In the preceding section we saw how to use refraction by lenses to design a telescope. We may also use reflection from mirrors to accomplish the same purpose.

The Principle of Reflection

The following figure illustrates the principle of reflection: the angle of incidence (measured from the perpendicular to the reflecting surface) is equal to the angle of reflection. The right side of the figure illustrates the use of a mirror to make a reflecting telescope.

Principle of reflection and the reflecting telescope

Here is a Java applet illustrating the use of a mirror (a diverging or convex mirror in this case) to form an image. For technical reasons, the largest optical telescopes are reflecting rather than refracting telescopes: it is easier to build and support large mirrors of high optical quality than large lenses.

Focus for Reflecting Telescopes

One problem that must be surmounted with a reflecting telescope is how to place an observer at the focus. In the example shown above, the focus is inside the telescope. This is called the prime focus, and in some large telescopes observations can be made at the prime focus. More commonly, various mirror arrangements are used to transport the light from the focus to an external observer. Two common ones are a Cassegrain focus and a Newtonian focus.

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