Astronomical equipment #3 : Catadioptric telescopes
Telescopes in a mixed system, as for their long focal length, are small in size. Their closed construction guarantees durability of the optics. The only disadvantage of these compact instruments is the high price.
Telescope Celestron C 9.25" - in the short tube there is a large diameter mirror
These are telescopes in which a lens that is a correction of optical defects was placed in front of the main mirror or sometimes the secondary mirror. The focus of the main mirror can be in this construction led out as the main mirror, which allows the placement of an eyepiece (the place where we attach the eyepiece to which we look) from the back of the instrument, as is the case in telescopes (lenticular telescopes). The big advantage of this design is the small length of the optical tube. A classic reflecting (Newton) or lens (refractor) telescope usually has a tube with a length close to the focal length of the instrument. In mixed systems, a short tube can contain even a long focal length, needed to obtain large magnifications, used, among others, for observing planets and the Moon. We distinguish two basic types of mirror-lens telescopes.
Schmidt–Cassegrain telescope
Construction scheme
This system has a corrector in the form of an aspherical Schmidt plate. Images created by this system have a light coma (optical defect consisting in not sharpening the image at its edges), however, it is smaller than in the classic Newton telescope. The aspherical corrector is expensive in production and therefore it is relatively expensive instruments. The most common are models with diameters from 6 to 14 inches, which makes them collect a lot of light and are therefore suitable for observing nebular objects.
Maksutov telescope
Construction scheme
In this system the concealer is a slightly concave meniscus lens. He has a better reduced coma. Due to the weight of the corrector is used in small constructions, usually up to 8 inches. These are very good planetary telescopes at fairly affordable prices. Unfortunately, the disadvantage of the structure is the long cooling time of the instrument, which can be troublesome in the winter (the telescope must cool down to the ambient temperature to give good images).
A large Schmidt-Cassegrain telescope
It is generally believed that apochromatic refractors (lenses with high correction of optical defects) are of the best quality. However, they cost a lot, and their diameter usually doesn't exceed 6 inches, which is why they aren't suitable for observing weak nebular objects. A 8-inch Newton reflecting mirror is an expense of around $ 300. This is the best choice when it comes to how much you can see at the lowest price. Mirror telescopes are, however, much more mobile and due to the combination of the features of both constructions perform well on both planets and nebulae. Such an instrument with a large diameter can easily fit into the trunk of most cars and one person will be able to move and fold it. To sum up: If we're looking for mobile equipment for years and we want to see all kinds of celestial bodies, the Schmidt-Cassegrain telescope will be the perfect choice.
Greetings to lovers of Astronomy!
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