[Davidsen et al. 1992] provide a full description of HUT, including its performance, as flown on the Astro-1 mission in 1990 December. Following Astro-1, the original 0.9-m, iridium-coated Zerodur primary mirror was replaced with the nearly identical Cervit backup mirror, which had been over-coated with silicon carbide (SiC) by the Optical Thin Film Laboratory at Goddard Space Flight Center. SiC has a reflectivity of 30--40% over the first order wavelength range of 820--1840 Å, nearly twice that of the original iridium coating, but in second order it is much lower, dropping to less than 10% below 600 Å ([Keski-Kuhi, Osantowski, Herzig, Gum, & Toft 1988]; [McCandliss 1994]). To retain good optical reflectivity for the acquisition TV camera, the SiC was deposited directly over the original iridium, and its thickness (1400 Å) was tailored accordingly. The resulting optical reflectivity is comparable to the 65% of the original iridium coating. Iridium was chosen originally for its high reflectivity combined with good stability in a space environment. SiC is susceptible to damage by high velocity atomic oxygen ([Herzig, Toft & Fleetwood 1993]), but is otherwise stable. To protect the mirror surface we avoided pointing the telescope within 20 of the shuttle velocity vector whenever the doors were open.
The near-normal-incidence Rowland circle spectrograph employed the same optical and mechanical design as that flown on Astro-1, except that the holographically ruled, 20-cm diameter f/2 grating was coated with SiC instead of osmium. The selection of apertures provided by the eight-position rotating mechanism at the spectrograph entrance was revised for Astro-2 to reflect the chosen scientific programs. Two positions for ground operations (a vacuum seal position and a large aperture for calibration) were retained. There were three circular apertures of , and diameter, and two long-slit apertures with dimensions of and . The long slits provide spectral resolutions of 3.5 and 7 Å, respectively, for diffuse sources. The remaining position, unused during Astro-2, contains a thin aluminum filter behind a diameter aperture, which provides a pure EUV bandpass (415--800 Å). The apertures were etched into a mirrored surface that reflects visible light from the surrounding star field to the TV acquisition camera. For Astro-2 the mirrored surface was polished to a higher quality optical finish to improve the throughput of the TV acquisition system.
The design of the Astro-2 HUT detector differed from that flown on Astro-1 only in the microchannel plate (MCP) mounting washers and the electron repeller grid mounted just in front of the photocathode surface of the MCP's. The new mounting washers were dimpled to provide increased conductance for pumping the region between the MCP's and the phosphor anode, and they were gold-plated to minimize the potential for vibration-induced problems with electrical contact. The new electron repeller grid consisted solely of two wires strung parallel to the dispersion direction, and located just outside the light path for the small aperture door states. The CsI photocathode was deposited on the MCP's in June 1993. Monitoring of the count rate produced by the 1849 Å line of a mercury calibration lamp imaged onto the detector by the grating throughout the pre-flight integration process showed a slow degradation in the photocathode response over time similar to that observed prior to Astro-1.
The HUT detector is limited to total count rates . To observe brighter targets, 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. The 1 cm aperture was not used during the Astro-2 mission. Software changes to the on-board Dedicated Experiment Processor (DEP) for Astro-2 permitted the large shutter doors to be partially opened to specified, reproducible, aperture areas. Two such partial openings, which exposed or of the primary mirror, were used routinely during Astro-2.