Salyut 4 (DOS 4) was a Salyutspace 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 attempted to make stays aboard Salyut 4 (Soyuz 17 and Soyuz 18 docked; Soyuz 18a suffered a launch abort). The second stay was for 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 to 2 keV (32 to 320 aJ). 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 survey. Data could be collected in 4 energy channels: 2 to 3.1 keV (320 to 497 aJ), 3.1 to 5.9 keV (497 to 945 aJ), 5.9 to 9.6 keV (945 to 1,538 aJ), and 2 to 9.6 keV (320 to 1,538 aJ) in the larger detectors. The smaller detector had discriminator levels set at 0.2 keV (32 aJ), 0.55 keV (88 aJ), and 0.95 keV (152 aJ).
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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 of 0.6 to 0.9 keV (96 to 144 aJ) 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.cm-2s-1 (35 fW/m²) in the 0.2 to 2.0 keV (32 to 320 aJ) range. Cyg X-1 was observed on several occasions. Highly variable flux, in both the time and energy domains, was observed.
After Salyut 6 manned operations were discontinued in 1981, a heavy unmanned spacecraft called TKS and developed using hardware left from the canceled Almaz program was docked to the station as a hardware test.
Salyut 1 was abandoned on Oct. 11, 1971, but several successor stations over the next 15 years helped pave the way for Mir.
The second Almaz, under the cover name “Salyut 3,” was successfully launched on Jun. 26, 1974, and its inaugural crew, Pavel Popovich and Yuri Artyukhin, docked with the station on Jul. 3 for a stay lasting a couple of weeks.
The successful Salyut4 was deorbited on Feb. 3, 1977, bringing the highest civilian honor, “Hero of the Socialist Labor”, to the chief designer of the spacecraft, Yuri Semenov, and one of the assembly technicians, V. Morozov (despite official objections that Morozov was not a member of the Communist Party)
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