Ultraviolet polarimetry using high altitude balloons.

The ozone and oxygen in the earth's atmosphere prevent all ground-based observing at wavelengths shorter than about 3000 A. With the largest available balloons and a large telescope one can get sufficiently above the ozone to make precise polarization measurements of stars and planets near 2820 A and 2200 A. Two telescope/gondola systems are in operation. The first actually is a prototype for spacecraft but it is also used on balloons, mounted in a gondola of the National Center for Atmospheric Research. It has two 7.5-cm reflecting telescopes (7.5 cm is the diameter of the primary mirror). The second system has a 71-cm Cassegrain reflector, two vidicon cameras, command and telemetry by radio link, and a startracker that guides the stabilized platform (+/-1 min of arc). The first measurements made were of the polarization of the moon at 2900 A in December 1965, and of interstellar polarization at 2820 A and 2200 A in May 1966.