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Telescopes |
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The telescope was invented at around the end of the 16th Century, although it is not clear exactly where or by whom.
The first person known to have used a telescope to look at the night sky was Galileo, in 1610.
There are two basic types of telescope. The first type, as used by Galileo, is known as a "refractor", and is the long tube with a lens at the end that is what most people think of as a telescope. When a ray of light passes through a substance (e.g. a glass lens) and is bent into a different path, this is called refraction, hence the name for this type of telescope.  ABOVE: The focusing of light by a refractor - because the light reaches a focus before it gets to the eyepiece, the image in an astronomical telescope is in fact upside-down. The second type of telescope is the "reflector", which uses a parabolic mirror to focus the light (a parabola is a type of curve, so the word "parabolic" describes how the surface of the mirror is curved).  ABOVE: The most common type of reflector is the Newtonian, invented by Sir Isaac Newton in 1668, in which the eyepiece is on the side of the tube. As you can see from the image above, the problem with a reflector is where to put the eyepiece. In the Newtonian design, a small mirror, known as the "flat", is suspended in the tube to deflect the light out to an eyepiece on the side of the telescope. This secondary mirror is too small to cause problems by blocking out any part of the image. There are many other reflector designs, for example the Cassegrain, which place the eyepiece in different places on the telescope. Modern mirrors are made from glass coated with a reflective layer of aluminium, but originally they were made from polished metal, which was much less effective and more difficult to make. All the large research telescopes of the 20th Century were reflectors, as these can be made much larger than refractors. The problem is that while a mirror can be supported from behind and from the sides, a lens can only be held by its edges. Consequently, very large lenses tend to sag and distort slightly under their own weight. Mirrors are also easier to make. Simply take a round block of glass, grind out the surface to the right curve, polish it, and coat with aluminium. It doesn't matter if there are any minor flaws in the glass block, as light won't actually be passing through it - the glass is really only there to support the reflective layer of aluminium. A lens, however, has to be ground to the right shape on both sides, and since the light is passing through it the glass has to be totally flawless. Hence, the world's largest refractor is still the 40-inch telescope at the University of Chicago's Yerkes observatory, completed in 1897 (I've given it as 40 inches rather than 1 metre, as they normally refer to it as "the 40-inch telescope"). The world's largest reflectors are the twin Keck telescopes in Hawaii, which have 10 metre mirrors. However, the Keck mirrors are constructed from 36 hexagonal segments instead of a single block of glass, so the largest instruments with single-piece mirrors are the Large Binocular Telescope in Arizona, which consists of two 8.4 metre instruments, Japan's 8.3 metre Subaru telescope in Hawaii, and the European Southern Observatory's cluster of four 8.2 metre telescopes in Chile. There is no doubt that larger reflectors will be built in future, and the ESO have even studied the feasibility of constructing one with a 100 metre mirror. |
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Last Updated: 1 Oct 07 |  |
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