The RF­15I solar X­ray photometer is equipped with two detector systems:

• Proportional argon gas­filled (350 Torr) detector of aperture 4.5 mm 2

• The Na(I) scintillation detector of 15.2 cm 2 aperture and crystal thickness

The proportional detector nominally registers the soft X­ray solar flux in the three energy channels 2-3-5-8 keV every 2 s. By 1998 the appropriate boundaries of energy channels moved to approximately 0.7-1.0-1.7-2.7 keV due to ageing of the calibration Fe 55 radioactive source. The scintillation detector registers every 0.125 s the hard X­ray flux in five energy bands 10-15-30-60-120-240 keV provided that appropriate rate thresholds are exceeded in each of the upper four energy ranges. In the lowest energy range 10-15 keV, the data are collected every 2 s simultaneously with the softer proportional detector channels. The normal to the detector's windows is pointed to the Sun within 10° accuracy thanks to spin­stabilisation of the spacecraft. One satellite revolution takes. 118 s.