Inaugurated in September 2004 and installed in Namibia, the HESS detector is a system of four 13 metre diameter telescopes, and the most sensitive high energy gamma ray detector in the world (gamma rays are a billion times more energetic than X-rays). These rays are very rare. Even for relatively intense sources, only one gamma ray per month and per square metre reaches Earth. As they are absorbed by the atmosphere, they could be directly detected by space instruments; however, these are too small to record a sufficient number of these very rare photons.
The HESS astrophysicists therefore use the atmosphere itself as a detector. When the gamma rays are absorbed by the atmosphere, they emit short flashes of a few billionths of a second of blue light, Cherenkov light. This light is collected by the HESS large telescope mirrors and ultra-sensitive cameras, and images of astronomical objects as they appear in the gamma spectrum can be made.
The HESS experiment is the result of several years of collaboration between over 100 researchers and engineers from Germany, France, the UK, Ireland, the Czech Republic, Armenia, South Africa and Namibia. The first data recorded led to a great number of discoveries, including the first image of a supernova shock wave at the highest energies.