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What is a Schmidt-Cassegrain (SC) telescope?

We have Newtonian telescopes and also Galilean ones. How is SC telescope different from other optical telescopes? What is its construction detail and what different mechanism does it work with?

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I don't think this question needs much more than this: starizona.com/acb/basics/equip_telescopes_scts.aspx –  Joan.bdm Jun 30 at 15:10
    
yes yes..this is good. –  MycrofD Jun 30 at 17:52

2 Answers 2

The differences are in the designs:

Newtonian:
enter image description here

Gallilean:
enter image description here

Schmidt-Cassegrain: enter image description here

Schmidt-Cassegrain telescopes are characterized by including 1 convex mirror, 1 concave mirror and 1 correction plate.

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There are many other variations on telescope designs others than the ones you have mentioned. They differ in the light path, the shape of the optics utilised, the use of mirrors and/or lenses, and the physical structure.

A Schmidt-Cassegrain is a particular type of Cassegrain reflector. A Cassegrain reflector uses two mirrors, a primary and a secondary. Both are curved, and the secondary is placed in front of the primary, which blocks some of the light to the primary. The primary directs light onto the secondary, and the secondary directs light through a hole in the centre of the primary. This 'folds' the lightpath, resulting in a short telescope, even at quite wide apertures.

A Schmidt-Cassegrain is a Cassegrain with particular design details. The primary mirror has a spherical curve, and the secondary usually has a spherical curve, though some manufacturers modify the design with a parabolic secondary to improve the performance. A key characteristic is the addition of a lens to correct for the optical aberrations produced by the spherical mirror(s). This lens is also used to mount the secondary mirror, so no vanes are required to hold it in place.

A Galilean telescope is a refractor, and so it uses lenses exclusively instead of mirrors. Lenses may be able to out-perform mirrors, but it is easier to manufacture high-quality large mirrors than large lenses.

Newtonian telescopes are also reflectors with two mirrors, but one is flat. It also blocks some of the light in the centre of the objective, but it is held in place by vanes instead of a lens. The flat mirror angles the light out the side of the telescope near the objective end of the telescope. While this may frequently place the eyepiece in a more convenient position to access it, especially when aiming near the zenith, it is also necessary to rotate the telescope in the mount to get the eyepiece in the right place.

There are pros and cons to all telescopes, and selecting a telescope requires knowing what you will be doing with it, and choosing something that complements that purpose well with its pros, and that has cons that are not relevant.

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This is a pretty good answer. Pictures would have improved its value. So a better answer is the mixture of this answer and the one given above by @krismath –  MycrofD Jul 5 at 9:02
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Yeah, I wanted to add some but didn't have drawings I had permission to use, so figured I would draw some later and add them. May still do that as the ones below aren't credited –  Jeremy Jul 5 at 10:56

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