an ` eye - piece ` system for a non - galilean afocal infra - red telescope designed to contribute to astigmatism correction , pupil match , and true afocality without undue increase in objective aperture requirements comprises a positively powered back group and a negatively powered front group consisting of a rear lens element with a convex rear surface and a front lens element with a concave front surface , there being a negatively powered gas lens between the rear and front elements of the front group . the objective system may be catadioptric .