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Berkeley ASTRON 10 - 7. Telescopes I, Refracting Telescopes

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Refracting Telescopes, 18 September 2013!Foreword:!1.For this lesson we will have to know that there are many telescopes and A.that they work in different ways. !We should feel boggled by the variety of telescopes, at least at the B.beginning of this lecture. !There are several ways to organize telescopes: reflectors vs. refractors is C.one way, but not the only way.!History of telescopes: !2.(1608) No one knows who invented the first telescope, but Hans A.Lippershey (an optician of Middleburg, Holland) is the most likely candidate. !These telescopes used refraction, which is bending far away light in a.order to focus it.!This works because astronomical objects are so far away. !1.As far as our optical devices are concerned, the rays of light from A.astronomical objects come from infinity and are basically parallel.!Even the moon is at infinity for telescopes–"there is no a.difference between focussing on the moon and focussing on the stars. !The telescope collects the light to give you an image of the sky, B.where stars appear as points of light and other celestial bodies are different shapes. !These telescopes were small, handheld, intricately decorated and 2.expensive.!NOTE: "refracting telescope," not "refractory telescope" !3.Something that is refractory melts at a high temperature, so A.technically both refracting and reflecting telescopes are refractory. !(1609) Galileo was the first astronomer to use the telescope extensively B.and scientifically. !Though Galileo made the best telescope in the world for the rest of his a.life, he never claimed to have invented the telescope. !lensHe said he heard about the concept and then made improvements on 1.the design.!He had a background in lenses that would allow him to work from A.a vague description. !Galileo would be considered a hero of astronomy for any one of his b.accomplishments, but transforming telescopes from a curio into a scientific instrument is the the most important.!Telescopes are now the way that astronomy gathers almost all of its 1.data.!One way Galileo accomplished this was to take scientific 2.measurements with the telescope, such as how long it took for an object to move out of the telescope's field of view and the proportion of an object to a field of view.!In general, he is a hero of science:!c.He had a wide ranging and glorious career which will not be 1.discussed in detail because this is an astronomy course.!He was a math professor at an Italian University.!A.He had many difficulties, and he overcame some of them. !B.He invented the concept of using testing/experiments to figure out C.how nature worked. !Unfortunately, Galileo's refracting telescopes had major color distortions C.which people had to fix (unsuccessfully at first).!These distortions were caused by the edge of the lens acting like a prism a.and bringing the various component colors of the light to different focuses. !You can only get one color in focus– the other two will be blurry. !1.The more steeply a lens is curved, the more it acts like a prism. !2.For a century and a half, people tried to reduce this distortion by creating D.shallow lenses, but telescopes with these lenses were too large, expensive, and unwieldy to be a good solution. !Hevelius' 150 foot long telescope needed a ship's mast to support itself, a.lightlightfocus focusfocusand many assistants manning guide wires to be positioned. It was ungainly, and also extremely expensive to make. !Another large seventeenth century refracting telescope was made in b.Italy, and it was similarly expensive and ungainly.!It was so much of a mess that the artist of this illustration of it didn't 1.bother to draw in the mast.!This method was soon abandoned. !c.From the mid 1700s to the mid 1900s, astronomers used two different E.lenses close to each other: one to cancel out the color dispersion of the other. !This breakthrough reduced color distortion into only a minor a.annoyance. !These telescopes were cheaper, mass produced, and more convenient.!b.Though the price of the lenses doubled (four surfaces needed to be 1.polished instead of two, and the two lenses needed to match each other), the rest of the telescope could be smaller, causing the price to be cheaper overall. !An optician named John Dollond mass produced these "Dollond 2.telescopes," and sold them to everyone he could. !NOTE: An English inventor named Chester Moore Hall actually A.thought of this idea. Hall tried to keep it a secret so he could make a lot of money from selling only a few telescopes for large sums of money. Unfortunately for him, but fortunately for astronomy enthusiasts, John Dollond found out about Hall's telescopes and won the patent for them. !Dollond also made his telescopes shorter and more portable, further 3.reducing the price. !You could mount some of the later Dollond telescopes on a table A.– a lot cheaper than the mounting of early refracting telescopes which required masts.!These telescopes were so cheap and plentiful that they could be bought by F.any institution or family in the upper middle class. This allowed astronomy to become more accessible as a hobby. !In fact, it was now fashionable among prosperous people to buy a a.telescope for recreational purposes. !These telescopes were good astronomical instruments, but were also 1.decorated impressively.!America's first famous women astronomer Maria Mitchell, discovered an b.asteroid with one of these telescopes.!By 1845, Americans learned how to make two lens refracting telescopes as G.well. !The first successful telescope maker was Henry Fitz of New York:!a.He discovered how to make telescopes with shorter focal lengths 1.which were especially good for comets, galaxies and nebulae. !His telescopes tubes were made out of mahogany and brass, and were 2.gorgeous. !The Clark family (Alvan Clark & Sons) then created bigger and better b.telescopes: !For example: !1.(1888) They made a 36 inch refractor at Lick Observatory on A.Mount Hamilton, San Jose.!It was built before Silicon Valley polluted the skies with light. !a.(1897) They also made a 40 inch refractor at Yerkes Observatory B.in Wisconsin, which is the world's largest successful refracting telescope.!There were many other telescope makers, some of which did not pu c.their names their telescopes. !People are still improving refracting telescopes: !H.There are new types of optical glass to get more kinds of color in tighter a.focus. !Some telescopes use three lenses instead of two


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Berkeley ASTRON 10 - 7. Telescopes I, Refracting Telescopes

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