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 large. Such telescopes worth the money, 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 take the ambient temperature faster, for example, you don’t have to wait long while preparing outdoor stargazing.
Presently, there are several optical schemes of catadioptric telescopes; a system of 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. One or another distortion will have a place in different optical designs, but generally, catadioptrics give a very good image. This kind of telescopes is good for deep sky and astrophotography.
It’s a mix of a refractor and a reflector: there are both mirrors, and lens in their optical train. This method allows creating relatively low-cost telescopes with a large objective. But, more important – they give a high-quality image. There are two most common types of catadioptric telescopes: Schmidt-Cassegrain and Maksutov-Cassegrain.
Catadioptric telescope: how it works?
The system of Schmidt-Cassegrain is based on spherical mirrors in a short closed case. A full-aperture correctional plate of Schmidt removes spherical aberration. The telescope has a wild field of view (up to 6 degrees), which is good for observation views. 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 able to form a better image than Schmidt-Cassegrain. Minuses of the system are a long time for thermostabilization and great weight.
Advantages of the catadioptric telescopes:
- Powerful catadioptrics are cheaper than two other types
- Good for both: deep sky and the planets
- There are models with a closed case – protection against dust
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 is expensive, but both spherical aberration and coma are removed well.
- Maksutov-Cassegrain. Spherical mirrors + large meniscus made of achromatic glass. The secondary mirror can be performed as a mirror coating on the inner side of a meniscus. There are little 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.