Two PSPCs were on board ROSAT. PSPC-C was the primary detector until it was destroyed on January 25th 1991 when it was accidentally pointed at the Sun. Thereafter the reserve, PSPC-B was used.
On detection by the PSPC an X-ray photon would first pass through a thin plastic entrance window into a chamber containing 65% argon, 20% xenon and 15% methane. This gas would absorb the photon by the photo-electric effect and emit a photo-electron. This electron is thermalized, and in the process ionizes other gas atoms producing an electron cloud. The number of electrons contained within this cloud is proprtional to the energy of the incident photon, hence the name Proportional Counter.
This secondary electron cloud drifts towards an anode, whereupon it ionises more atoms due to the increase in electric field strength. This process causes the charge in the cloud to be increased by a factor
. The effect of this charge on impact with the anode is to produce a charge pulse which induces a signal in a grid of cathode strips. The arrangement of the cathodes in a grid enable the position of the incident photon to be determined.
The combination of the X-ray mirror assembly and the PSPC results in a spatial resolution of 
arcseconds (Briel & Pfeffermann 1986).