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Beyond the earth's atmosphere, the diffuse UV ``sky'' is relatively uncontaminated by zodiacal emission. For this reason, a careful characterization of the UV sky can yield important information on the Galactic and Extragalactic radiation fields and, ultimately, on their origins. Despite recent concerted efforts, the strength and spatial distribution of the UV background remain controversial topics. Estimates range from a few hundred photons s$^{-1}$ cm$^{-2}$ sr$^{-1}$ with no obvious spatial distribution to several thousand photons s$^{-1}$ cm$^{-2}$ sr$^{-1}$ with a strong gradient toward the Galactic midplane (see the reviews by Henry and Bowyer in the 1991 ARAA).
As part of the December 1990 Astro-1 Spacelab mission on the Space Shuttle Columbia , the Ultraviolet Imaging Telescope obtained 361 NUV ($\sim$2500 \AA) images and 460 FUV ($\sim$1500 \AA) images of the sky -- each with a 40 arcmin field of view and a resolution of $\sim$2 arcsecs. Analysis of the UIT images has yielded positive detections of NUV and FUV backgrounds in both the daytime and nighttime skies. The daytime backgrounds can be attributed to stray light from the Sun, while some of the nighttime backgrounds can be attributed to stray light from UV-bright stars just beyond the imaged fields of view. The remaining nighttime background levels amount to $\sim$3000 photons s$^{-1}$ cm$^{-2}$ sr$^{-1}$ in the NUV and $\sim$5000 photons s$^{-1}$ cm$^{-2}$ sr$^{-1}$ in the FUV. The resulting FUV/NUV flux ratios are consistent with light arising from OB-type stars in the Galactic disk which is then scattered by interstellar dust grains in the field of view. Due to the sparse sampling of the entire sky, we were not able to determine any spatial dependence for the UV background emission. This capability may come with the re-launch of UIT as part of the STS-67/Astro-2 mission (scheduled for November 1994).
