A scope that is a combination of lenses and mirrors allowing the basic newtonian design to be “folded” so that a catadioptric telescope is more compact than an equivalent newtonian. This combination of lenses and mirrors may also be used to achieve other effects such a building a scope with a very wide aperture but short focal length making it very fast. ie having a low ‘f’ number. There are several different such configurations including Cassegrain; Schmidt Cassegrain, and RASA
Celestron SCT
A hugely popular range of range of Schmidt Cassegrain telescopes ranging from 5″ to 14″. The bigger seller is the C8 (8″)