H.E.S.S. - The High Energy Stereoscopic System

H.E.S.S. is a system of four imaging atmospheric Cherenkov telescopes in the Khomas Highland in Namibia, near the Gamsberg mountain. Namibia is known for its excellent observation conditions, i.e. clear sky, low humidity, and a very low level of light pollution. Also the high altitude and the position in the Southern hemisphere, which offers optimal viewing conditions of many interesting objects including the Galactic plane and the Galactic centre, make the Khomas Highland especially suited for gamma-ray observations.

H.E.S.S. measures gamma rays at energies between 100 GeV and 100 TeV and thus operates at the upper end of the electromagnetic spectrum. At these energies, gamma rays probe the non-thermal universe: the most violent processes leave their signatures in the light of very-high-energy gamma rays such as supernova explosions whose shock fronts travel through the interstellar medium with velocities of 10,000 km/s or jets of relativistic particles launched in the vicinity of giant black holes. The unprecedented sensitivity of the H.E.S.S. telescopes allow the measurement of gamma-ray sources with intensities down to a few per mill of the flux of the Crab Nebula - the standard candle in gamma-ray astronomy and the brightest steady source in the gamma-ray sky.

Since gamma rays cannot penetrate the Earth's atmosphere, H.E.S.S. detects them indirectly via their interactions with the air molecules. In these interactions a cascade of secondary particles is produced, a so-called air shower. The flash of Cherenkov light emitted by the air shower - much too faint and short to be visible with the naked eye - can be detected by sensitive telescopes on the ground. Large mirrors (diameters of 13 m) reflect the light into a fine-grained camera. The camera field of view is 5° in diameter (about 10 times the diameter of the moon). Mirror and camera are carried by a steel structure, which is designed for high rigidity and weighs about 60 tons per telescope.

The four H.E.S.S. telescopes have been constructed between 2000 and 2003. The complete array was inaugurated in 2004 and has been taking data since then. At the moment, H.E.S.S. is upgraded by the addition of a much larger telescope in the center of the array, therby reducing the energy threshold and increasing the sensitivity of the experiment. The H.E.S.S. II telescope is under construction and first light is expected end of next year.

H.E.S.S. is operated by an international collaboration of mostly European countries.
Starting in 2009, the Institute of Astro- and Particle Physics of the Leopold-Franzens-University Innsbruck has joined H.E.S.S.; the Innsbruck group contributes in the field of data analysis and interpretation of Galactic as well as extragalactic phenomena.