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why do astronomical telescopes use mirrors instead of lenses?
The earliest telescopes lenses, like those used by Galileo in the early 17th century, were made of glass. Newton in 1704 showed that a curved mirror could also act like a lens. Both glass and mirror telescopes developed in parallel throughout the intervening history. Both have advantages and disadvantages. The overwhelming advantage of mirrors in today's monster telescopes, diameters now measured in meters instead of centimeters, is that they are technically easier to make and are much, much less expensive to manufacture. So it's more a matter of engineering and economics rather than of inherent merit.
(published on 06/11/09)
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