Through the telescope

William Herschel (1738-1822), the German-born astronomer who lived and worked in Britain, built this large reflecting telescope in 1789. It had a curved polished metal mirror 4 feet (1.2 meters) in diameter and a focal length of 40 feet (12.2 meters). Regarded as one of the great technical feats of the time, it remained in use in its original form for more than 20 years.

The development of the telescope was the key to the great advances in astronomy that took place during the seventeenth and eighteenth centuries. Galileo introduced refracting telescopes, which use lenses to produce a magnified image. Newton, and most of the astronomers who followed him, employed reflecting telescopes, in which the chief optical components are curved mirrors. The principle of the reflecting telescope is similar to that of the large dish-shaped aerials that today’s radio-astronomers use to collect microwave radio signals emitted by stars and other celestial objects.
Galileo did not invent the telescope, although he was one of the first to use the instrument for astronomy. His main contribution stemmed from his improvements to the making and selection of lenses (his later telescopes had a magnification of about 33 times). The most important function of an astronomical telescope, however, is not to magnify. Stars are so far away that they never appear as more than points of light, even when viewed through the most powerful telescopes. For this reason an astronomical telescope’s chief function is to increase the amount of light that can enter the eye from a distant object such as a star. That is, to make objects visible that cannot be seen with the unaided eye.

Power and brightness

The extra light-gathering power of a telescope enables an astronomer to see thousands of stars that are invisible to the naked eye. This light amplification is related to the diameter of the object lens (in a refractor, or the mirror in a reflector) of the telescope. It is equal to the area of the object lens divided by the area of the pupil of the human eye (taken to be about 0.25 inch across when the eye is conditioned for viewing in the dark). For example, a telescope with a lens or mirror 6 inches (15 centimeters) in diameter has a light-gathering power of 625. It can be used to see stars of brightness down to 12 on the magnitude scale. This is about 250 times less bright than a star of magnitude 6, which can just be seen with the unaided eye by someone with good eyesight. The magnitude scale is logarithmic; bright stars have low, or negative, magnitudes (the sun, the brightest object in the heavens, has a magnitude of —27); dim stars have high positive magnitudes.
The “power” of a refracting telescope is apparently limited only by the size of its objective lens. But in practice, this limit is soon reached because of the difficulty of manufacturing large glass lenses. Also, image quality deteriorates because large lenses suffer from chromatic and other aberrations. The result is in out-of-focus images surrounded by colored fringes of light. Aberrations can be corrected to some extent by combinations of lenses made from different types of optical glass, but such achromatic lenses are extremely expensive in large sizes. Reflecting telescopes, as introduced and developed by Isaac Newton, avoid these difficulties. Mirrors can be made larger than lenses, although even mirrors become difficult to make when they exceed about 13 feet (4 meters) in diameter.

Telescope mountings

There are two principal ways of mounting an astronomical telescope so that it can be aimed at any point in the heavens. The simpler, and less expensive, type is an altitude-azimuth mounting, usually referred to as an alt-AZ mount. The telescope is mounted on a horizontal axis so that it can be pointed up or down at any angle from the horizon to the zenith, that is, at any altitude. The instrument (on its horizontal axis) is then mounted onto a vertical axis so that it is free to turn and point in any direction of the compass. From north (0°) through east (90°), south (180°), west (270°) and back around to north through a whole 360° turn—that is, it can turn to any azimuth. With such a mounting, a star’s position is given in terms of its altitude (angle above the horizon) and azimuth (bearing around from north).
The second main type of telescope mounting is called an equatorial mounting. It is mechanically similar to the alt-AZ mount, but with an important difference. The vertical axis is inclined at an angle equal to the latitude of the telescope’s site, so that the axis points to the celestial pole; it is termed the polar axis. When the telescope is aimed at right angles to the polar axis, it therefore points at the celestial equator and continues to do so as it is turned about the equatorial axis. The altitude axis becomes a declination axis, indicating the angle the telescope’s line of sight makes with the celestial equator.

This image of Jupiter was taken by the Wide Field & Planetary Camera-2 on the Hubble Space Telescope. The Hubble Space Telescope can resolve details in Jupiter cloud belts and zones as small as 200 miles (320 kilometers) across. This detailed view is surpassed only by images from spacecraft that have traveled to Jupiter.

