Telescopes |
Today's "telescopes" come a wide variety from orbiting probes that scan the sky in X-rays and other wavelengths to an underground tank of cleaning fluid that serves as a "neutrino" telescope. There are even odd arrays of instruments, which loosely qualify in the definition of "seeing at a distance," intended to detect gravity waves. Huge dishes collect radio waves from across the universe and in recent years have become a commonly recognized symbol of modern science. But certainly the most common and most commonly considered are the optical telescopes used in astronomy. There are two main types with numerous variations:
Despite occasional and fanciful references to Atlantis or ancient Egypt or China, the first true telescope can be traced to Middleburg, Zeeland (southwest Holland) in 1608. Although the credit goes to Hans Lippershey, a spectacle-maker, some claim that the invention actually stemmed from a discovery of his apprentice who put two difference kinds of lenses together and was astonished when some distant object appeared closer. This was a refractor , meaning that it operated by refracting or bending light through glass lenses. It is the easiest kind of telescope to make, at least in smaller versions, and was popularized as the pirate's spyglass . Galileo Galilei made and used this kind of telescope for astronomical purposes.
Lippershey's first telescope was likely only a foot or two long, and magnified only 3 or 4 times. At the request of his government, he soon paired two small telescopes into what we would today call field glasses, similar to but considerably less sophisticated than modern binoculars. (True binoculars also use prisms to "fold" the light path and allow for a more compact design -- that is why the two tubes are not straight but offset or "dog-legged.")
Modern refractors collect light in a large objective
lens. You can imagine this as a huge eye lens, but which collects
far more light than just your eye. The objective lens refracts or
bends the light from a distant object, such as a star, into a
cone. The light from distant objects travels in essentially
parallel rays, but are focused down to a single point after they
pass through the objective lens. Where the cone converges in one
spot is the focal point. After that, the light diverges again
into a smaller cone and then into the eyepiece. The eyepiece
consists of one or more lenses which put the light waves back
into parallel rays so that your eye can focus them. The effect is
that whatever the telescope is focused on appears larger and
brighter. The telescope has effectively become your eye, with
much greater power for detail and able to see much fainter
objects that just your eye alone.
Refracting telescopes, while simple and excellent for many purposes, are limited. Not only are large objective lenses very expensive, they cannot be easily made or effectively used at large sizes because their own weight distorts them. The largest in the world is the Yerkes Observatory refractor in Wisconsin, whose objective is 40 inches across.
Older styles of refractors suffered from a type of
distortion known as chromatic aberration. Without the correcting
lenses common today, refractors focus different colors at
different places, resulting in a sometimes blurred image. Sir
Isaac Newton realized that he could correct this by using a
mirror to reflect light rather than a lens to refract it. He
announced his invention in 1672, and his basic design (shown here),
is still in use today. Interestingly, however, he built only a
small model and never used it seriously. The first useful
astronomical reflecting telescope was built in 1722 by John
Hadley.
In the cutaway view above, light from a distant star comes down the tube and is reflected off the objective mirror. The mirror is constructed in such a way that it reflects the light much the same as the refractor bends it. The problem is that it sends back in the same direction, requiring a small secondary mirror to reflect it off the the side for the eyepiece. You don't look straight through the telescope, so this takes a little getting used to.
Two variants of the reflecting telescope are of particular interest to students and amateur astronomers. They are the Cassegrain (or more commonly today, the Schmidt-Cassegrain) and the Dobsonian.
The Cassegrain, named for the French sculptor who
invented the design in 1672, is a variation of Newton's reflector.
Instead of directing the image at a right angle to the tube,
Cassegrains design reflects it back down the tube an out a small
hole in the middle of the large objective mirror. As a result,
the observer looks through the telescope just as in a
refractor. In a more modern refinement (the Schmidt-Cassegrain),
the front of the tube is covered with a glass correcting plate
that serves as a very weak lens to correct certain problems with
the mirror (known as spherical aberration). Often the secondary
mirror is mounted on the correcting place itself. The Schmidt-Cassegrain
is also known as a catadioptric reflector, signifying that it
uses both a mirror and a lens as part of the objective. This are
the most popular design for serious amateur astronomers.
Another popular variation of the Newtonian reflector
is the Dobsonian, designed by John Dobson, a former monk and
astronomy enthusiast in San Francisco. The innovation of the
Dobsonian is the simple but sturdy mounting, allowing relatively
large and heavy mirrors to be used with ease. Although not
particularly good for use with high power or photography,
Dobsonians (or Dobbies as they are called) are excellent for
public and hobby viewing. They also are relatively inexpensive
and easy to make.
One further consideration is how the telescope is mounted and turned. Since celestial objects move across the sky in arcs, some means has to be provided to follow these moving targets. The two main mounting types are the "Alt-azimuth" and the "Equatorial." Alt-azimuth mounts move the telescope on two axis.One turns the telescope around the horizon (azimuth) and the other moves it up and down (altitude). Simple and inexpensive, this mounting is virtually useless for high-power use or photography. Serious astronomical work demands the smooth operation of a more sophisticated mounting -- the equatorial. The equatorial mounting is essentially an alt-azimuth mounting tilted such that the azimuthal axis is parallel with the Earth's axis. Having set this up and located an object, following the object can be done simply by rotating the axis slowly, which usually is done with a precise motor.
Viewing objects at high power means that any slight jiggle or vibration will be highly amplified. Consequently, mounting should be sturdy and massive to damp out unwanted vibrations from the ground or wind.
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