The Schmidt-Cassegrain telescope is a design which allows for both a concave mirror as well as a secondary convex lens. Both optics are aligned about a common focal axis. The advantage of this is that a telescope may maintain a long focal point while reducing the tube size for easier maintenance and transport. They also can handle greater magnification from the eyepiece. The primary mirror has a central hole, by which light passes from the secondary convex mirror and focused into the eyepiece or camera located behind the primary mirror.
These telescopes are almost entirely for astronomical purposes, especially for amateur astronomers. They are also one of the most popular professional telescopes used by the astronomical community. The design dates back to the 1600’s, and is named after Laurent Cassegrain. There are many subsequent designs named after their inventor, such as James Gregory. His design, referred to as the Gregorian reflector, is similar to a Cassegrain design, although uses an off axis configuration. Many of these other designs are commonly referred to as Catadioptric telescopes.
There are many manufacturers of Cassegrain systems, including here in the US are Celestron, Meade and others for amateur purposes. They are also manufactured by international companies, especially in Japan, China, Russia and Germany. There are numerous suppliers of these instruments available on the internet, worldwide. These types of reflector, due to the complex optics, are generally more expensive than a conventional Newtonian reflector or a basic refractor telescope. These telescopes come in many primary mirror apertures, ranging from 4 to about 16 inches for amateur to advanced users. The larger aperture instruments are generally for advanced amateurs or educational institutions. Larger apertures up to 36” are largely for professional research institutions. Due to the weight, most Cassegrain telescopes should be permanently mounted, although some of the smaller instruments can be easily transported.