Figure 5.5. Major types of telescopes. From Astrophotography for the Amateur, by Michael A. Covington (Cambridge, 1999).
Larger refractors show appreciable color fringing around bright stars unless they use an apochromatic (apo) lens that corrects chromatic aberration more effectively. Apo refractors are a relatively new development. They are popular with astrophotographers because they can cover a very wide field - even a full 6 x 6-cm frame of medium-format film - with an image that is sharp all the way to the edges. Apo refractors are neither cheap nor lightweight, and 6 inches (15 cm) is about the practical limit for a portable instrument.
The Newtonian (invented by Sir Isaac Newton) is one of several kinds of reflector (mirror-based telescope). Newtonians are popular with amateur telescope makers because only one precise optical surface has to be made, the main mirror, which is paraboloidal. Image quality is often excellent and cost is relatively low. Like the refractor, the Newtonian is bulky because the tube length equals the focal length.
Low-cost Newtonians on a simple altazimuth mounts are called Dobsonians ("Dobs"); they were popularized by John Dobson of the San Francisco Sidewalk Astronomers.
The classical Cassegrain, invented by Guillaume Cassegrain in 1672, has a tube much shorter than its focal length. Not only does the folded design save space, but the convex secondary mirror magnifies the image, multiplying the effective focal length. Classical Cassegrains are popular with observatories because the eyepiece is at the lower end of the telescope; you don't have to climb around on ladders the way you do with large Newtonians.
The Schmidt-Cassegrain telescope (SCT) is even more compact. It is a cata-dioptric ("cat") telescope, which means it uses both lenses and mirrors.
In its present form, the Schmidt-Cassegrain was developed by Tom Johnson of Celestron in the 1960s. Both mirrors are spherical,1 hence inexpensive to manufacture.
In front of the secondary mirror is a corrector plate that has a slight but complex curvature. Invented by Bernhard Schmidt in 1930, the Schmidt corrector plate corrects the aberrations that would otherwise result from using spherical mirrors.
A Schmidt-Newtonian has the mirrors of a Newtonian (except that the primary is spherical) and the corrector plate of a Schmidt. Telescopes of this type are new on the scene and usually have short f -ratios; the corrector plate reduces off-axis aberrations.
Another catadioptric design is the Maksutov-Cassegrain ("Mak" for short), popularized by Questar in the 1950s and by Meade (as the ETX) in the 1990s. Adapted from a 1944 design by D. D. Maksutov, it uses all spherical surfaces and can be made to a high degree of precision.2 The secondary mirror is an aluminized spot on the corrector plate and does not get out of collimation. Maksutov-Cassegrains are heavier and more expensive than comparable Schmidt-Cassegrains, but they have a reputation for optical superiority.
Maksutov-Cassegrain designs are also used in "mirror lenses" for cameras, but the details of the design are somewhat different. A telescope is designed to form as sharp an image as possible at the center of the field. A camera lens sacrifices some sharpness at the center to get better sharpness at the edges.
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