Dall-Kirkham telescope

A Dall-Kirkham telescope is a type of reflector telescope that has a concave elliptical primary mirror and a convex spherical secondary. It has some aberration not present in a classical Cassegrain reflector or a Ritchey-Chrétien telescope but is easier to manufacture, the mirror being easier to grind, a tradeoff that motivates the design's use in some amateur telescopes.