HESS gamma ray telescope. Panoramic image of two of the telescopes in the HESS (High Energy Stereoscopic System) array on the Gamsberg plateau, Namibia. This array uses four of these Imaging Atmospheric Cherenkov Telescopes (IACTs) to detect the flashes of Cherenkov radiation produced when high-energy gamma rays and cosmic rays hit the Earth's atmosphere. Each telescope has hundreds of mirrors (round discs), each 60 centimetres in diameter and made of aluminised glass with a silica coating. HESS is used to investigate gamma ray sources such as supernova remnants and pulsars and has been operational since December 2003.