Telescopes enable astronomers to gather light from distant celestial objects using a series of lenses and/or mirrors. Early astronomers observed this light directly from the telescope's eyepiece. Nowadays, the gathered light is passed into auxiliary instruments where it can be studied in detail. By analyzing the light, astronomers strive to learn about the nature of the celestial objects that produced it.
Refracting telescopes (refractors), like gigantic spyglasses, use clear glass lenses to bend (refract) light rays into a focused image of the object being studied. At least two lenses, located at the end of the telescope opposite the eyepiece, must be used to make a focused image. As light passes through the first lens, the colors of light separate, similar sunlight passing through a prism. Another lens or lenses are needed to lessen this chromatic aberration and and sharpen the image. Refractors are typically built such that the telescope tube length is long compared with their lens diameter, or aperture.
The maximum size of a refractor is limited to an aperture of about 40 inches. A lens much larger than this will slump and deform under its own weight. This lens can only be supported around the edges and cannot be reinforced by a backing structure, since light must pass through it. It is technically very difficult to cast perfectly clear disks of glass of this size. Building, supporting, and housing the proportionately long telescope tubes refracting telescopes require are additional challenges.
Reflecting telescopes (reflectors) use a precisely curved mirror, located at the dome floor end of the telescope. This mirror reflects and focuses light at the sky end of the telescope tube, where the eyepiece was traditionally located. Nowadays, the eyepiece has been replaced by a variety of instruments in conjunction with computers, and astronomers observe from a control room which may be adjacent or many miles away. Other mirrors inside the telescope may reflect the light to alternative focus locations (foci), where specialized instruments are located. See the Shane telescope webpage for further discussion of different foci and their associated instruments.
Reflectors solve many of the problems inherent in refractors. Because reflectors use a mirror at the base end of the telescope, the mirror can be supported from below, unlike the lenses at the top of a refracting telescope tube. More support means that larger aperture (mirror diameter) telescopes can be built. In addition, reflecting the light rather than refracting it eliminates the chromatic aberrations of light passing through a lens. The object of study appears less distorted, and additional correctional lenses are not needed. Elimination of the lenses also decreases the length that a telescope tube needs to be. Decreasing the size of the tube reduces the cost and technical challenges of building the both telescope and its dome.
The glass of a reflector's mirror does not need to be as free of optical distortion than the lens glass of a refracto, since the light does not pass through it.. Only the surface of the mirror needs to be perfect. So casting mirror glass is less technically challenging than casting lens glass. Of course, the mirror surface must be painstakingly ground and polished to perfection and the reflective aluminum mirror surface carefully applied. Single mirrors as large as 330 inches have been cast. Even larger reflectors have been built using multiple mirrors nested in a mosaic. The Keck twin reflectors, for example, each use a mosaic of 36 hexagonal mirrors to provide an aperture of 10 meters, or 393.7 inches! Even larger reflectors are now being designed, most notably the Thirty Meter Telescope (TMT). Larger apertures mean more light can be gathered, so fainter objects may be detected and studied.
In the late 1800s, A.A. Common, using the Crossley Reflector, discovered that astronomers could see more detail in the night sky in photographs than they could by direct observation. Similar to making a time-exposure with a camera, more light can be gath