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Telescopes: reflecting, refracting, catadioptric

Hale 200" reflecting telescope

The Hale Telescope at Mt. Palomar in California was the largest telescope in the world from 1948 to 1974.  Today, each of the twin 10-meter Keck telescopes atop Mauna Kea in Hawaii are twice its size.

 

 

Although there is some disagreement as to exactly who invented the telescope, the credit is often given to Hans Lippershey, a Dutch spectacle maker. Lippershey was supposedly watching two children playing in his shop with lenses. The children noticed that when they lined up two lenses and then looked through both of them in alignment, a weather vane on a nearby church appeared larger and clearer. Lippershey tried it himself, and then placed a tube between the lenses to hold them in alignment; thus the first telescope was born. He called his invention a "kijker", meaning "looker" in Dutch, and in 1608 applied for a patent with the Belgian government. Though he was paid very well for his invention, he was not granted a patent because it was felt that the simple device could not be kept a secret.

Today all modern instruments fall into one of three main categories: refractor, reflector, or a combination of the two: the compound, or catadioptric, telescope. They all rely on the same principle: the faint image of a distant object is condensed or focused into a much smaller area by a lens (refractor) or a mirror (reflector).  The image at the telescope's focus is brighter than the original image falling on the telescope's mirror (reflector) or lens (refractor) by the number of times the area of the mirror or lens exceeds the area of the focused image. For example, a 10-meter, 394-inch reflector forms a focused 1/4" image 2½ million times brighter than the original image on its 10-meter surface.

 

In 1845 the third Earl of Rosse had an immense 72" reflecting telescope built for himself at Birr castle in central Ireland. With a 3-ton metal mirror suspended between 45-foot stone walls, it was the largest telescope in the world for three-quarters of a century. With his "Leviathan" Lord Rosse was able to see the spiral structure in the gas clouds or nebulae discovered some 80 years earlier by Charles Messier. 80 years later Edwin Hubble identified them as galaxies like our own island universe, the Milky Way. 

 

 

Theoretically, a telescope can magnify its brighter, focused image by as many times as it has been brightened during the process of focusing. This subsequent magnification of a focused image is performed by the eyepiece, a lens or series of lenses placed at the focal point. A telescope almost always has many eyepieces used for different magnifications and their concomitant fields of view.

Magnification, however, is not just limited by an image's brightness at the eyepiece.  Magnification is more often curtailed by instability in the atmosphere through which it is viewed.  It's like lying on the bottom of a stream and looking through the moving water at overhanging tree branches. The view of the branches is degraded by the amount, turbulence and dirtiness of the water.  If you liken this moving water to the atmosphere through which all celestial objects are viewed, you can see why the world's largest telescopes are built on the tops of mountains painstakingly chosen for the lack of turbulence in the air above them. Atmospheric turbulence is also why stars twinkle and planets do not. Stars twinkle because their beams of light are so narrow that they are periodically shifted or interrupted by atmospheric turbulence.  A planet's incoming light beam is just too wide for atmospheric turbulence to interrupt it.  This is also why the higher a celestial object is in the sky, the clearer it is: the closer it is to the horizon, the more air you have to look through to see it. 

The planets in our solar system—of all celestial objects—demand the most magnification (100 to 1000 times) to resolve their surface details.  Except for the planets, however, the original object is usually so faint that the telescope's main job is just making it bright enough to see. Whereas a telescope can make an image millions (or, with time exposure and a CCD, billions) of times brighter, it is often only magnified 50 to a few hundred times because it was originally very faint but not that small. An extreme example of this is our nearest galactic neighbor, the Andromeda Galaxy. At a distance of 2.3 million light years, it's the most distant object we can see with our naked eye.  Yet if we could see its entire, very faint structure unaided, it would appear almost six times wider than the full Moon!  It is certainly big enough, but not at all bright enough, to see well without a telescope.

Telescopes also perform a third function in addition to brightening and magnifying an image.  If the viewed object is an "extended" image like a planet or a galaxy, features within that image may need to be seen in detail and distinguished or separated one from another.  This is particularly true of binary or multiple star systems whose component stars are very close to each other and need to be "resolved."  Whereas a telescope's ability to brighten an image is proportional to the area of its lens or mirror (that is, to the square of the diameter of its "aperture"), its resolving power is directly proportional to just the diameter of its aperture.

This Telescopes-reflecting-refracting-catadioptric page and the entire website are excerpted from You and the Universe, a handmade, personalized fine art book on astronomy, mythology and astrology through which is woven each recipient's complete astrological reading.

 

go to . . .

reflecting telescopes
refracting telescopes
compound-catadioptric telescopes
charge-coupled devices (CCDs)
adaptive optics
astronomy
Galileo's first telescopes

 

 

 

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© Carl Woebcke: Telescopes, reflecting, refracting and catadioptric, 1991-2017. All rights reserved.