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Riddle:
How is playing a game of darts like looking for planets around other stars?
To read the answer, hold a mirror to one side of your computer monitor so that you see the monitor's image. Or print this page and look at its reflection in a mirror.
Answer:
Catchy, huh? But, Dr. Marc, what does this strange answer mean?
And what do mirrors have to do with looking for other planets, Dr. Marc?
 Almost all telescopes nowadays use mirrors, not lenses, to gather and focus the light from celestial objects. We have just seen, though, that mirrors reverse the image. The riddle's answer you saw on your monitor (or paper) looks very different from its image in a mirror. So why use mirrors?
Well, there are several excellent reasons. The optics (that is, the mirrors or lenses that focus light) of a good telescope must be nearly perfect. The mirror or lens must be just the right shape and the surface must be perfectly smooth and polished. In the case of a lens, even the inside must be perfect. Otherwise, the image appears distorted. Also, the larger the light-gathering surface (that is, the lens or mirror), the brighter the image. The brighter the image, the fainter the stars that can be seen.
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A lens (like in a pair of eyeglasses or a magnifying glass) must be very large and very thick (thus very heavy) if it is to be very powerful. Not only must the front and back surfaces be perfect, but the inside must also be perfect, because the light passes through the whole lens. Any flaws in the lens will ruin the image--like looking through some kinds of shower doors or warped windows.
A mirror, on the other hand, can be very thin, even if it is large, as long as it has the right curved shape. And there is only one surface to clean and polish. It is much easier to make a large, near-perfect mirror than to make a large, near-perfect lens.
