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We describe an instrument that images the Alcator A tokamak plasma in the soft x-ray energy range with high temporal resolution. Radiation from a cross section of the plasma column is detected by an array of seventeen surface barrier detectors which provides 1 cm poloidal spatial resolution over a 16 cm field of view. The instrument’s nominal sensitivity lies in the spectral range from 0.27 to 25 keV; this range includes much of the radiation emitted by the Alcator A plasma, whose core temperature is typically in the 0.6 to 1.1 keV range. The frequency response of the detector-electronics system extends from dc to 170 kHz. We discuss: (1) some design considerations and instrument calibrations, including the instrument’s measured response to 0.28, 1.5, 8.0, and 17.5 keV x-rays and its theoretical response to a bremsstrahlung spectrum; (2) the detector-electronics noise levels under various conditions; (3) the instrument’s response to an electrically modulated x-ray source; (4) the detector x-ray response as a function of bias voltage and temperature; (5) the detector linearity of response to 1.5 keV x-rays; and (6) the stability of the detector x-ray response despite numerous and prolonged vacuum breaks. Sample data, obtained from Alcator A discharges, are presented.