A schematic drawing of the HUT instrument is shown in Figure 1.
The optical system consists of a 90 cm diameter 
primary mirror
and a prime-focus, near-normal-incidence, Rowland-circle spectrograph.
These elements are held in position by a rigid metering structure of low-expansion Invar.
The entire assembly is housed within an aluminum environmental control
canister, which protects against contamination on the ground and
provides a thermally-controlled, optically-shielded environment during
observations.
The parabolic primary mirror is fabricated of Zerodur
and figured to provide 
image quality at the focal plane.
It is coated with iridium, which provides 
20% reflectivity in the
primary 912--1216 Å range (and in the 500--600 Å range as well) and
is stable in the low-Earth orbital environment.
Three motorized mechanisms in the Invar mirror-mounting structure
provide the capability to focus and align the instrument during the mission.
The prime focus spectrograph is housed in an evacuated stainless steel
enclosure, whose vacuum is maintained at 
torr by redundant Vac-Ion pumps.
The spectrograph contains a 20 cm diameter 
spherical concave
grating that was holographically generated
(600 lines mm
) and overcoated
with osmium.
The depth of the sinusoidal groove profile is chosen to provide optimal
first-order efficiency at 
1000 Å. Light passing through the
spectrograph entrance aperture is diffracted and focused onto a photon-counting
CsI-coated microchannel-plate intensifier at a dispersion of
41 Å mm
. The intensifier employs a chevron
pair of 80:1 microchannel plates (MCPs) with 10 
m diameter pores and 25 mm active area, operated at a gain of 
. Charge
pulses from the MCPs impinge on a P20 phosphor, whose visible light output is coupled via fiber optics to a one-dimensional 1024 element self-scanned photodiode array (Reticon)
clocked at 1 scan 
. A typical charge pulse covers
photodiodes (200 
m) at threshold. Each pulse is centroided by an
onboard computer, yielding a
pixel
size of 12.5 
m (half the diode width or 0.51 Å in first
order), and is accumulated in a 2048 element spectral array.
The detector resolution is 
1 Å, but the aberrations of the 
optical system limit the effective resolution for a point source to about 3 Å in first order. The plate scale at the detector is 
, so when the pointing is stable (
) the resolution is not significantly degraded by image motion. The design and performance of an early version of the HUT
detector have been described in more detail by [Long et al. 1985].
To enable the instrument to be operated on the ground, a small mercury calibration lamp was included in the spectrograph. This lamp provides a line at 1849 Å that is imaged onto the detector by the grating. Monitoring this source at various stages throughout the shuttle integration confirmed the operation of the entire system, while the observed count rate provided a record of the sensitivity of the spectrograph at one wavelength over time.
An eight-position rotating mechanism at the spectrograph entrance
provides a choice among six aperture sizes and/or filters for
observations, as well as two positions for ground operations (a vacuum
seal position and a large aperture for calibration). Point source observations are generally made through one of two circular apertures of 18
and 30
diameters. Extended source observations are generally made through one of two long-slit apertures with dimensions of 
and 
, providing resolution of 3.3 and 6 Å, respectively.
One of the observing positions contains a thin aluminum filter behind a 30
diameter aperture, which
strongly rejects the first order FUV light and provides a pure EUV
bandpass (415--700 Å). Finally, one position contains a 
aperture and a CaF
filter, which provides the capability to exclude Lyman-
(and all wavelengths shorter than 
Å) from the spectrograph.
The apertures were etched into a mirrored surface that reflects
visible light from the surrounding star field through a re-imaging lens
system and filter wheel onto the focal plane of a silicon
intensified target (SIT) vidicon camera.
The camera views a 
field surrounding
the spectrograph aperture with 
resolution.
Digitized video images are analyzed by the onboard processor and
compared with pre-defined target and guide star positions.
Final target identification is made onboard by the
payload specialist (an astronomer/astronaut chosen from the Astro
instrument teams),
who completes the acquisition by
adjusting the Instrument Pointing System
(IPS) on which HUT and the other Astro UV telescopes are mounted.
Commandable adjustments to the camera gain and integration time, as
well as
selection of an appropriate neutral density filter wheel position, permit
acquisition of targets with visual magnitudes ranging
from -4 to +17.
HUT incorporates two custom-built microprocessors ([Ballard 1984]
that execute commands directly from the high-level language FORTH. The spectrometer processor (SP) handles the high-rate data from the Reticon, applying commandable pulse width and amplitude limits to the digitized Reticon output for noise rejection, calculating centroids of pulses, and creating telemetry messages in several possible formats. Of these output modes, two are principally used for the science data. The high-time-resolution mode provides a time-tagged list of each individual photon event, with an accuracy of 
. This mode can be employed for sources whose total count rates do not exceed 
. For higher rate sources, histogram mode is employed, in which a cumulative 2048 element histogram is output to the telemetry every 2 s. This cumulative histogram is also downlinked every 60 seconds in high-time-resolution mode. Thus when data dropouts occur, only the pho
ton arrival time information is lost, and the spectral data are preserved. The dedicated experiment processor (DEP) performs all the functions of command and control of the HUT instrument, and also digitizes, stores and analyzes the video images from the target acquisition camera. The DEP also provides communications with the crew and with the science team at the Payload Operations Control Center in Huntsville, Alabama, via the Spacelab Experiment Computer on the shuttle.
The HUT detector is limited by phosphor persistence, pulse size and
scan rate to total count rates 
5000 counts s
. To observe
targets with higher count rates, principally the brightest O and B
stars, the HUT telescope aperture may be reduced from the nominal
full aperture size of 5120 cm
. A 50% reduction is accomplished by closing one of
two semi-circular shutter doors. Further reduction to 1%
(50 cm
) and 0.02% (1 cm
) of full aperture can be made by
closing both shutter doors and opening either of two small apertures
provided for this purpose. In these apertures the spectrograph aberrations become negligible, and the resolution improves to a detector-limited value of 
Å (for stable pointing). The smallest aperture was not employed
during the Astro-1 mission.