What Is A Catadioptric Telescope?

| May 19, 2015

Catadioptric telescopes are telescope designs that employ a combination of mirrors and lenses. The name itself refers to this construction: “catoptric” = mirrors “dioptric” = lenses. The result includes fast focal ratios and controlled aberration. Among amateur astronomers, there are four primary types:

  • Schmidt-Cassegrain
  • Maksutov-Cassegrain
  • Maksutov-Newtonian
  • Schmidt-Newtonian

Each of these telescopes’ names reflect a particular feature of the astronomical device, whether telescope or camera. Advantages of the designs include reduced aberration, increased image stability, a compact design, and low maintenance.

Catadioptric Lens

Catadioptric Lens

For the Cassegrain variant, there’s a primary mirror with a central hole and a convex secondary mirror. Light is reflected straight back (180°) through the hole and into the focuser and eyepiece (ocular). The Newtonian variant features a parabolic primary mirror and a diagonal secondary mirror. Light is reflected off the second mirror at a 90° angle, into a focuser and eyepiece located on the side of the tube. The Schmidt variant adds a thin corrector plate that is thicker in the middle and edge. This design helps minimize spherical aberration, such as coma. And a Maksutov variant features a thicker corrector plate, with deeper curves than the Schmidt. The purpose is the same however. It’s to reduce spherical aberration common in reflecting telescopes.

What is the difference between a Schmidt and Maksutov?

Taking these definitions into consideration, then the above combinations, such as Schmidt-Cassegrain, simply feature elements of each, i.e. a Schmidt corrector plate and Cassegrain design. The most common catadioptric telescopes are

  • Schmidt-Cassegrain (SCT)
  • Maksutov-Cassegrain (MCT)

The Schmidt-Cassegrain telescope rivals the Dobsonian telescope in popularity among amateur astronomers. Its compact size, relatively large aperture, and portability make it ideal for hobbyists, both beginner and pro. On the market, SCTs are available in apertures ranging from 4″ to 20″.

The Maksutov-Cassegrain telescope has recently become more widely available and is similar in design to the Schmidt-Cassegrain. They too are compact in design, but feature longer focal lengths and high focal ratios. One consequence of the MCT is that the Maksutov corrector plate’s thickness requires a longer cool-down period than other telescopes, say a Schmidt design.

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