The optical system of the TUS space experiment

The TUS space experiment has been proposed to address some of the most important astrophysical and particle physics problems. It is aimed to study the energy spectrum, composition and angular distribution of Ultra High Energy Cosmic Rays (UHECR) at in the region of the GZK cutoff. The TUS mission is now planned for operation at the Small Space Apparatus (SSA) as an additional payload of the Foton-4 Russian-ESA satellite. The TUS optical system is an important part of the detector which will measure the fluorescence light radiated by EAS of the UHECR. It consists of a 7-segment Fresnel mirror-concentrator of 2 sq. m. The photo receiver at the mirror focal surface comprises 256 PMT pixels. In this paper the TUS optical system is described. Production of its technological prototype is in progress.