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The first type of telescope, and the one people are most familiar with, is the refractor. The objective of a refractor is a glass lens at the end of a tube; it focuses the image by bending (refracting) its light rays. The lens of a telescope work the same way as the lens of a magnifying glass. When light enter glass at an angle it changes direction slightly and once again as it leaves the glass. By carefully shaping the lens, it can be made to bend the light rays of an image in such a way as to re-form the image on the other side of the lens. The glass lens of a refracting telescope forms an image at the oppose end of the tube. The eyepiece lens then refocuses the image for the user.
Galileo was not the inventor of the refractor although he was the first person to direct it skywards. At first the only way to improve the refractor was by increasing its focal length. Some early models grew to well over 100-foot length. Fortunately, design improvements were made and such cumbersome lengths are not longer necessary. The largest refractor is the Yerkes Observatory in Williamsbay, Wisconsin. It has a 40-inch objective and a focal length 760 inches.
Why use a refractor? The most important reason is that glass will pass on more light than the most efficient and reflective mirror. A refractor will always beat out a reflector with a similarly sized objective. The other major advantage is maintenance. Most refractors are so simple in design and construction that little or no maintenance is needed. The mirror of a reflector occasionally needs delicate adjusting and it also will have some sort of coating to increase its ability to reflect light. The coating, which is either silver or aluminum, tarnishes and must be periodically replaced.
Despite these advantages, most skygazers use reflectors. The major technical limitation of a refractor is its chromatic aberration. A glass lens bends different colored light to slightly different angles. This causes a series of colored rings around an object being viewed. This rainbow effect becomes more disturbing with the larger lenses.
Refractors tend to be bulky and unwieldy as a result of their high focal ratios. The focal ratio of a telescope is the ratio of its focal length to its object size. A refractor with a large objective must also have a large tube. For instance, an 8" f/15 (8-inch objective, focal ration 15) refractor needs at least five feet between the object lens and its eyepiece. For a telescope that size, you will also need an appropriately sized mounting and counterweight.
The final problem of a refractor is cost. The cost of refractor is roughly twice that of similarly–sized reflector. Large lenses are particularly expensive.
So let us return back to the original question. Why buy a refractor? I would recommend a refractor in two cases. If you want a small telescope, primarily for planet watching, a refractor is excellent. Most of the refractor’s problem increase as its objective size increases. A one or two-inch refractor will provide some very fine viewing of the Moon and planets.
A medium-size refractor is very suitable for a permanent location, especially one which will be used by a large number of people I occasionally go over to a public observatory with a refractor similar in size to my reflecting telescope. Thanks to its clear lens and stable mounting, the refractor provides much larger and brighter images than my reflecting telescope.