Ultraviolet astronomy has made enormous progress in the 14 years since the launch of the International Ultraviolet Explorer ( IUE) ( [Boggess et al. 1978]), and is poised for further advances as a result of the launch of the Hubble Space Telescope. Both of these telescopes employ now-standard technology, including magnesium fluoride over-coated aluminum surfaces, which provide good reflectivity for wavelengths longer than , and sealed-window detectors with magnesium fluoride or lithium fluoride windows, which provide transmission longward of 1150 and 1050 Å respectively. Multiple reflections combine with the detector window transmission to limit these ultraviolet telescopes to a wavelength range which includes, at the short end, the Lyman- line of hydrogen (), but does not extend to the higher order lines in the Lyman series or the Lyman limit ().
The 912--1216 Å spectral region also contains the principal transitions of molecular hydrogen (the Lyman and Werner bands) and important transitions from commonly occurring ionization stages of other abundant elements; for example, O VI . Furthermore, this wavelength region can provide a sensitive measure of the effective temperature of the hotter stars, for which the flux distribution longward of Lyman- is a poor discriminator of . Of course, observations shortward of the Lyman limit are also interesting, though for most sources the high opacity of interstellar hydrogen is expected to reduce drastically the observed flux just below 912 Å.
The astrophysically rich 912--1216 Å region has been explored previously in only fairly limited ways. Copernicus ([Rogerson et al. 1973]) obtained high-resolution spectra of the brightest stars in the 950--1450 Å range, primarily to study the interstellar medium ([Spitzer & Jenkins 1975]). The Voyager ultraviolet spectrometer ([Broadfoot et al. 1977]) has been employed to obtain low-resolution spectrophotometry of a number of sources from 500 to 1700 Å ([Holberg 1990], [Holberg 1991]). Finally, several rocket-borne experiments have carried out a small number of observations in the far ultraviolet (e.g., [Brune, Mount, & Feldman 1979]; [Carruthers, Heckathorn, & Opal 1981]; [Woods, Feldman, & Bruner 1985]; [Cook, Cash, & Snow 1989]).
The Hopkins Ultraviolet Telescope (HUT) was designed to perform moderate resolution spectrophotometry that would reach faint sources (e.g., quasars at V16) throughout the far-ultraviolet band from 830 to 1850 Å, with special emphasis on obtaining maximum performance in the 912--1216 Å band. In achieving this goal it was possible to make HUT sensitive to extreme ultraviolet (EUV) radiation as well, covering the range 415--925 Å in second order, without compromising or significantly complicating HUT's primary function. HUT was originally proposed to NASA in response to an Announcement of Opportunity for Spacelab missions aboard the space shuttle ([Davidsen et al. 1978]). An early description of the HUT design was given by [Davidsen et al. 1981], and a more extensive discussion may be found in [Davidsen & Fountain 1985]. A detailed exposition of HUT's EUV capabilities is given by [Davidsen et al. 1991a].
HUT was launched aboard the space shuttle Columbia as a component of the Astro-1 mission on 1990 December 2. It performed nearly flawlessly throughout the 9 day mission, obtaining almost 40 hours of observing time on 77 different sources. The first result from HUT, a limit on the lifetime of the neutrino in the decaying dark matter hypothesis of [Sciama 1990], [Sciama 1991], has been presented by [Davidsen et al. 1991b]. Other early results from HUT are presented in papers by [Blair et al. 1991], [Feldman et al. 1991], [Ferguson et al. 1991] [Kriss et al. 1992], [Long et al. 1991], and [Moos et al. 1991]. In this paper we describe the HUT instrument and give a brief account of its performance and calibration during the Astro-1 mission.