Through the telescope

The development of the telescope was the key to the great advances in astronomy that took place during the seventeenth and eighteenth centuries. Galileo introduced refracting telescopes, which use lenses to produce a magnified image. Newton, and most of the astronomers who followed him, employed reflecting telescopes, in which the chief optical components are curved mirrors. The principle of the reflecting telescope is similar to that of the large dish-shaped aerials that today’s radio-astronomers use to collect microwave radio signals emitted by stars and other celestial objects.

Galileo did not invent the telescope, although he was one of the first to use the instrument for astronomy. His main contribution stemmed from his improvements to the making and selection of lenses (his later telescopes had a magnification of about 33 times). The most important function of an astronomical telescope, however, is not to magnify. Stars are so far away that they never appear as more than points of light, even when viewed through the most powerful telescopes. For this reason an astronomical telescope’s chief function is to increase the amount of light that can enter the eye from a distant object such as a star. That is, to make objects visible that cannot be seen with the unaided eye.

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Power and brightness

The extra light-gathering power of a telescope enables an astronomer to see thousands of stars that are invisible to the naked eye. This light amplification is related to the diameter of the object lens (in a refractor, or the mirror in a reflector) of the telescope. It is equal to the area of the object lens divided by the area of the pupil of the human eye (taken to be about 0.25 inch across when the eye is conditioned for viewing in the dark). For example, a telescope with a lens or mirror 6 inches (15 centimeters) in diameter has a light-gathering power of 625. It can be used to see stars of brightness down to 12 on the magnitude scale. This is about 250 times less bright than a star of magnitude 6, which can just be seen with the unaided eye by someone with good eyesight. The magnitude scale is logarithmic; bright stars have low, or negative, magnitudes (the sun, the brightest object in the heavens, has a magnitude of —27); dim stars have high positive magnitudes.