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The Diffuse X-Ray Spectrometer (DXS) experiment is scheduled to be flown as an attached Shuttle payload in December 1992. As of July 1992, it has completed pre-flight testing at Goddard Space Flight Center and being prepared for shipment to Kennedy Space Center for launch. DXS is designed to measure the spectrum of the low energy (0. 15 — 0.28 keY) diffuse x-ray background with — 10 eV energy resolution and 15° spatial resolution. During its 5-day Shuttle mission, DXS is to measure the spectrum of ten 15° x 15° regions lying along a single 150°-long great circle arc on the sky. DXS has two large area Bragg x-ray spectrometers to cover the wavelength range 44 —84 A using lead stearate Bragg crystals. The spectrometers are of a novel design and have a very large area—solid-angle product, so as to permit measurement of the wavelength spectrum of the cosmic low-energy diffuse x-ray background with good spectral resolution. The bulk of these x-rays are almost certainly from a very hot (T 106 K) component of the interstellar medium that occupies a large fraction of the interstellar volume near the Sun. Astrophysical plasmas near 1O K are rich in emission lines, and the relative strengths of these lines, besides providing information about the physical conditions of the emitting gas, also provide information about its composition, history and heating mechanisms. Each DXS detector consists of a curved panel of Bragg crystals mounted above a position-sensitive proportional counter. The spectrum is dispersed across the counter and all portions of the spectrum are measured at the same time. This eliminates the serious problem in conventional Bragg spectrometers of false spectral features being introduced by time-varying background. On the other hand, while all wavelengths are measured at the same time, the various wavelengths come from different directions in the sky. The spectrometers are therefore rocked back and forth about an axis perpendicular to the dispersed direction to obtain complete spectral coverage along an arc of the sky. This paper describes the DXS instrument concept and design and presents calculations of the anticipated data. It also provides a brief description of the DXS Shuttle payload and its operations
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