Telescope

I

Introduction

Telescope, device that permits distant and faint objects to be viewed as if they were much brighter and closer to the observer. Telescopes are typically used to observe the skies.

For hundreds of years, telescopes were the only instruments available for studying the planets and stars. Even today, space probes can reach only our closest neighbors in the heavens, and scientists continue to rely on telescopes to learn about distant stars, nebulas, and galaxies. Telescopes are the fundamental research instruments that enable astronomers to tackle scientific questions about the birth of the universe (see Big Bang Theory; Cosmology); the emergence of structure in the early universe; the formation and evolution of stars, galaxies, and planetary systems; and the conditions for the emergence of life itself.

Most telescopes work by collecting and magnifying visible light that is given off by stars or reflected from the surface of planets. Such instruments are called optical telescopes. Conventional optical telescopes use a curved lens or mirror to collect light and bring it to a focus, a point in space where all the light rays converge (see Optics). A small magnifying lens, called an eyepiece, placed at the focus allows the image to be viewed. In astronomical research, cameras or other instruments placed near the focus make a precise recording of the light gathered by a telescope. The visible light collected by a telescope is divided into component wavelengths, or colors, through a process called spectroscopy. This powerful technique, which uses a prism or diffraction grating, essentially “decodes” starlight to yield information about an object’s temperature, motion and other dynamics, chemical composition, and the presence of magnetic fields.

Light rays, however, are just one part of what scientists call the electromagnetic spectrum (see Electromagnetic Radiation). Just as stars emit visible light, they also give off other types of electromagnetic radiation, including radio waves, microwaves, infrared light, ultraviolet light, X rays, and gamma rays. All these forms of electromagnetic radiation are emitted as waves.

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Rapid advances in astrophysics and optical technology, coupled with the advent of the space age, broadened telescope technology in the last quarter of the 20th century. Astronomical telescopes today come in a wide variety of shapes and sizes, dictated largely by the portion of the electromagnetic spectrum the telescope is designed to view. Telescopes today view the entire spectrum of electromagnetic radiation sweeping the universe. Each new advance in wavelength coverage has dramatically altered our view of the universe.

Many telescopes are Earth-based, located in astronomical observatories around the world. But only radio waves, visible light, and some infrared radiation can penetrate Earth’s atmosphere and reach the surface of our planet. To overcome this problem, scientists have launched telescopes into space, where the instruments can collect waves from the other regions of the electromagnetic spectrum (see Space Telescope).

II

Optical Telescopes

There are two main kinds of optical telescopes—refracting and reflecting. Refracting telescopes use a lens to magnify objects; reflecting telescopes use a curved mirror.

A

Refracting Telescopes

Refracting telescopes, or refractors, use a glass lens to bend, or refract, starlight and bring it to a focus. The lens is convex, meaning that the center of the lens is its thickest part, and the lens becomes thinner toward its edges. A convex lens bends light at the edge of the lens to a greater angle than light coming through the center, so all of the rays converge to a focus. The distance between the lens and the place where the rays converge is called the focal length of the lens. A refracting telescope’s light-gathering power is proportional to the size of the objective, or main, lens and to the ratio of the focal lengths of the objective lens and the eyepiece.

Refracting telescopes are typically hampered by chromatic aberration, which causes different colors of light to come to a different focus because every color has its own degree of refraction. Chromatic aberration causes the image of a star or planet to be surrounded by circles of different colors.

Another fundamental limitation of refractors is that lenses with diameters beyond 40 in (100 cm) are impractical because they weigh more than half a ton and sag under their own weight, distorting the starlight. They cannot be supported from behind, as optical mirrors are.

B

Reflecting Telescopes

A reflecting telescope uses a precisely curved mirror instead of a lens to collect starlight. The mirror is concave—that is, shaped like the inside of a dish—a shape that brings reflected light waves to a focus at a point above the mirror. Reflecting telescopes are especially useful for gathering light from dim objects. A reflecting telescope’s light sensitivity increases with the square of the diameter of the telescope’s mirror, so doubling the mirror’s diameter increases light-gathering power by a factor of four. Not only can a larger telescope see fainter objects, but it can also obtain the data in a fraction of the time required for a smaller telescope. Larger reflecting telescopes can typically detect objects that are a millionth or a billionth the brightness of the faintest star seen by the human eye.

The ideal mirror for a reflecting telescope has a parabolic or hyperbolic shape that brings distant light rays to a precise focus. Such mirrors are difficult to make because the curvature of the mirror’s surface changes with its distance from the center, unlike a simpler, though not as precise, spherical reflector. A telescope mirror is cast from special molten glass that will not significantly expand or contract with temperature once it cools and hardens. Pyrex glass has been commonly used, and newer materials include borosilicate glass and a glass-ceramic composite.

The molten glass is cast as a mirror blank, a flat, thick disk that approximates the size of the finished mirror. It then must cool slowly to avoid cracking. Once cooled, the flat mirror blank’s surface is ground and polished to the right shape using a computer-controlled polishing tool that rubs a liquid slurry of fine abrasive across the glass. This process must be extraordinarily accurate—differences in the surface must be smaller than a fraction of the width of a human hair. A fine layer of aluminum is deposited on the glass to create a reflective surface.

A technique that reduces some of the time needed to grind a mirror to shape was developed in the 1990s. The glass is spun into the desired shape while it is still molten. Rotational forces move some of the glass toward the edge of the spinning container and into a shape called a paraboloid. After it cools, a spin-cast mirror does not require laborious grinding to remove excess glass.

Astronomers seek ever-larger mirrors to increase the power and efficiency of telescopes. However, huge mirrors are expensive and difficult to make, and they are challenging to move while tracking celestial targets. One particularly daunting problem is that a solid glass mirror is heavy. The 200-in (508-cm) Hale telescope on California’s Mount Palomar weighs 14 tons.

In the 1990s a daring and innovative design broke the mirror size barrier. Each of the twin Keck telescopes, located in Mauna Kea, Hawaii, combined 36 hexagonal 72-in (183-cm) mirrors together, like bathroom tiles, to behave like one immense 400-in (1,016-cm) mirror having four times the collecting power of Palomar (see Mauna Kea Observatory).

In some telescopes designed in the 1990s, the mirror’s weight has been dramatically reduced by sandwiching a honeycomb pattern of glass ribs between a thin, but rigid, concave mirror and a flat back plate. Engineers have even developed meniscus mirrors—mirrors that are too thin to support their own weight. An adjustable framework supports the meniscus mirror, and servomechanical actuators, controlled by computer, continually adjust the shape of the mirror as it tracks celestial targets. Actuators are also critical to the operation of segmented mirror telescopes, like Keck, that require that a number of smaller mirrors operate as if they were one large mirror.

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