Reading: "Optical Telescopes" by Chris Impey

The first type of telescope to be built was the refractor. It uses a lens to bend, or refract, light rays to a focus. 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, but 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 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.

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 could duplicate his best effort with a dollar's worth of components from a drug store.) 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. 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 six 8.4-meter diameters mirrors placed in telescopes in Chile, Hawaii, and Arizona.

Alternatively, a large collecting area can be built up out of a number of smaller mirrors. This approach was first used at the Multiple Mirror Telescope in 1980 in southern Arizona. In the early 1990s, the multiple mirrors of the Multiple Mirror Telescope was replaced with a single 6-meter mirror. Two of the largest optical telescopes in the world are at the Keck Observatory on Mauna Kea in Hawaii, funded by a bequest from the world's largest privately owned oil company. Each of the two Keck telescopes has 36 hexagonal segments, each with a diameter of 1.8 meters. The effective diameter of the telescope is 10 meters. The individual mirrors are controlled by an active system that maintains the mirror's shape to a high degree of accuracy.

Amateur astronomers can share in this excitement by building their own telescopes. Many amateurs (sometimes working in local clubs) grind their own mirrors and other optical elements -- 15 cm or 20 cm or larger -- by methods like that used by Newton. With such telescopes, they can see lunar craters in detail, Saturn's rings, the colored gas of various gas clouds, and many other wonders of the universe. With systematic study of the skies using a small telescope, amateur astronomers can discover comets and dying stars and track the behavior of variable stars. You don't have to be a professional astronomer to contribute to the journey of astronomical discovery.