The Berkeley spectrometer aboard the ORFEUS payload achieved a variety of “firsts” during its inaugural mission in September 1993. The instrument utilizes spherical gratings with mechanically ruled varied line-spacing, and curved microchannel plate detectors with delay-line anode readout systems, to cover the 390 – 1200 Å band at a resolution of λ/5000. The instrument will be discussed, and its performance illustrated with calibration and in-flight spectra. Science highlights from the ORFEUS-I mission will be presented (oral presentation only). The payload will be available for use by guest investigators during the ORFEUS-II mission currently scheduled for late 1996.

The extreme ultraviolet (EUV) diffuse background is the most poorly known of any of the diffuse astronomical backgrounds. Only upper limits to this flux exist, obtained with spectrometers with very crude (from ≈ 15 to 30 Å) resolution; these limits are generally one to two orders of magnitude larger than the expected sources of cosmic flux. A variety of source mechanisms have been postulated to radiate in this bandpass; the most discussed is the hot phase of the interstellar medium. A speculative possibility is that hot dark matter in the form of massive, radiatively unstable neutrinos in our Galaxy will produce a unique line in this bandpass. We describe an instrument employing a new type of spectrometer which will provide ~5 Å resolution and unprecedented sensitivity for diffuse EUV radiation. The instrument will be carried aboard the newly developed Spanish Minisat satellite.