Observations of Galactic Gamma-ray Sources

The High Energy Stereoscopic System (H.E.S.S.) is a system of four imaging atmospheric Cherenkov telescopes which commenced full operation in the Khomas Highland of Namibia in December 2003 , at an altitude of 1800 m. The experiment is run by an international collaboration of mostly European institutes. It is built for very-high-energy (VHE) gamma-ray astronomy, exploiting the energy range above 100 GeV up to several tens of TeV. Gamma rays are measured by means of Cherenkov light emitted in air showers of secondary particles that form whenever a primary gamma ray hits the earth’s atmosphere and is being absorbed. Using Cherenkov images of air showers one can deduce the energy and direction of the primary particle. The H.E.S.S. Cherenkov telescopes are operated in moonless nights yielding a total observation time of roughly 1000 h per year. In normal data taking mode, five to ten objects are tracked per night with a typical cosmicray event rate of 300 Hz. The observations proceed in stereoscopic mode: events are recorded if at least two out of the four telescopes have triggered on the same air shower . The telescopes itself have a 60-t steel structure with altitude-azimuth mount. Each has a tessellated mirror surface