Annu. Rev. Astron. Astrophys. 1991. 29: 89-127 Copyright © 1991 by Annual Reviews. All rights reserved

2.1 HI 1216 Å Lyman a Radiation

The Lyman line is the radiation seen along the left edge of Figure 4. Its origin is terrestrial and solar system, not cosmic. It is vastly brighter than appears in Figure 4, because there it has been greatly attenuated by the presence, in the optical system, of a CaF₂ filter designed to block it. That filter also totally excludes radiation with wavelengths shorter than Lyman (L). The loss of this radiation for the present study is the price paid here to be free of intensely bright L radiation scattering within the spectrometer. The problem, of course, is that there is no such thing as a perfect grating; any grating will scatter some of the L to other wavelengths, where it might be misinterpreted as true cosmic continuum background at the nominal wavelength.

Even if L is admitted to the spectrometer, it is to some extent possible to correct the data at other wavelengths for the scattered L. Edelstein (21) is able to strongly suppress the reflectivity of optics at and near 1216 Å, which will permit study of shorter wavelength radiation with much less significant interference from scattered L; in addition, gratings with superior scattered-light properties are now available.

The source of the annoying L is sunshine. The solar system is bathed in (L) sunshine, even at night. Solar L photons scatter from the hydrogen upper atmosphere of the Earth, multiply scattering to the night side of the Earth. Furthermore, solar L photons scatter back from the interstellar neutral hydrogen gas that is flowing through the solar system.

It is possible to reduce the L sky brightness substantially by removing the spectrometer to the far reaches of the solar system, as was done for the Voyager far-ultraviolet spectrometers, discussed below.