On the optical design of VHE gamma ray imaging Cherenkov telescopes

Optical characteristics of some operational and future VHE atmospheric imaging gamma ray telescopes are discussed. It is shown that due to large longitudinal size of a TeV air shower in the atmosphere, which is comparable to its average distance to a measuring telescope there is a type of optical aberration, that is proportional to diameter of the used reflector (depth of field). The use of imaging telescopes with essentially larger than 10 m diameter reflectors leads to deterioration of image quality which can strongly affect the accuracy of measured image parameters and the gamma-hadron separation power. It is shown that the edges of a reflector of spherical design in the case of large relative aperture A (A ≥ 1) have to be bent towards the reflectors axis in order to provide the smallest spot size.