Reflectors are used not only to examine the visible region of the electromagnetic spectrum but also to explore both the shorter- and longer-wavelength regions adjacent to it (i.e., the ultraviolet and the infrared). The name of this type of instrument is derived from the fact that the primary mirror reflects the light back to a focus instead of refracting it. The primary mirror usually has a concave spherical or parabolic shape, and, as it reflects the light, it inverts the image at the focal plane. The figure below illustrates the principle of a concave reflecting mirror.

The primary mirror is located at the lower end of the telescope tube in a reflector and has its front surface coated with an extremely thin film of metal, such as aluminum. The back of the mirror is usually made of glass, although other materials have been used from time to time. Pyrex (trademark) was the principal glass of choice for many of the older large telescopes, but new technology has led to the development and widespread use of a number of glasses with very low coefficients of expansion. A low coefficient of expansion means that the shape of the mirror will not change significantly as the temperature of the telescope changes during the night. Since the back of the mirror serves only to provide the desired form and physical support, it does not have to meet the high optical quality standards required for a lens.

Reflecting telescopes have a number of other advantages over refractors. They are not subject to chromatic aberration because reflected light does not disperse according to wavelength. Also, the telescope tube of a reflector is shorter than that of a refractor of the same diameter, which reduces the cost of the tube. Consequently, the dome for housing a reflector is smaller and more economical to construct. So far only the primary mirror for the reflector has been discussed. In the figure, one might wonder about the location of the eyepiece. The primary mirror reflects the light of the celestial object to the prime focus near the upper end of the tube. Obviously, if an observer put his eye there to observe with a modest-sized reflector, he would block out the light from the primary mirror with his head. Isaac Newton placed a small plane mirror at an angle of 45 inside the prime focus and thereby brought the focus to the side of the telescope tube. The amount of light lost by this procedure is very small when compared to the total light-gathering power of the primary mirror. The Newtonian reflector is popular among amateur telescope makers.

A contemporary of Newton, N. Cassegrain of France, invented another type of reflector. Called the Cassegrainian telescope, this instrument employs a small convex mirror to reflect the light back through a small hole in the primary mirror to a focus located behind the primary. Figure 5 illustrates a typical Cassegrain reflector. Some large telescopes of this kind do not have a hole in the primary mirror but use a small plane mirror in front of the primary to reflect the light outside the main tube and provide another place for observation. The Cassegrain design usually permits short tubes relative to their mirror diameter.

Most large reflecting telescopes that are currently in use have a cage at their prime focus that permits the observer to sit inside the telescope tube while operating the instrument. The five-meter reflector at Palomar Observatory, near San Diego, Calif., is so equipped. Reflectors, like refractors, usually have small guide telescopes mounted parallel to their main optical axis to facilitate locating the desired object. These guide telescopes have low magnification and a wide field of view, the latter being a desirable attribute for finding stars or other remote cosmic objects.

Excerpt from the Encyclopedia Britannica without permission.