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How the
telescope came about...
 The
telescope was invented in Holland but as some controversy exists over the actual inventor.
In 1609 the Italian astronomer Galileo exhibited the
first telescope on record. The German astronomer Johannes Kepler discovered the
principle of the astronomical telescope with two convex lenses. This idea was employed in
a telescope constructed by the German Jesuit astronomer Christoph Scheiner about 1630. The
invention of the achromatic, or colorless, object glass in 1757 by the British optician
John Dollond and the improvement of optical flint glass, which began in 1754, soon
permitted the construction of improved refracting telescopes. Dollond's lenses were only
7.5 to 10 cm (3 to 4 in) in diameter, however, so these telescopes all had modest
dimensions. Pierre Louis Guinand, a Swiss optician who became associated with the German
physicist Joseph von Fraunhofer, discovered methods of making large disks of flint glass
in the late 18th century. Guinand's discovery permitted the manufacture of telescopes as
large as 25 cm (10 in) in diameter.
The next successful manufacturer of telescope
lenses was the American lens maker and astronomer Alvan Clark, who gradually achieved the
highest rank as a maker of telescope lenses. A concave mirror is used to form an image in
the reflecting telescope. Early in the 17th century, an Italian Jesuit, Niccolo Zucchi,
was the first to use an eye lens to view the image produced by a concave mirror, but the
Scottish mathematician James Gregory first described a telescope with a reflecting mirror
in 1663. The English mathematician and physicist Sir Isaac Newton constructed
the first reflecting telescope in 1668, but viewing was difficult in this type of
telescope because the eyepiece and the head of the observer cut off a large portion of the
incident, or incoming, rays. Gregory removed this difficulty in his design by interposing
a second concave mirror, which reflected the rays to the eyepiece. Henry Draper, one of
the few early American astronomers to construct a reflecting mirror, successfully used a
prism that reflected all light instead of a flat mirror.
The French physician and astronomer Giovanni D.
Cassegrain invented a telescope about 1672 that used a convex mirror instead of a concave
one. The English astronomer Sir
William Herschel successfully tilted the mirror in
his telescope and placed the eyepiece so that it did not block the incident rays. The
mirrors for reflecting telescopes were usually made of speculum metal, a mixture of copper
and tin, until the German chemist Baron
Justus von Liebig discovered the method of depositing
a film of silver on a glass surface. Silvering has been superseded by aluminum coating,
which lasts much longer.
In 1982 Canadian physicist Ermanno Borra helped
revive interest in building reflecting telescopes with mirrors created by spinning pure
liquid mercury in a concave dish. This technique had been proposed as early as the 17th
century, but the technology required to spin the dish without shaking it was not present
until the late 20th century. The movement of the dish forces the mercury up against the
sides, forming a perfect, smooth paraboloid , even if the surface of the dish is not
perfectly smooth or perfectly shaped. Liquid mercury mirrors can be made much larger than
other mirrors without jeopardizing the quality of the reflective surface, but liquid
mercury mirrors cannot be tilted as far as other mirrors, making observations of objects
on the horizon impossible. Makers of conventional mirrors have adopted the technique of
spinning liquids to form paraboloids. Molten glass is spun into shape until it cools and
hardens in a process called spin casting, then smoothed and polished.
The Keck telescope incorporates an important
design innovation. The surface of the instrument's mirror consists of 36 individual
hexagonal segments, each of which can be positioned by three actuator pistons.
Segmentation not only reduces the weight of the instrument, it also makes polishing the
giant mirror a much easier task. A second telescope at Mauna Kea, Keck II, is planned to
begin operation in late 1996.
The Hubble Space Telescope has the advantage of
being above the earth's distorting atmosphere. Launched in 1990 with multiple mechanical
and electronic problems, the telescope was repaired in December 1993. In the late 1980s a
group of astronomers at the Mullard Radio Astronomy Observatory at Cambridge University in
England began applying radio astronomy interferometry techniques to optical astronomy. The
Cambridge astronomers used three fairly small telescopes to gather light from the double
star Capella, which consists of a very bright star and a dim star that orbit
each other so closely that not even the Hubble Space Telescope can produce an image
showing two separate stars. By combining the beams of light from the three telescopes and
analyzing the interference patterns, the astronomers produced an exceptionally clear image
that shows both stars.
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