Light and other kinds of electromagnetic radiation coming from the universe outside the Earth must travel enormous distances through space and time to reach observers. Only the brightest and nearest stars can be seen with the unaided eye. To see farther and to clarify and measure what is seen, a telescope is needed. The word telescope is derived from the Greek words tele, “from afar,” and skopos, “viewer.” Even a simple homemade telescope can clearly show Saturn’s rings, Jupiter’s bands and red spot, stars, nebulae, and nearby galaxies not visible to the unaided eye. The ability to study the distant planets and other structures in the universe with these powerful yet remarkably simple instruments has revolutionized mankind’s understanding of the natural world.
All telescopes gather radiation from distant objects over a large area and focus it, thereby increasing the intensity of the radiation and allowing the objects to be magnified. Sophisticated telescopes are used to view radiation in all parts of the electromagnetic spectrum from long-wave radiation and radio waves to infrared radiation and light and much shorter wave radiation, including ultraviolet and X rays.
This radiation travels through space at the speed of light in the form of waves of electric and magnetic fields. Because of its basic similarity, all such radiation can be focused by reflecting it off a
The first telescope developed, and the one most widely used, is the optical telescope, which gathers visible light radiation. There are three basic types of optical telescopes: refractors that use lenses, reflectors that use mirrors, and catadioptrics that use a combination of both lenses and mirrors.
The refracting telescope has a closed tube. At one end of the tube is the object glass, usually made of two or more lenses, that admits light emanating from the object observed. The light rays are refracted by the lenses to a point of focus at the lower end of the tube where the eyepiece is located.
The reflecting telescope focuses light rays with a large curved concave mirror that is generally made of glass covered with a thin coating of aluminum. In the simplest reflector, called the Newtonian reflector after its inventor Sir Isaac Newton, light is collected by a primary curved mirror at the bottom of the tube and reflected forward to a secondary mirror. The secondary mirror is flat and mounted at a 45-degree angle that deflects the converging light rays 90 degrees to the eyepiece.
The light-gathering power of a telescope is determined by the diameter of its objective mirror or lens. This light-gathering power determines how faint an object the telescope can observe. Telescope magnification is determined by the ratio of the objective focal length to that of the eyepiece. Focal length is the distance from objective to focal point. Thus, the longer the focal length the greater the magnification.
In order to make long focal length telescopes more compact, the secondary mirror can also be made curved as well. In such reflectors–which are called Cassegrain reflectors after N. Cassegrain, the French optician who invented them–the secondary mirror is a convex front surface mirror that reflects light collected by the concave primary mirror straight back down the tube through a hole in the center of the primary mirror. The combined action of the two mirrors dramatically increases the telescope’s effective focal length over its actual length.
All telescopes suffer from optical defects called aberrations. Aberrations are distortions in the image. Refractors suffer from chromatic aberrations caused by the varying degree that light rays of different wavelengths are bent by the lens.
By using compound lenses made of different types of glass, suchchromatic aberrations can be eliminated. Reflectors also have various aberrations that occur when light from the side of the viewing region is not precisely
focused. To correct both sets of aberrations, some telescopes use thin lenses called catadioptrics. Catadioptrics are used for photographing wide areas of the sky with low distortion.
Mountings and Size
In general a telescope can be pointed in all directions if two mutually perpendicular axes of rotation are provided. In both large and small visual telescopes these axes are often made vertical and horizontal in what is known as an altazimuth mount. For some telescopes, the equatorial mount is frequently used. In this mount one of the axes, known as the polar axis, is made accurately parallel to the axis of the Earth. The axis perpendicular to this is known as the declination axis. This type of mounting has the great advantage that any object can be followed from east to west by driving the polar axis at the uniform rate of one revolution in 24 hours.
A typical reflecting telescope used by an amateur astronomer may have a primary mirror measuring 4 to 8 inches in diameter. Reflecting telescopes used by professional astronomers usually have mirrors that measure more than 60 inches in diameter. One of the largest is a 236-inch telescope in the Caucasus Mountains of Eastern Europe that began operating in 1976. From 1948 until 1976 the largest reflecting telescope in the world was the Hale instrument at the Palomar Observatory near Pasadena, Calif., with its 200-inch mirror. Other reflecting telescopes more than 150 inches in diameter are located at observatories near Tucson, Ariz.; La Serana, Chile; and Siding Spring, Australia.
