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A Russian satellite devoted to high-energy astronomy, launched in Dec. 1989 into a highly eccentric orbit (apogee: 30 Earth radii, perigee: 3000 km) with a period of 4.09 days and inclination 49°. The payload includes two types of instruments.

The narrow-field instruments – SIGMA, ART–P, and ART–S – are designed to observe point sources at a variety of energies. SIGMA is a coded-mask telescope imaging low-energy γ-ray sources in the energy range 30–2000 keV; it can also make spectral and temporal measurements. ART–P is an X-ray imaging telescope with a coded mask and four independent gas proportional counters; operating over the energy range 3–100 keV, it is designed to provide X-ray images of localized sources. ART–S is a hard X-ray/γ-ray spectroscope constructed from four gas proportional counters; operating over the spectral range 3–150 keV, it performs spectral and time analysis of relatively bright localized sources.

The wide-field telescopes – Phebus and Konus – are designed to observe the entire sky and detect transient events, especially gamma-ray bursts. Both have detector elements distributed around the spacecraft for all-sky coverage, and provide the location as well as a spectral and time analysis of the bursts. Phebus operates over the energy range 0.1–100 MeV, Konus over the range 0.02–20 MeV.

The payload also includes the Tournasol experiment, which has a narrow field of view but can also observe transient events because it is on a gimbal mount that can slew automatically to the direction of a burst localized by Konus. Mounted on the gimbal are two optical detectors and four proportional counters, designed for the optical detection of γ-ray bursts and the X-ray spectral analysis (2–20 keV) of any burst decay ‘tails’.