The first flight of the Hopkins Ultraviolet Telescope (HUT) aboard the Astro-1 space shuttle mission in 1990 December clearly demonstrated the value of far-ultraviolet spectrophotometry for many diverse areas of astrophysical research ([Davidsen 1993]). HUT is designed to observe faint sources at moderate resolution (3 Å) in the (first order) wavelength range 820--1840 Å. It thus overlaps with the spectroscopic capabilities of the Hubble Space Telescope ( HST) at Å, and provides an important complementary capability for studies of the little-explored, but astrophysically interesting 912--1200 Å region. By employing just two reflections, special optical coatings, and a windowless detector system, HUT achieves a sensitivity equal to that of the Faint Object Spectrograph (FOS) on HST at about 1200 Å ([Keyes et al. 1995]), and maintains high sensitivity down to the Lyman limit ([Davidsen et al. 1992]).
Prior to the flight of Astro-1, spectroscopic studies at far-UV wavelengths were essentially limited to high resolution work on very bright stars with Copernicus ([Rogerson 1973]) and very low resolution observations with Voyager ([Holberg 1991]). More recently there has been some further work at these wavelengths with ORFEUS ([Grewing et al. 1991]; [Hurwitz & Bowyer 1991]). HUT carried out the first extensive moderate resolution work at wavelengths from Lyman- to the Lyman limit.
The success of HUT on Astro-1 was tempered somewhat by the relatively short mission duration (9 days) and by difficulties encountered with the Spacelab Instrument Pointing System (IPS). NASA's decision to re-fly HUT on the Astro-2 mission provided the opportunity to correct these deficiencies and also to make significant improvements in HUT's performance which are described here. A factor of 2.3 increase in sensitivity was achieved, primarily through replacing the primary mirror and diffraction grating with new ones coated with previously-unavailable, ion-sputtered silicon carbide. IPS hardware and software problems were corrected, resulting in excellent pointing performance, with stability obtained routinely. An extended-duration mission totalling 16 days, the longest space shuttle flight conducted to date, enabled us to capitalize fully on these improvements.
A guest investigator program for Astro-2 expanded the scope of scientific topics addressed with HUT. Altogether, about 20 scientific programs were successfully conducted. We obtained 385 observations of 265 different celestial targets, with a total on-source integration time of 205 hours. The excellent performance of HUT/Astro-2 was highlighted by our success in observing the quasar HS 1700+64 (, , ), toward which the opacity of singly-ionized intergalactic helium was measured ([Davidsen, Kriss & Zheng 1995]). In this Letter we describe the changes made to HUT and its performance on Astro-2, including a preliminary, but detailed calibration.