The Salyut-4 X-ray instrument, often called the Filin telescope, consisted of four gas flow proportional counters, three of which have a total detection surface of 450 cm² in the energy range 2–10 keV, and one of which has an effective surface of 37 cm² for the range 0.2–2 keV. The field of view was limited by a slit collimator. 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 is considered along with in-flight operation in three modes: inertial orientation, orbital orientation, and survey. 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.