3 Å, and, for a small number of sources, in the
extreme ultraviolet (415--912 Å) beyond the Lyman limit at a resolution
of 
1.5 Å. The objects observed include quasars, galaxy clusters,
active and normal galaxies, cataclysmic variables, globular clusters, supernova remnants,
planetary nebulae, white dwarfs, Wolf-Rayet stars, Be stars, cool stars
with active coronae, comet Levy (1990 XX), Jupiter and Io. HUT has provided the first
spectrophotometry in the sub-Lyman-
region for most of
these sources. In this paper we describe the HUT
instrument and its performance in orbit. We also present a HUT observation of the DA white dwarf G191-B2B and derive the photometric calibration curve for the instrument from a comparison of the observation with a model stellar atmosphere.
The sensitivity reaches a maximum at 1050 Å, where 1 photon cm
s
Å
yields 9.5 counts s
Å
, and remains within a factor 2 of this value from 912 to 1600 Å. The instrumental dark count measured on orbit was less than 
counts s
Å
.