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 V
16) 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.