions uniquely different from ground-based astronomical equipment. The hostile environment, the weight limitations, the remote commanding, the platform stability, and the need for extreme pointing accuracy (to accommodate the diffraction-limited optical system) demand that special attention be given to both hardware and operating techniques. Stratoscope II, an unmanned balloon-borne 36-inch telescope, was floated at an 80 000-foot altitude during one night in May, 1968. The ground-commandable telescope acquired, tracked, and photographed several stellar objects. Pointing inaccuracy was 0.015 on 5th magnitude stars and 0.05 on 7.5 magnitude stars, this despite a temperature gradient across the telescope tube which degraded the images that are both photographed and tracked by the telescope. Details on telescope configuration, pointing method, focusing, and thermal problems are given.