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Salyut 4

Salyut 4 (DOS 4) was a Salyut space station launched on December 26, 1974 into an orbit with an apogee of 355 km, a perigee of 343 km and an orbital inclination of 51.6 degrees. It was essentially a copy of the DOS 3, and unlike its ill-fated sibling it was a complete success. Three crews made stays aboard Salyut 4 (Soyuz 17, Soyuz 18 and Soyuz 21), including one of 63 days duration, and an unmanned Soyuz capsule remained docked to the station for three months, proving the systems' long-term durability. Salyut 4 was deorbited February 3, 1977.


The Salyut 4 X-ray instrument, often called the Filin telescope, consisted of four gas flow proportional counters, three of which had a total detection surface of 450 cm² in the energy range 2-10 keV, and one of which had an effective surface of 37 cm² for the range 0.2-2 keV. The field of view was limited by a slit collimator to 3° × 10° FWHM. The instrumentation also included optical sensors which were mounted on the outside of the station together with the X-ray detectors, and power supply and measurement units which were inside the station. Ground based calibration of the detectors was considered along with in-flight operation in three modes: inertial orientation, orbital orientation, and sruvey. Data could be collected in 4 energy channels: 2-3.1 keV, 3.1-5.9 keV, 5.9-9.6 keV, and 2-9.6 keV in the larger detectors. The smaller detector had discriminator levels set at 0.2 keV, 0.55 keV, and 0.95 keV.


Among others, observations of Sco X-1, Cir X-1, Cyg X-1, and A0620-00 were published from the Filin data. A highly variable low energy (0.6-0.9 keV) flux was detected in Sco X-1. Cir X-1 was not detected at all during a July 5 1975 observation, providing an upper limit on the emission of 3.5e-11 erg/cm²/s in the 0.2-2.0 keV range. Cyg X-1 was observed on several occasions. Highly variable flux, in both the time and energy domains, was observed.