phys0001_chapter05 - Chapter 5 Telescopes and...

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Chapter 5 Telescopes and Electromagnetic Waves Galileo was the first person who used a telescope to observe the sky. We will see what a telescope is and the nature of light and electromagnetic waves. Why Do We Need a Telescope? Most objects in the sky are very dim. Telescope effectively collects more light and converges it for our viewing. Thus, a telescope with larger collecting area, which usually means larger main mirror or lens, is more powerful. This is the main reason why we prefer large telescopes. Large telescopes also improve the resolution . We can see more clearly even if the apparent size of an object is the same. This is similar, but not equal to, seeing an object in or out of focus. There are three kinds of telescopes: the refracting telescopes ( refractors ), the reflecting telescopes ( reflectors ) and the catadioptric telescopes . In refractor, a lens is used to bend light to a point, called the focus , for viewing. In reflector, a mirror is used instead. Catadioptric telescopes use both a lens, called correcting plate, and a mirror. The following figure shows the three basic optical designs of telescopes. Usually, apart from the main lens or mirror, there are more mirrors to bring the focus to some convenient position, as shown in the diagrams above. Common misconceptions: 1. People usually think that larger telescope will magnify the object more. The magnification is the ratio of the apparent sizes of the object. For example, if the angular size of the object is one arc minute and through a telescope, its apparent size becomes 30 arc minutes, then we say the magnification is 30. The
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magnification of any telescope can be changed very easily (by changing the eyepiece). Even for the largest telescopes, the magnification is seldom over 500,
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This note was uploaded on 10/14/2010 for the course BSC phy1001 taught by Professor Prof during the Spring '10 term at HKU.

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phys0001_chapter05 - Chapter 5 Telescopes and...

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