Types of Telescopes
Astronomy benefits greatly as the technology of producing higher quality and more powerful telescopes become increasingly more advanced. Three types of telescopes are used by astronomers: the reflecting telescope, the refracting telescope and the catadioptric, (a combination of the first two).
Reflecting telescopes are also called Newtonians. Reflecting telescopes use mirrors instead of lenses. They are usually built with two mirrors. One is the primary and one is the secondary. The primary mirror is the largest one. Light gathered there is reflected to the secondary mirror and then to the eyepiece. The eyepiece magnifies the image.
The fact that a telescope has mirrors is a positive factor because mirrors do not have chromatic aberration like lenses. Lenses are found in refracting telescopes which bend light differently depending on the wavelength. Chromatic aberration is a situation where colors are not correctly bent by the lens. They all do not combine at the focus well. Therefore, a halo, or fringe, occurs around objects.
Mirrors produce better images than lenses because they reflect all wavelengths of light equally. A reflecting telescope is less expensive than the other two kinds. Because of the simpler design and lower cost, reflecting telescopes can incorporate bigger reflectors. The size of the main mirror or lens of a telescope is called the aperture or opening. The size of the aperture determines how much light can be gathered in by the telescope. The bigger the area of the aperture is, the more light can be let in.
Reflecting telescopes do have some disadvantages. One defect is called a coma. It causes an image to look like a comet around the edge. Images can be fuzzy if the temperature of the mirror and the air outside is different. Collimation refers to the proper alignment of the two mirrors. The mirrors can get out of alignment easily, by handling the telescope or moving it to another location. The mirrors do have to be cleaned frequently also.
Refracting telescopes use lenses instead of mirrors. The maintenance necessary for this type of telescope is less than with the reflecting one. One of the greatest advantages of the refracting telescope is that the lenses do not have to be collimated. The lenses in the refracting telescope do not move. They are fixed. The design of a refracting telescope makes this type much heartier than the reflecting telescope. It will withstand handling better.
Refracting telescopes do not have a secondary lens, like the secondary mirror of the reflecting type. This means that the light is not blocked on its way to the eyepiece. Thus, the images are better. The lens is not affected by the temperature outside or air currents because the air tube is closed. The image can be clearer. The eyepiece is located at the end of the tube. This makes it easier for viewing. On a reflecting telescope, the eyepiece is located on the top of the tube.
Refracting telescopes can be expensive. They suffer from chromatic aberration. Making a large lens in this type of telescope is difficult because of the weight involved. The telescope itself is usually rather heavy, making it difficult to transport.
Catadioptric telescopes are a hybrid of the two other types. They are shorter. That means they can be moved more easily. They also have less chromatic aberration. However, they are more expensive than reflecting telescopes. They also have to be collimated, although less often than a reflecting type. They still have the obstruction caused by a secondary mirror.
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