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The Baryonic Extragalactic Structure Tracer (BEST) is a SMEX-class mission that is designed to map the hot million-degree diffuse intergalactic and interstellar gas with high spectral resolution. It consists of an imaging X-ray spectrometer that can, over a 1-2 year mission, map the entire sky and conduct deep pointed observations of selected regions to profoundly extend our understanding of hot matter in the Universe. BEST will be able to detect and characterize the missing baryons in the current epoch, which are primarily in moderately overdense intergalactic regions and are predicted to account for 10 - 20% of the soft X-ray background, and also determine the properties of the hot Galactic halo and the hot Galactic gas, crucial to understanding the evolution and dynamics of our Galaxy and its interstellar medium.

The Far Ultraviolet Spectroscopic Explorer (FUSE) satellite was launched into orbit on June 24, 1999. FUSE is now making high resolution ((lambda) /(Delta) (lambda) equals 20,000 - 25,000) observations of solar system, galactic, and extragalactic targets in the far ultraviolet wavelength region (905 - 1187 angstroms). Its high effective area, low background, and planned three year life allow observations of objects which have been too faint for previous high resolution instruments in this wavelength range. In this paper, we describe the on- orbit performance of the FUSE satellite during its first nine months of operation, including measurements of sensitivity and resolution.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.