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10.20 Optical Telescopes

The first type of telescope to be built was the refractor. It uses a lens to bend, or refract, light rays to a focus. Glasswork matured in Europe in the 16th Century and by the end of the century artisans had learned how to grind convex lenses that could magnify an image. Although he did not invent the refracting telescope, Galileo first used this type astronomically in 1609. The second type, the, the reflector, uses a curved mirror to reflect light rays to a focus. Isaac Newton built the first reflecting telescope in 1668. The first large telescopes were refractors and the largest ever built was the 40-inch telescope at Yerkes Observatory in Wisconsin, opened in 1897. Soon much larger reflecting telescopes were being constructed; the 100-inch telescope at Mount Wilson Observatory in California opened in 1917.
 

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The Large Binocular Telescope on Mt. Graham near Safford, Arizona. This is the largest current optical telescope in the world, employing two 8.4 meter diameter mirrors. Click here for original source URL.

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Albert Einstein and the staff of the Observatory in front of the 40-inch Refractor, Yerkes Observatory — Williams Bay, Wisconsin. This is the largest refracting telescope ever put into use. Click here for original source URL.

Almost every major research telescope now uses the reflector design. There are three reasons for this. First, each little segment of a lens acts like a prism, splitting white light into its component colors. Since red and blue wavelengths are bent by different amounts, there is no single place in a refracting telescope where light of all colors is in focus. Reflecting telescopes avoid this problem because a mirror reflects all wavelengths to a single focus. Second, large refractors have large and heavy lenses, which must be supported around the edge, and they may sag slightly in the middle, distorting the image. Reflectors use a curved mirror, which can be supported across its back surface. The largest refractor ever built had a diameter of 1 meter, whereas astronomical mirrors as large as 8.4 meters across have been constructed. That's a mirror larger than the average living room! Finally, large refractors are long and thin, so they suffer from flexure — bending of the telescope structure that can distort the image.
 

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The 4 main telescopes of the VLT, and the 4 auxiliary telescopes in Paranal, Chile. Click here for original source URL.

Driven by the need for more light-gathering power and better resolution, astronomers have built larger and larger optical telescopes ever since Galileo first used his small refracting telescope. (You can duplicate his best effort with a dollar's worth of components from a drug store, and the Galileo scope upgrades his design with modern components for just $25.) This is an exciting time for optical astronomy. After many decades when the Palomar 5 meter telescope was the largest available, a dozen telescopes of 8 meters or larger diameter have recently been built or are under construction. This is a factor of 200,000 more collecting area than Galileo's telescope.

Today, there are two main approaches to building large telescopes. The first is to construct large, single mirrors, trying to make them as big, light, and accurate as possible. One of the most ingenious facilities sits under the football stadium at the University of Arizona. Glass chunks are placed in an enormous rotating oven. As the oven is heated, the glass flows freely, and as the oven spins, the mirror takes a roughly parabolic shape. A special kind of borosilicate glass is used because it has a low coefficient of thermal expansion so the mirror will not change its shape once it& rsquo's in the telescope and the ambient temperature rises and falls. As the oven cools, the mirror solidifies in its final shape, ready for polishing and aluminizing. This facility produced the largest mirror to be cast in the United States in over 50 years. The largest single mirrors ever produced are 8.4-meter diameters mirrors placed in telescopes in Chile, Hawaii, and Arizona.
 

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Rear view of a primary mirror at W.M. Keck Observatory on Mauna Kea. Click here for original source URL.