Silicon carbide coatings on the primary mirror and grating of HUT improved its performance relative to Astro-1 by more than a factor of two for the flight of Astro-2. We achieved a peak effective area of at 1160 Å with an inverse sensitivity of . Most of the first-order wavelength range of 820--1840 Å had an effective area exceeding the peak area of the Astro-1 instrument. Spectra of point sources showed a resolution of 2--4 Å. Dark counts were less than outside the SAA; grating-scattered geocoronal Ly radiation made a comparable contribution to the background during orbital night and dominated the background rates during orbital day.
We derived our photometric calibration using multiple observations of the white dwarf HZ 43 and a white dwarf model atmosphere. Comparison of flux-calibrated observations of several other white dwarfs spanning a broad range in temperature to model atmosphere calculations shows our calibration to be accurate to 5%. The instrument's sensitivity declined throughout the flight by as much as 27% at the short end of the wavelength range; most of this degradation occurred during the first third of the mission. A similar decrease in reflectivity seen in one of the witness mirrors in the telescope housing suggests either in-flight contamination or some oxidation of the silicon carbide coating.
The near nominal performance of HUT throughout the 16-day mission plus the improved pointing stability relative to Astro-1 led to a total of 385 observations of 265 individual targets. A total on-target integration time of 205 hours over the 14 days of science operations gives an observing efficiency of better than 60%, a phenomenal performance for a low-earth-orbit observatory.
We are grateful to our many colleagues and students on the HUT team at The Johns Hopkins University and the Applied Physics Laboratory, who have contributed greatly to the success of this project. We thank the payload processing team at Kennedy Space Center for their expert handling of the integration and pre-flight testing of the Astro-2 payload. We also thank Ritva Keski-Kuha and the staff of the Optical Thin Film Laboratory at Goddard Space Flight Center for their efforts in producing the successful SiC coatings of our grating and primary mirror. Finally, we thank the crew of STS-67 and the ground controllers at Johnson Space Center and Marshall Space Flight Center for all their efforts in making this mission a success. This work was supported by NASA contract NAS 5-27000 to the Johns Hopkins University.