Schmidt–Cassegrain telescope

Schmidt–Cassegrain telescope (SCT) An adaptation of the Schmidt camera for visual use. In addition to the concave spheroidal primary mirror it has a convex spheroidal secondary mirror, which reflects the converging beam through a hole in the primary to a Cassegrain focus. This results in a compact instrument which has become very popular for amateur use. The optical performance is not usually as good as that of a Newtonian telescope or conventional Cassegrain instrument because of compromises in the design, and the comparatively large central obstruction caused by the secondary mirror, which reduces contrast.