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.Instrumentation