Refractor Telescopes

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A Short History Of Refractor Telescopes

A refracting telescope, the first type of telescope, uses lenses to magnify objects. Refractor telescopes are reported to have been invented in the Netherlands, though there is some evidence they had been in use in other areas. In 1608 the opticians Hans Lippershey and Zacharia Jannsen came up with the initial designs. There is evidence of others coming up with the telescope around the same time, Jacob Metius being one who applied for a patent around the same time as Lippershey, and some controversy remains on who the actual original inventor was. Regardless of who credit is given to, the early inventors realized they could magnify objects by looking through two lenses.

Galileo Galilei invented his version of the refracting telescope in Venice in 1609. He presented the telescope to the Doge and was rewarded for his efforts. He made improvements over the first telescopes and was the first to use the telescope for astronomical purposes. He used his telescope to map the moon, discover moons of Jupiter, see sun spots, and Venusí phases.

 

The Galilean telescope design used a concave eyepiece with a convex lens which resulted in the image being right side up. In 1611 the design was changed and further improved by Johannes Kepler. He changed the eyepiece to a convex lens. While this caused the image to be inverted, it made the field of view wider and improved eye relief. The idea of the Keplarian telescope design was first used in a telescope by Christoph Schenier, a Jesuit astronomer, in 1630.

There were some problems with the early refracting telescopes. They had chromatic and spherical aberration. This caused the telescopes to have a long focal length, some up to 150 feet. This problem was corrected by John Dollond, an optician from Britain, around 1758. He was the first to patent the achromatic refracting lens, though some give credit for this development to Chester Moore Hall. He is said to have come up with the idea in 1733. The achromatic refracting lens was constructed with two different pieces of glass, crown and flint glass. The crown and flint glass had different dispersions which corrected some of the aberrations. Refractor telescope design has changed little since this improvement. The achromatic lenses, however, kept telescopes relatively small because of the limited size of the disks of flint glass.

 

A Swiss optician discovered how to make the flint glass larger, allowing for refractor telescopes to be made with a diameter of up to ten inches. The optician was Pierre Louis Guinard. His discovery was not made until late in the seventeen hundreds. Since his time, much advancement has been made in glass manufacturing that has further reduced aberration. Apochromatic refractors are made with extra-low dispersion glass that makes the image sharper and reduces chromatic aberration.

The largest refracting telescope for scientific research was built by an American optician, Alvan Clark. The telescope is at the Yerkes Observatory of the University of Chicago in Wisconsin. It is a one meter refractor built in 1897.

 

Refractor Telescopes 2012