Reflection telescopes are a type of optical telescope that uses curved mirrors to form the image you see. They were first invented in the seventeenth century. Up till that time, the most common type of telescope was the refracting telescope – the sort you think of when people mention these devices. It used lenses, which suffered from problems with color and brightness viewing some objects. While there are other types of distortion that come with reflecting telescopes, they do permit large lenses and additional optical elements to improve the quality of the image.
There are several different reflecting telescope designs, which have been improved upon over the course of history. However, that doesn't mean the older designs are obsolete. They're still popular for some uses, including for people building telescopes at home.
Newtonian Telescopes
Designed by Sir Isaac Newton in the late seventeenth century, this type of reflecting telescope is still in use today. It's a simple, inexpensive design for any given primary mirror size, making it very popular among home builders.
Cassegrain Telescopes
This reflecting telescope design allows for a very compact instrument. It's sometimes used in small telescopes and in camera lenses, and a variation of this design used in these cases eliminates some distortions that commonly occur. However, removal of these distortions does mean that the image can be a little dim. There are several variations of this type of reflecting telescope. The Ritchey-Chretien telescope uses two hyperbolic mirrors and is free of a number of aberrations that the original tends to suffer from. The Dall-Kirkham is easier to grind mirrors for than the above two variations on the Cassegrain design, but the image degrades quickly when it's off-axis. This is much more noticeable at longer focus ratios, so these telescopes tend to be at about f/15 or less.
Gregorian Telescopes
Invented by Jame Gregory, this type of reflecting telescope uses a concave secondary mirror, creating an upright image. This is useful for observing things on the ground. However, this design isn't often used anymore, except in smaller spotting scopes.
Off Axis Telescopes
Since the secondary mirror can obstruct incoming light, several designs attempt to eliminate it, or move it away from the axis of the primary mirror. Versions of these designs appeared as early as the late eighteenth century. Some problems they have include geometrical aberrations caused by a tilted mirror, increased astigmatism, and high coma. However, some models, like the Schiefspiegler type, have defects that are less noticeable at a larger focal ratio, and are good for observing the planets, moon, and similar objects.
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Refracting telescopes are probably what you think of when someone mentions these instruments. They gather and focus light using curved lenses at either end of the scope, as well as some in between. The refracting telescope...
