Performance and preliminary calibration of the Hopkins ultraviolet telescope on the Astro-2 mission

Abstract

An improved version of the Hopkins Ultraviolet Telescope (HUT) made its second flight on the Astro-2 mission aboard the Space Shuttle Endeavour from 1995 March 2--18. The longer mission duration and greatly improved pointing stability relative to Astro-1 made possible 385 observations of 265 celestial targets at far-ultraviolet wavelengths. Observing efficiency exceeding 60% over 14 days of science operations yielded 205 hr of on-source integration time, a factor of 5.1 increase over Astro-1. We describe changes to the instrument following Astro-1 and the in-flight photometric calibration, which is based on a comparison of our observations of the hot DA white dwarf HZ 43 with a model atmosphere whose parameters were derived from optical observations. The peak effective area is 24.1 cm2 at 1160 A, where the inverse sensitivity is 7.09 x 10-13 ergs cm-2 count-1. This is an improvement by a factor of 2.3 over Astro-1, largely attributable the installation of new optics coated with ion-sputtered silicon carbide. Observations of several other white dwarfs indicate that the calibration is accurate to about 5%, after correction for modest, but significant, time-dependent degradation during the mission. The spectral resolution varied from 2 to 4 A over the first-order wavelength range of 820--1840 A. The wavelength scale is established to better than 1 A. As on Astro-1, dark counts and scattered light were extremely low. Airglow line intensities were much lower because of the lower level of solar activity. When all factors are considered together, HUT performance on Astro-2 was a full order of magnitude better than that achieved on the highly successful Astro-1 mission.