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Telescopes - The Eyes of our Solar System



The History of Telescopes:

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One of the Earliest Telescopes

The first telescopes were produced in 1608 and were designed by Hans Lippershey and Zacharias Janssen as a type of spyglass. These early refracting telescopes had a convex objective lens and a concave eyepiece and focusing was achieved by sliding the pieces in and out. Galileo greatly improved upon this design and is often credited as being the first to invent the first astronomical telescope. By the year 1616, Niccolo Zucchi had designed the first reflecting telescope, but the design was impractical. By 1688, Sir Isaac Newton had solved the problems of Zucchi's reflector by adding a small flat diagonal mirror to reflect the light to an eyepiece mounted on the side of the telescope. During 1688, Laurent Cassegrain initiated the design of a reflector with a small convex secondary mirror to reflect light through a central hole in the main mirror. In 1733, Chester Moore Hall went into telescope history with the first achromatic lens refractor and John Dolland independently came up with the same design five years later producing telescopes and marketing them. John Hadley was working to improve reflecting telescopes in 1721 by creating larger paraboloidal mirrors, but it would be 1857 before Leon Foucault came up with a good process for coating them, known as silvering. Believe it or not, it would be 1932, before long lasting aluminized coatings on reflector mirrors would become a standard! (1)




Reflectors and Refractors
The two basic telescopedesigns are refracting telescopes and reflecting telescopes. Refracting telescopes are the simpler of the two, using two transparent glass lenses tocollect and focus the light it gathers. Reflecting telescopes use two mirrors, a primary mirror that is precisely curved to gather light, and secondary a mirror which reflects the light to a focus where it can be observed. The majority of telescopes used in modern astronomical research are reflecting telescopes. There are two main factors that make reflecting telescopes more popular. The first reason is that reflecting telescopes require only one surface of the mirror to be precisely shaped, and the quality of the glass underneath is not important, whereas with refracting telescopes both sides of the lens must be precisely shaped, and the glass must be of a high quality. Secondly, the size of refracting telescopes is dictated by the size of the lens, as large glass lenses are very heavy and unwieldy and can only be held in place by their edges. Since the lens is at the
top of the telescope, the glass is difficult to stabilize and prevent from deforming. Reflecting telescopes, on the other hand, is mounted at the bottom of the telescope, presenting less of a problem in regards to stabilization and preservation of the lens. Another problem that refracting telescopes have that reflecting telescopes do not is called chromatic aberration, in which the lens brings different colors of light into focus at slightly different places. While the size of refracting telescopes was in the past limited by the weight of the glass used for the lens, recent technological innovations have allowed scientists to build lighter-weight mirrors and larger reflecting telescopes. Despite these technological advances, refracting telescopes are still better and are more widely used for astronomical research. (2)

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Telescope History – 20th and 21st Century
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The first radio telescope was built by Grote Reber in 1937, and many types of telescopes were developed in the 20th century for a wide range of wavelengths from radio, to gamma-rays. Interest in radio astronomy grew after the Second World War when much larger dishes were built including: the 250 ft. Jodrell bank telescope in 1957, the 300 ft. Green Bank Telescope in 1962, and the 328 ft. Effelsberg telescope in 1971. The huge 1000 ft Arecibo telescope came along in 1963, and it is so large that it is fixed into a natural depression in the ground. The central antenna can be steered to allow the telescope to study objects up to twenty degrees from the zenith.(3)




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The first Gamma Ray telescope was the Compton Gamma Ray Observatory built in 1991. Very high-energy gamma-rays can be detected from the ground via the Cerenkov radiation produced by the passage of the gamma-rays in the Earth’s atmosphere. Several Cerenkov imaging telescopes have been built around the world including: the HEGRA in 1987, STACEE in 2001, HESS in 2003, and MAGIC in 2004. (4)






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X-ray telescopes, known as Wolter telescopes, have been in use in satellites since the 1940′s. Some of the OAO satellites conducted X-ray astronomy in the late 1960s, but the first dedicated X-ray satellite was the Uhuru in 1970 which discovered 300 sources. More recent X-ray satellites include: the EXOSAT in 1983, ROSAT in 1990, Chandra in 1999, and Newton in 1999. (5)





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Ultra-violet telescopes began as early as 1962. The International Ultraviolet Explorer in 1978 systematically surveyed the sky for eighteen years, using a 45 cm (18 in) aperture telescope with two spectroscopes. Extreme-ultraviolet astronomy (10-100 nm) is a discipline in its own right and involves many of the techniques of X-ray astronomy. The Extreme Ultraviolet Explorer in 1992, was a satellite which operated at these wavelengths. (6)











The Most Famous Telescope In History
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In 1962, the USA’s National Academy of Sciences recommends building a large space telescope. In 1977, Congress votes to fund the project and construction of Hubble Space Telescope begins. Construction of Hubble Space Telescope was completed in 1985. The launch of Hubble was delayed due to the 1986 Space Shuttle Challenger disaster. Hubble was launched on the space shuttle on April 25, 1990. (1)





Twin Telescopes
Twin telescopes utilize two separate mirrors in two separate telescopes to produce one precise image of the universe. This was a new idea that was start in 2001 with the first combined image produced by the Keck Observatory. The two telescopes at the Keck Observatory, Keck I and Keck II, successfully combined their images in March of 2001. The telescopes were completed in 1993 and 1996 respectively. The telescopes are still in operation today and produce some of the best and most accurate images of the universe. (7)



One Telescope, Two Mirrors

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The Large Binocular Telescope at the Mount Graham Observatory is one of the largest and most powerful telescopes in the world. It was used to discover a galaxy cluster 2XMM J083026+524133 in 2008, over 7 billion light years away from Earth. In 2007 it detected a 26th magnitude afterglow from the gamma ray burst GRB 070125. (7)



Citations:

(1) http://www.nasa.gov/audience/forstudents/9-12/features/telescope_feature_912.html

(2) http://www.astronomytoday.com/astronomy/telescopes.html

(3) http://www.nrao.edu/index.php/learn/radioastronomy/radiotelescopes

(4) http://science.hq.nasa.gov/kids/imagers/ems/gamma.html

(5) http://www.britannica.com/EBchecked/topic/650496/X-ray-telescope

(6) http://www.coolscopes.com/ultraviolettelescopes.html

(7) http://www.scientificamerican.com/article.cfm?id=first-light-for-twin-tele