An optic scheme of a catadioptric telescope

Catadioptric telescopes

Catadioptric telescopes are compact with high optical capabilities. Since a light beam refracts several times inside the optical tube before going out into the eyepiece, the focal length of such telescopes can be very high. Such telescopes are worth the money, because they produce a high-quality image. Often they can be equipped with a computerized mount.

Due to their design, catadioptrics produce different image distortions. Optics for catadioptric telescopes can be constructed inside an open or a closed pipe. The closed optical design is a bit heavier, but it protects lenses and mirrors from dust and damage. Telescopes with an open design adjust to the ambient temperature faster, for example, you don’t have to wait long while preparing for outdoor stargazing.

Presently, there are several optical schemes of catadioptric telescopes; Maksutov-Cassegrain is recognized as the best. Of course, image quality depends not only on the optical design but also on the quality of its optics and adjustment. Multiple distortions are possible with each optical design, but generally, catadioptrics provide a very good image. This kind of telescope is good for deep sky and astrophotography.

It’s a mix of a refractor and a reflector: there are both mirrors, and a lens in their optical train. This method allows creating relatively low-cost telescopes with a large objective. But, more importantly – they provide a high-quality image. There are two common types of catadioptric telescopes: Schmidt-Cassegrain and Maksutov-Cassegrain.

Catadioptric telescope: how it works?

The Schmidt-Cassegrain system is based on spherical mirrors in a short closed case. A full-aperture correctional plate of Schmidt-Cassegrain removes spherical aberration. The telescope has a wild field of view (up to 6 degrees), which is good for observations. Residual distortions are the curvature of the field and a lump.

In a Maksutov-Cassegrain system, the special meniscus is responsible for the correction of distortions. It allows reducing almost all aberrations and is able to form a better-quality image than Schmidt-Cassegrain. Cons of the system are a long time for thermostabilization and the telescopes tend to be heavier than other types.

Advantages of the catadioptric telescopes:

  • Powerful catadioptrics are cheaper than two other types
  • Good for viewing both deep sky and the planets
  • There are models with a closed case – protection against dust
  • Compactness

Disadvantages of the catadioptric telescopes:

  • Long-time for thermostabilization
  • High cost
  • They need an accurate adjustment, so are better for professionals

Optical schemes of the catadioptric telescopes

  • Volosov, Slefogt. This scheme is difficult for manufacturing, hence it is expensive, but both spherical aberration and the comatic aberration is removed well.
  • Maksutov-Cassegrain. Spherical mirrors + large meniscus made of achromatic glass. The secondary mirror can be created as a mirror coated on the inner side of a meniscus. There are few distortions.
  • Klevtsov. Spherical mirrors + meniscus and Manzhen reflective lens. Open pipe. Obvious distortion due to the mounting of the secondary mirror on stretch marks.
  • Schmidt-Cassegrain. Aspherical base + spherical mirrors. Schmidt managed to correct the spherical distortions, but a coma has remained.
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