Scattered light from geocoronal Ly accounts for the remainder of the background in a spectrum, and for the largest apertures it can dominate over the dark count during orbital daylight. The holographic grating used in the HUT spectrograph has excellent scattered light characteristics with no ghosts. Far from the line center, scattered light is roughly uniform across the spectrum at a level of Å-1 times the integrated intensity of the incident emission line. This is approximately a factor of ten lower than for comparable ruled gratings. Scattering profiles for geocoronal Ly were measured through each aperture used on Astro-1 during orbital night in observations of blank fields. The fitted profile for the 18 circular aperture used on Astro-1 is shown in Figure 4-1. The profile is expected to be similar in the 20 circular aperture on Astro-2.
In the pipeline processing of HUT data only the roughly uniform contribution to the scattered light is subtracted from each spectrum. This is computed from the residual count rate after dark-count subtraction in an airglow-free region spanning 845-885 Å below the 912 Å cutoff due to neutral hydrogen in the local interstellar medium. For faint sources the wings of the Ly scattering profile can be significant. Removing this emission accurately requires fitting the line profile template to the raw data, and this is best done by the user interactively in off-line processing.