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 l arge 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 es sentially larger than 10 m diameter reflectors leads to deterioration of image quality which can strongly affect the accuracy of measured im age parameters and the gamma-hadron separation power. It is shown that the edges of a reflector of spherical design in the case of large rel ative aperture A (A greater than or equal to 1) have to be bent toward s the reflectors axis in order to provide the smallest spot size.