The Argunov Cassegrain telescope is a telescope design first introduced in 1972 by P.P. Argunov. All optics are spherical, and the classical Cassegrain secondary mirror is replaced by three air-spaced lens elements. The element farthest from the primary mirror is a Mangin mirror, which acts as a second surface mirror, having a reflective coating applied to the surface facing the sky. Argunov systems only employ spherical surfaces and avoid the practical difficulties of making and testing aspheres. However, this seeming benefit is marginal, as it is almost as difficult to make a true, zone-free sphere of precise radius of curvature as it is to make an asphere of equivalent precision.
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