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Telescope Designs

Listed below are three of the most common designs of telescopes available to amateur astronomers:

The Refractor

The Refractor. The simplest and oldest type of telescope, the refractor was invented by Galileo in 1609. The refractor uses two lenses; light rays strike the object glass, and are refracted (bent) to come together at the focus, F. The second lens, or eyepiece, is positioned at F. As you can see, the light has to travel a long way to reach the focus; thus, most refractors have a long focal length (the distance between the objective and the focus). The focal ratio, written as f/*, is the focal length divided by the aperture of the objective. The aperture is simply the diameter of the objective or mirror, usually written in millimeters or inches. Since refractors have such long focal lengths, they become bulky as the aperture increases, and more expensive to produce. The world's largest refractor, The Yerkes 40-inch, is 60 feet long!


The Newtonian Reflector

The Newtonian Reflector. One of the most popular and practical telescope designs, the Newtonian Reflector was produced The Dobsonianby Sir Isaac Newton in 1671. The light rays enter the telescope tube and bounce off the primary mirror, which is parabolic in shape (like the shaving mirror that you may have in your bathroom) and located at the back end of the telescope. The light rays bounce again off the diagonal,or secondary mirror, which is flat and inclined at 45º to the main mirror. Finally the light  rays come to the focus, where the eyepiece is located. Reflectors are much easier and cheaper to manufacture, so you can get more aperture in a smaller tube. A popular spin on the reflector is the Dobsonian, designed by John Dobson in the 1970s. The Dobsonian (pictured here) is a  reflector that rides in an altitude/azimuth, or alt/az mount. It pivots in altitude (up and down) and azimuth (side to side) making it extremely portable and easy to set up and use. Reflectors can also be mounted on equatorial mounts.

The Schmidt-Cassegrain The Schmidt-Cassegrain. The "SCT" has grown in popularity over the last decade, espcially among astrophotographers. In the Cassegrain design, the primary mirror is spherical rather than parabolic like the reflector. It also has a hole cut out for the focus. The light rays enter through the corrector plate (the Schmidt portion of the SCT), a thin, almost flat lens. The corrector plate corrects spherical abberation, which is the spherical mirror's inability to bring all the light rays into the same focus. After bouncing off the primary mirror, the light rays hit the secondary mirror, which in the SCT is convex. Focus in the SCT is achieved by moving the primary mirror itself, rather than the position of the eyepiece.


The Equatorial Mount The Equatorial Mount is the other main type of telescope mount and is more complex than than the alt/az mount. It can be motor driven or guided by hand, and it tracks the motion of the stars, following the earth's rotation as it spins on its axis. The German equatorial mount is shown here. To understand how an equatorial mount works, you must be familiar wth right ascension and declination. These are key concepts in astronomy. Right ascension and declination are simply the lines of longitude and latitude, respectively, projected out into the celestial sphere. Just think of them as references for mapping the sky, the same way we use longitude and latitude to map the Earth. While 0º longitude is arbitrarily located in Greenwich, England, 0º right ascension is located in the constellation of Pisces, at the point of the Spring Equinox. 0º declination is located on the celestial equator, which corresponds to the Earth's equator.

For an equatorial mount to track properly it must be polar aligned (that is, the polar axis must be aligned to Polaris, the North Star). If you're located at 38º N latitude, for example, then the polar axis will be angled at 38º from the ground. This is because Polaris is 38º above your horizon, due north. If you're standing atop the North Pole, Polaris will be 90º above your horizon, or straight overhead. This is because the Earth's North Pole is pointed directly toward the position of Polaris in space. Once the polar axis is aligned with Polaris, an object can be located in the sky by moving the scope along its right ascension and declination axes. The declination axis spins around the polar axis, while the right ascension axis is the telescope tube itself. The advantage that an equatorially mounted telescope has over an alt/az mounted telescope is that the equatorially mounted, polar aligned 'scope need only be adjusted in one direction, that of right ascension, once an object is located.

Credits and copyrights for the images on this page: The telescope diagrams were adapted from Norton's Sky Atlas 2000.0 18th Edition, edited by Ian Ridpath, published by Longman Scientific and Technical. The Dobsonian image is from Meade.

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