The two largest refracting telescopes are the 36-inch instrument at Lick Observatory of the University of California, located on Mount Hamilton, and the 40-inch telescope of the Yerkes Observatory of the University of Chicago, located in Williams Bay, Wis. The focal length of the Yerkes Observatory refractor is 63 feet.
An obstacle to building ever larger telescopes is the distortion of large lenses and mirrors caused by gravity. In 1978 an innovative reflector called the Multiple Mirror Telescope (MMT) began operation at the Smithsonian Astrophysical Observatory in Arizona. Instead of one large mirror, the MMT features six mirrors arranged to focus together. The six-mirror combination acts like a single mirror 21 feet in diameter.
Similarly, the Keck Telescope on Mauna Kea in Hawaii, completed in 1991, has a 33-foot series of mirrors forming a mosaic of hexagons. Astronomers operating the New Technology Telescope of the European Southern Observatory in La Silla, Chile, use a special computer system that frequently pushes and tugs on the mirror to keep it from sagging under its own weight.
The Nordic Optical Telescope in the Canary Islands has the thinnest and lightest mirror of any comparably sized telescope in the world.
A telescope’s resolution is its ability to delineate distant objects that appear close in the sky–increases proportionally to the diameter of the objective. A 6-inch telescope theoretically can resolve stars 0.6 second of arc apart. (A second of arc is a tiny unit of measure; for example, a penny must be 2.5 miles away before it appears as small as 1 second of arc.) This resolving power limits useful magnification to 60 power for every inch of the objective’s diameter.
Infrared, UV, and X-Ray Telescopes
Orbiting telescopes are used to observe the ultraviolet (UV), far infrared, and X-ray portions of the electromagnetic spectrum. The Infrared Astronomical Satellite, placed in orbit in 1983, carried a 22.5-inch infrared telescope. Because all matter emits infrared radiation if warm, technologists had to cool the telescope to near absolute zero with liquid helium so its internal heat radiation would not mask radiation it was collecting from deep space objects. Among its many discoveries was a disk of gas surrounding a star, from which planets may be condensing.
The Hubble Space Telescope, launched aboard the space shuttle Discovery on April 24, 1990, has special infrared-, UV-, and X-ray-sensitive instruments for the study of structures and systems too faint to be seen clearly with ground-based telescopes. Because the telescope orbits miles above the Earth and its distorting atmosphere, scientists hoped it would be able to capture and magnify light from about 20 billion light-years away. Just days after the launch, however, NASA engineers discovered major flaws in the telescope’s mirrors. Despite this setback, the telescope remained operational and sent back, among other things, evidence of a black hole and information about very young stars to engineers on Earth.
For shorter wavelengths, those in the X-ray region of the spectrum, ordinary mirrors will not work. X rays tend to penetrate conventional mirrors rather than be reflected by them.
Only if X rays are bounced off mirrors at a small, glancing angle can they be focused. X-ray satellites, such as Einstein, launched in 1978, and Exosat, launched in 1983, carried telescopes with deeply concave metal mirrors shaped so that they could focus X rays onto detectors
The first radio telescope was built in 1937 by Grote Reber, an American electrical engineer. It looked a little like the reflector of an optical telescope, but it was much bigger: 31 feet in diameter. Its reflector was made of wire screen instead of polished glass or metal. A much larger one, 250 feet in diameter, was built at Jodrell Bank, England, in 1957, and a 328-foot radio telescope began operating in West Germany in 1971. One such telescope, 1,000 feet across, was constructed in the 1960s at Arecibo, Puerto Rico, and fills an entire valley. Although it cannot move, its focal point can be scanned on large cranes.
Radio telescopes made vast new regions of the universe observable on Earth because radio waves penetrate dust and gas that obscure light. For long-wavelength radio waves, however, even the largest telescopes have
resolutions not much better than the unaided eye, though they have enormous power to detect weak or distant radio emitters.
