The case shown in Fig. 22.9 illustrates the principle of the reflecting telescope. When the object is a very long distance away from the mirror the rays from any particular point on it are practically parallel when they reach the mirror. Consequently, an image is formed at the principal focus. The first telescope of this type was made by Sir Isaac Newton in the seventeenth century (Fig. 22.11). In order to see the image conveniently a small plane mirror, M is placed at 45° to the axis of the concave mirror and just in front of the principal focus.
This reflects the rays to one side, and the image may now be viewed through a lens. Newton’s first telescope had a mirror of diameter about 25 mm. One of the largest reflecting telescopes in use today, called the Hale Telescope, is at Mount Palomar Observatory in California. It has a mirror of diameter of just over 5 m which is made of special glass coated with aluminium. The mirror took several years to make and was installed at the top of Mount Palomar, where the cloud- and dust-free atmosphere gives excellent visibility. Such a large mirror collects enough light energy to make it possible to see or photograph very distant stars and nebulae. The world’s largest reflecting telescope, with a mirror 6 metres in diameter, is in the Soviet Union. At present (1983), plans have been prepared at the Royal Greenwich Observatory for a telescope with six mirrors which together have a light-collecting capacity equivalent to a mirror 18 metres in diameter. See also Fig. 22.13.