telescope

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telescopesearch for term

An optical instrument that gathers and focuses light for use in astronomical study. Telescopes are usually shaped as tubes, and they use a series of either mirrors or lenses to focus light onto a camera, CCD, photographic plate, or eyepiece. Two major types of telescopes dominate astronomy: refracting telescopes and reflecting telescopes.

refractors

A refracting telescope uses an objective lens to gather and focus light to a focal point. The aperture size of the telescope is limited by the purity and weight of the glass lens. Several of the most magnificent telescopes are refractors. Yerkes Observatory's 40-inch refractor is the largest. Not only are the instruments mechanically and aesthetically beautiful, but they were also responsible for some of the pioneering astronomical research in the first part of the 20th century that paved the road for present-day discoveries.

reflectors

A reflecting telescope uses a primary mirror to gather and focus light to a focal point. These telescopes are used today almost exclusively in astronomical observatories. The telescope aperture can range from a few centimeters to several meters, since the mirror diameter is limited by the flexible nature of glass, mirror weight, and design of the telescope. Some telescopes overcome these problems with a segmented primary mirror.

In order to help point a telescope in the correct direction, astronomers have developed two systems of coordinates for locating objects in the sky. The first, called the alt-az system, uses altitude and azimuth coordinates. Telescopes that use this coordinate system are said to have alt-az mounts and are used mostly by amateur observers. Professional astronomy demands telescope motion to be able to track stars and faint objects in the sky for long hours. To accommodate this, the telescope must turn opposite the direction of the Earth. The equatorial system of coordinates, used by telescopes with equatorial mounts, solves this problem. The coordinates in an equatorial system are right ascension and declination.