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

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On January 12, 1986, Space Shuttle Columbia (in the last shuttle flight before the Challenger accident) carried into low Earth orbit (340 km ~ circular) two separate experiments, created by two different university groups for the study of cosmic diffuse ultraviolet background radiation. In the experiments, collectively called UVX, the instruments were rigidly co-aligned to insure that the same part of the sky was observed by both. The Berkeley group had previously reported many extraordinarily bright patches of diffuse ultraviolet background, at high galactic latitudes. The Johns Hopkins group had consistently reported much lower intensities for the diffuse background at all moderate and high Galactic latitudes. The purpose of the UVX experiment was to address this fundamental contradiction.

The result of this parallel experiment with comparisons is described below; but first we consider the Johns Hopkins UVX data in detail because we shall use it as a template to address the technical difficulties that are faced by all observers of the diffuse cosmic ultraviolet background.

Figure 4 shows the Johns Hopkins data for target 9, called SPECTRUM, which has been described by Murthy et al (81, 82). The corresponding identical Berkeley target is called by them either number 8 (67, 68) or number 9 (47).

Figure 4

Figure 4. A sample of spectral data from the Johns Hopkins UVX cosmic background experiment. Time increases up the page; wavelength is from Lyman alpha (bright emission line at the extreme left, 1216 Å) to 3200 Å at the right edge of the photograph. Intense airglow is seen early in the spatial scan at the longest wavelengths (lower right). As the spatial scan proceeds, stars (horizontal bands at long wavelenghts) enter and leave the field of view, and two airglow lines of the variable intensity are seen near Lyman alpha. Zodiacal light (faint vertical bands at far right) appears throughout the spatial scans at an intensity of ~ 1500 units. The vertical discontinuity in the middle of the figure shows that most of the residual signal is due to instrumental dark current in the two spectrometers.

This SPECTRUM target is a region from Galactic latitude 86° (start of scan), to 74°, at Galactic longitude 335°. Time increases up the page. The scan represents about 20 minutes of data taken while the spectrometers scanned a fairly blank region of the sky. What is shown in Figure 4 is a spectrum of the entire area, obtained by scanning the slits of the spectrometers in latitude with time Slit width was 0.3° on the sky, while spectral resolution was 17 Å in the short-wavelength (1200-1700 Å) spectrometer and 27 Å in the longer-wavelength (1650-3100 Å) spectrometer. Wavelength increases from 1216 Å (the very bright Lyman a emission line that is observed throughout the whole course of the scan) on the left to 3200 Å at the right edge of the figure.

Using Figure 4 we now discuss the various sources of noise that the observer of cosmic diffuse background must contend with. This is done in some detail to appreciate the difficulty of the observations and therefore to assess reliability when we examine the data, to determine which should be trusted.

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