The Schmidt-Cassegrain was invented in 1940 by James Gilbert Baker as a modification of Bernhard Schmidt’s 1931 Schmidt camera. As in the Schmidt camera this design uses a spherical primary mirror and a Schmidt corrector plate to correct for spherical aberration. In this Cassegrain configuration the convex secondary mirror acts as a field flattener an relays the image through the perforated primary mirror to a final focal plane located behind the primary. Some designs include additional optical elements (such as field flatteners) near the focal plane.
This design is very popular with consumer telescope manufacturers because it combines easy to manufacture spherical optical surfaces to create an instrument with the long focal length of a refracting telescope with the lower cost per aperture of a reflecting telescope. The compact design makes it very portable for its given aperture, which adds to its marketability. Their high f-ratio means they are not a wide field telescope like their Schmidt camera predecessor but they are good for more narrow field deep sky and planetary viewing.