Schmidt-Cassegrain telescopes
Undoubtedly the best and most compact model for astronomical viewing are the well-known Schmidt-Cassegrain telescopes. These telescopes included a corrector plate and folded overall optical path for fantastic viewing of the skies.
The Schmidt-Cassegrain telescopes were developed on the basis of camera designed by Bernhard Schmidt. The first Schmidt-Cassegrain telescopes were granted a patent in 1946 on application physicist Roger Hayward. There is a primary mirror and a Schmidt corrector plate to tackle spherical aberrations in these telescopes. The secondary mirror is convex and the focal plane is located behind the primary mirror. Some models include additional optical implements like field flatteners to improve the viewing.
Today, Schmidt-Cassegrain telescopes have become a great consumer item as it is easy to manufacture and great for both the hobbyist and the professional astronomer. These telescopes are also cheaper when compared to refractor telescopes of the same focal length. The Schmidt-Cassegrain telescopes are also known for viewing the sky in a narrow field of view and extreme portability.
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In fact, the Schmidt-Cassegrain telescopes come in two types – the compact and the non-compact ones. In the first type, the corrector plate is closer to the primary mirror's focal point. This helps in the telescopes being built with short lengths of tube though narrowing the field of view. In the non-compact variety of the Schmidt-Cassegrain telescopes, the tube is comparatively longer though the aberrations are much less. However, Schmidt-Cassegrain telescopes are not better than the Newtonian reflector type of telescopes.
Incidentally, the basic version of the Cassegrain reflector telescope before Schmidt the corrector was added to it had a primary mirror as a parabolic reflector and the secondary mirror as a hyperbolic reflector. The light from an object was reflected back from the secondary mirror through a hole to the primary one.
Today, the Schmidt-Cassegrain telescopes are used in all kinds of satellites and for all kinds of purposes. There are several variations of this telescopic model too. One of the most popular is the Schiefspiegler telescope. It includes oblique and tilted reflectors that prevent the secondary mirror's shadow from falling on the primary one but this arrangement forces other problems in these models of the Schmidt-Cassegrain telescopes.
Then, there is the Dall-Kirkham model that includes an elliptical and concave primary mirror along with a spherical and convex secondary. This is an easier model to make but does not improve the field curvatures thereby affecting the overall image quality. The problem becomes less with larger focal lengths but overall, the Schmidt-Cassegrain telescopes remain the popular version.
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