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Reflecting
Telescopes
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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.
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Principle of reflection and the reflecting telescope
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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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