It is generally circular in shape except for a break-out towards the south. In addition, there are many bright, filamentary structures. The HRI image shows the angular structure of the Cygnus Loop. Thus, the two detectors were complimentary with the PSPC providing the best spectral information and the HRI providing the best angular resolution. A photon enters through the window and ionizes an atom of the gas producing an electron with most of the energy of the photon.
Photons enter a channel and photoionize an atom in the wall.
With the applied voltage, the channel then acts as a small photomultiplier tube producing a final charge pulse which is detected. Cosmic Journeys Part of the NASA OSS Structure and Evolution of the Universe theme.