H.E.S.S. observes gamma radiation. When this very high-energy electromagnetic radiation hits the Earth's atmosphere, it triggers a so-called particle shower with a large number of charged particles. These particles are still so energetic that they can move faster than light in the atmosphere (light propagates more slowly in air than in vacuum). Charged particles also excite other atoms and molecules to emit light waves as they pass.
What happens then is very similar to what is observed in airplanes when their speed exceeds the speed of sound: the individual waves superimpose themselves on a front, which leads to a supersonic bang in airplanes. Particle showers are referred to as Cherenkov radiation, a flash of light that lasts a few billionths of a second, too short to be detected by the human eye.
H.E.S.S.'s mirrors and their high-speed cameras, on the other hand, can image this radiation. An image is taken which shows the direction of the air showers and thus also the direction to the source of the gamma photon.
The intensity of the image is a measure for the energy of the photons. If only one telescope observes a particle shower, it is difficult to determine the geometry of the shower and the exact direction of origin. Therefore, several telescopes are usually combined and these allow a stereoscopic determination of the air shower geometry.
In the case of H.E.S.S., four 12 m diameter telescopes are located in a square with 120 m side length and a 28 m diameter telescope is in its center. Each telescope can be rotated about two axes so that objects in the night sky can be tracked automatically. A video from the Helmholtz Alliance For Astroparticle Physics demonstrates the mobility of the telescopes.
The 12 m telescopes are each composed of 382 mirrors with a diameter of 60 cm each and achieve a total mirror area of 108 m². The 28 m telescope, on the other hand, consists of 875 hexagonal mirrors with a diameter of 90 cm each and a total mirror surface of 614 m². Two so-called actuators are attached to each mirror, motors that allow remote control of the mirrors. Before each observation, the mirrors are automatically aligned to optimize the image of the observed object.