Cassegrain telescopes are made up of a combination of two coaxial reflectors. They were invented in the late seventeenth century by Laurent Cassegrain, and combine a secondary convex mirror with a primary concave mirror. Both are symmetrically aligned around the optical axis of the telescope, and the primary mirror usually has a hole in its center. This lets light get to the eyepiece, or sometimes to a light detector or camera. The secondary mirror is hyperbolic, and the primary parabolic.
Some Cassegrain telescopes use a support spider to hold up the second mirror. However, this can cause a star-shaped diffraction effect. Instead, smaller telescopes (and camera lenses), uses a clear glass plate to hold up the secondary mirror. This prevents the distortion. However, some light gathering power must be sacrificed to fix this problem.
The above describes the “classic” Cassegrain telescope. Since its development, however, refinements of this design have occurred. One is the Ritchey-Chretien, which uses two hyperbolic mirrors instead of one hyperbolic and one parabolic mirror. The benefit of this is that spherical aberration and coma are removed. That makes this type of Cassegrain telescope good for photographic and wide field observation. Most professional reflector telescopes use this design, which was invented around the turn of the twentieth century.
The Dall-Kirkham Cassegrain telescope was created in 1928. It uses a convex, spherical secondary mirror and a concave, elliptical primary. This is easier to grind mirrors for than a classic Cassegrain telescope or the Ritchey-Chretien type. However, it doesn't correct for field curvature or off axis coma. That means that image degrades quickly off axis. This is harder to notice if the focal ratio is longer, so Dall-Kirkham Cassegrain telescopes are usually not any faster than f/15.
Sciefspiegler telescopes are an odd variant of the Cassegrain. The name means “oblique” or “skewed reflector”. Tilted mirrors are used to prevent the problem of the secondary mirror shadowing the primary. This eliminated diffraction patterns, but creates several new aberrations.
In addition to the above, reflector telescope Cassegrains, there are also Catadioptric, or compound telescopes that use this type of reflector. The Schmidt-Cassegrain telescope was developed based on the Schmidt camera, which had a wide view field. These telescopes are popular among amateur astronomers. The Maksutov-Cassegrain telescope is a variation on the Maksutov, which uses a transparent corrector lens. The primary mirror is spherical and the secondary mirror is a section of a hollow sphere. The Argunov-Cassegrain, another variation, uses entirely spherical optics. The classical secondary mirror is replaced, in this type, by three lens elements instead.
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