The dark count in the HUT detector system is principally due to charged particle events. These particle-induced events account for about 65% to 95% of the detector background during orbital night. The balance of the background is due to scattered light from airglow emission, and its level depends upon the aperture selected for the observation as well as the position of the shuttle in its orbit and the pointing direction. Measurements made during three long observations on Astro-1 with the spectrograph aperture closed detected dark count rates ranging from 3.55 times 10-4 4.70 times 10-4 count pixel-1 s-1 spread uniformly across the spectrum. The mean dark rate was $3.94 × 10-4 count pixel-1 s-1.Similar count rates are anticipated for Astro-2.