To overcome this drawback, astronomers developed a new type of telescope that concentrated signals picked up by physically separate telescopes. Such interferometers work by reconstructing the shape of emitted radio waves, which are “sampled” by radio telescopes at various points. The resolution of such interferometers is comparable to that of a single radio telescope whose diameter is equal to the separation between the individual telescopes that make up the array.
One such array, constructed in the 1970s, is the Very Large Array (VLA) in New Mexico. The VLA consists of 27 radio telescopes, or antennas, spread over 24 miles. Each antenna is an 82-foot-wide dish mounted on a large pedestal, which is in turn attached to a transporter that moves the 200-ton antennas on rails laid out in a Y shape. The entire array can point to any part of the sky and, by changing the locations of the antennas, view a large object in the sky or focus at higher resolution on a small one. The maximum resolution of the VLA is about 1 arc second, which is comparable to that of optical telescopes. The signals from each antenna are carried by cable to a central computer, which electronically combines them into a single image. By combining the signals from radio telescopes scattered across the globe, very high resolutions are possible.
The Very Long Baseline Array (VLBA) was the world’s largest astronomical instrument in the mid-1990s. It consisted of ten 82-foot dishes across 5,000 miles in the United States. With such Very Long Baseline Interferometers (VLBIs), resolutions of a few thousandths of an arc second have been achieved.
It is likely that the telescope was invented independently and accidentally many times before Galileo turned it on the heavens in 1609. Glass was made in Egypt as early as 3500 BC, and crude lenses have been unearthed in Crete and Asia Minor believed to date from 2000 BC. Euclid wrote about the reflection and refraction
of light in the 3rd century BC, and in the 1st century AD the Roman writer Seneca noted that the glass globe filled with water referred to by the Greek dramatist Aristophanes could be used as a magnifying glass.
The 11th-century Arab scientist Alhazen published the results of his experiments with parabolic mirrors and the magnifying power of lenses. Alhazen’s works were translated into Latin in 1572, but much earlier Roger Bacon had recognized the usefulness of lenses.
The invention of the printing press in the 15th century, followed by the ever-increasing need for spectacle lenses by scholars, probably made inevitable the final invention of the telescope and its widespread use. It is clear that the oft-repeated statement that the telescope was first invented in 1608 by Hans Lippershey in the United Netherlands, is incorrect. Lippershey made a number of telescopes in 1608 and sold them to the government of the United Netherlands, which was interested in their military applications. His request for a 30-year privilege or patent was denied on the grounds that “many other persons had a knowledge of the invention.” Telescopes were on sale in France, Germany, Italy, and England in 1609.
Galileo heard of Lippershey’s work and reinvented the telescope, using basic optical principles. His first telescope magnified three diameters and consisted of a convex, or outward-curving, lens and a concave, or inward-curving, lens fitted into opposite ends of a tiny lead tube. The results were so gratifying that Galileo made several larger telescopes, grinding his own lenses.
His largest telescope was about 1.7 inches in diameter and had a magnifying power of 33 diameters. With these simple instruments he discovered the mountains and craters of the moon’s surface, the satellites of Jupiter, the starry nature of the Milky Way, and the fact that Venus undergoes phases like those of the moon. His observations showed that Venus is spherical, and goes around the sun, contrary to Ptolemaic theory.
Rarely has a new scientific instrument had a more dramatic effect than that of Galileo’s telescope. It not only advanced scientific knowledge by enormous strides but also stirred vast waves in philosophy
and religion by upsetting the traditional picture of a universe centered on a stationary Earth. In 1659 the Dutch scientist Christiaan Huygens discovered the true nature of Saturn’s rings using a telescope measuring 23 feet (7 meters) in length, which he had designed and built himself. In 1663 James Gregory, a Scots mathematician, designed the first reflecting telescope–the Gregorian reflector. In 1672 England’s Isaac Newton built what is now known as the Newtonian reflector, and, that same year in France, N. Cassegrain designed and built the Cassegrain reflector.
The telescope is one of the greatest inventions of all time. They have helped us understand the planets around us and the planets beyond. In the years to come, we will see things that we thought could only be since fiction. Below are just a few of the many things that we can see with this marvelous invention