The dark count level was low, averaging counts s Å and counts s Å, respectively, for the two detector voltages used during the mission. The dark count increased by as much as a factor of 30 during passages through the South Atlantic Anomaly, but remained too low to have a significant effect on observations of bright targets planned during these periods.
Due to its large apertures, HUT is particularly sensitive to airglow. Flying near solar minimum for Astro-2 led to a dramatic decrease in many airglow lines relative to the near-solar maximum conditions of Astro-1. Optically thick lines such as Ly and O I were reduced by a factor of two to three in intensity relative to Astro-1. Typical orbital night intensities for Ly were 1.5 kR (1 Rayleigh = ) with variations depending on the particular line of sight. Optically thin lines such as O I were down by nearly an order of magnitude relative to Astro-1.
Most airglow lines are weak enough that they only affect regions of the spectrum corresponding to the width of the aperture used for a particular observation. For the frequently used aperture this is 15 Å. Line profiles derived from observations of blank fields suffice to characterize and remove these lines when rescaled to the observation of interest. Light from geocoronal Ly scattered within the spectrograph is a more serious problem. While the holographically ruled grating has extremely low scattering properties, of the total intensity far from the line center, the strength of the Ly airglow emission gives a scattered component to the background that is comparable to the detector dark count during orbital night. During orbital day and in the near wings of Ly it dominates the background.
Observations of blank fields during orbital night were summed and fitted with a smoothly varying function to derive the Ly scattering profile for each aperture used during the Astro-2 mission. Figure 6 shows the scattering profile obtained for 7146 s of integration on blank sky in four separate pointings through the aperture. Since the cores of Ly and of O I are affected by dead-time corrections and a continuously decreasing QDE, the best match for removing the scattered Ly profile is obtained by the scaling the model profile to the near wings in the 1159--1211 Å and 1221--1273 Å ranges.