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

5.7 More Observations

The purpose of parts of the preceding discussions concerning various past experiments was to emphasize that truly reliable data concerning the diffuse ultraviolet background are hard to come by. In particular, one cannot simply accept data as they are presented; one must look very critically at the circumstances under which they were obtained to judge the likelihood that the data deserve serious attention. Consider now four additional data sets.

5.7.1 OAO-2 The pioneers of the study of diffuse ultraviolet background radiation were Lillie & Witt (62), and their conclusions concerning the ultraviolet albedo a and the scattering parameter g, for interstellar grains are still widely used. My concerns (33) about their data have centered on the extremely small (8.5 x 10^-6 sr) OAO-2 field of view, and the time-variable dark current due to low Earth orbit through the radiation belts. The OAO-2 was intended for the study of point sources, not low-surface-brightness extended backgrounds. Such experiments, in low Earth orbit, are prone to problems of false patchy backgrounds, and a patchy background is exactly what Lillie & Witt report. In view of this their conclusions must be confirmed by other experiments that are optimized for the study of diffuse backgrounds rather than point sources.

5.7.2 BERKELEY ARIES ROCKET Jakobsen et al (53) made rocket measurements, at altitudes below 250 km and with broad-band photometers, of a region at constant galactic latitude but having spatially variable hydrogen column density. Intensities of 600 to 3000 units are reported, and correlations, of varying strength, of intensity with column density, are reported at all three wavelengths (1590, 1710, and 2135 Å) observed. Murthy, Henry & Holberg (83) have used the Voyager ultraviolet spectrometer to make observations at the beginning and at the end of the Jakobsen et al scan areas. They obtain only the usual Voyager upper limits. The interpretation of the differences between the two studies will appear in upcoming work by Murthy et al (83). The reason is either that the Jakobsen et al results are incorrect, or that different sources are involved at the different wavelengths.

5.7.3 PROGNOZ The Prognoz spacecraft had an orbit that took it very far from Earth. Solar system L was admitted to the spectrometer but could be corrected for. The data have been presented in a number of different forums, and it has been difficult to follow the details of data selection and treatment. The latest presentation (119) shows excellent agreement between some of the data, and some of the data of Paresce et al (92).

The authors also point to a possible origin for the contamination which we suggested above is present in the data of Paresce et al (91, 92). The Russian authors refer to a photograph showing a ``huge cloud of heavy molecules which surrounded the combined Apollo-Soyuz spaceship and produced strong scattering in the experiment of Paresce et al (92)''.

5.7.4 TD - 1 The TD-1 field of view was very small (1.7 x 10^-5 sr), and time-variable dark current was a serious problem. The TD - 1 data are mentioned here only for reference (75, 76).