Cassegrainian telescope
From Glossary of Meteorology
A coaxial optical system used to transmit and/or collect optical energy.
The concave primary mirror's surface is parabolic in shape and has a central hole into which light passes to or from a smaller, convex secondary mirror with a hyperbolic surface. The focus of the optical system is behind the hole in the primary mirror when collimated light illuminates the primary mirror.
