Astronomy 101 - Part 1 - Cassegrain

   In 1672, Jacques Cassegrain (1652-1712) proposed his telescope design, but probably never constructed any. The first known Cassegrain telescope was built by James Short (1710-1768). There are two common Cassegrain telescopes available on the market: - the Schmidt-Cassegrain ("SCT") and the Maksutov-Cassegrain ("Mak-Cas").

   The Schmidt-Cassegrain, like this Meade 10-inch LX200, utilizes a concave primary mirror to gather light and reflect it to a convex, adjustable secondary mirror. The light is then reflected through a hole in the primary mirror to the eyepiece. This design helps to remove coma, a problem with Newtonian reflectors. It also places the focus at a more symmetrical position, allowing for the easier use of cameras.

   The Schmidt part of the system has to do with a specially shaped corrector plate that covers the front end of the tube assembly. This corrector plate is coated, like a camera or binocular lens, to allow light to transmit as freely as possible to the eyepiece. Putting the Schmidt and Cassegrain features together, creates a telescope that is compact for its aperture size, yet yields high quality deep-sky and planetary views. The Schmidt-Cassegrain telescope (SCT) is one of the most popular designs due mostly to these two features.

   The Maksutov-Cassegrain used to be considered rare, but lately has made a bid for widespread popularity, showcased by the iOptron MAK 152mm Telescope. Instead of an aspherical corrector plate like the one used in the Schmidt, the Maksutov utilizes a crescent-shaped lens. This corrector plate is shaped like a shallow bowl, which acts as a secondary mirror. Meade, Vixen, and iOptron are the main manufacturers of new Mak-Cass telescopes.

Maksutov Cassegrains and Schmidt Cassegrains are mostly found on either fork-type or equatorial mounts.

Meade’s ETX-90 & ETX-125 are examples of how popular the Maksutov Cassegrain has become. Advantages of the Maksutov-Cassegrain:*