
IN A Newtonian telescope either a right-angle prism or a diagonal mirror of plane-parallel glass serves to turn the light focused by the concave mirror at right angles into the eyepiece. If a mirror is used, its front surface must be aluminized or only about four per cent of the light will reach the eye, the remainder passing through the diagonal glass to be wasted. When observing the sun, however, the large mirror of the telescope collects far more light than the eye can accept with comfort. Then an unaluminized diagonal that wastes light in this manner actually becomes desirable. The difficulty in using such a diagonal is that a fraction of the light is reflected back from the inside of the rear surface and reappears at the front to form a second image of the sun nearly as bright as the primary one beside it.
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