On an equatorial mounting, a telescope is free to turn about two axes at right angles to each other. Movement counterclockwise around the polar axis correlates with the right ascension of, say, a star (its angle around the celestial equator). The movement about the equatorial axis corresponds to the star’s declination (its angle above or below the celestial equator), expressed as a positive angle (above) or as a negative angle (below).
Many equatorial mounted telescopes have graduated scales called the right ascension circle and the declination circle. Using these, an astronomer can set the scales to the known right ascension and declination of a star, planet, or comet. The telescope is then correctly aimed at the object and can be kept trained on it simply by turning the telescope slowly about the polar axis in order to compensate for the earth’s spin. This compensation must be at a rate of 15° every sidereal hour, or 1° every 4 minutes. Some telescopes have a motor drive synchronized with a sidereal clock to maintain the aim automatically. The largest modern telescopes are controlled by computer.

Photographing the stars

Before the latter part of the nineteenth century, astronomical observations were recorded by positional readings of altitude and azimuth or declination and right ascension; and by means of drawings wherever there was anything of significance to be seen. Photography has revolutionized this often laborious process to such an extent that observatories are now used almost exclusively for obtaining photographs, which are analyzed later, and usually elsewhere.
Early photographs were, almost inevitably, of relatively poor definition, despite long exposure times. Developments in photographic science have, however, benefited astronomy as much as—if not more than—any other field. Special emulsions can be made that are sensitive to infrared or ultraviolet light; extremely fine-grain films can be used for extended time exposures; these factors can achieve remarkable definition in the images that result.
So valuable is photography, in fact, that some very large telescopes have been built that cannot be used visually at all (the Schmidt telescope at Mount Palomar, in the United States, is an important example). This telescope has the additional merit of being able to photograph a relatively large area of sky at one time on a curved photographic plate with an area of slightly more than 10 square feet (1 square meter). This is in marked contrast to most optical telescopes, in which the field of view is narrow.

During long time-exposures, the telescope must track the stars being photographed (which appear to move because of the earth’s rotation) with absolute precision to avoid blurring the image. Sometimes an independently moving object, such as a satellite, enters the photographic field. When this happens, a line of light appears on the photograph, marking the object’s path.

Invisible radiation

Sight is our most acute and sensitive form of long-distance perception. We know about the stars because we can see them, and because of this, optical telescopes are the traditional instruments of astronomy. Nevertheless, it must not be forgotten that light is only one form of electromagnetic radiation, and that it forms only a narrow band in the electromagnetic spectrum.
On either side of the band of visible radiation (wavelengths of which range from approximately 4,000 A for light at the violet end of the visible spectrum, to 7,000 A for light at the red end) are electromagnetic radiations which are invisible, but which can nevertheless be perceived. Ultraviolet (of shorter wavelength than light) can be perceived indirectly, by the effect it has on the pigmentation of human skin. Infrared radiation (of longer wavelength than light) can be felt as radiated heat. Film emulsions can be prepared that react to both of these, so that otherwise invisible sources emitting either can be “observed” photographically. Similar photographs can be used to identify the ultraviolet or infrared components of objects that emit light as well.
Of shorter wavelength, and higher frequency than ultraviolet, are X rays and gamma rays. At the other end of the electromagnetic spectrum, with longer wavelength and shorter frequency than infrared, are microwaves and radio waves. Although all these radiations exist in space, only some of them (fortunately) can penetrate the earth’s atmosphere. These are visible light, some ultraviolet, some infrared, and a substantial range of radio waves, from wavelengths of approximately 107A, which is 1 centimeter, to almost 1 kilometer, or 10,3A.

The portable equatorial telescope (right) was designed in 1775 by the British instrument-maker Jesse Ramsden, who introduced many innovations to optical instruments used in astronomy. There are four main types of telescopic instruments. A Galilean telescope {far right, A) is a refracting telescope with a converging object lens and a diverging lens in the eyepiece. Together they produce an upright magnified image (similar optics are still used in opera glasses). Later astronomical telescopes (B), like Ramsden s, used two converging lenses, which give an inverted image. In the Newtonian reflecting telescope (C), a curved mirror focuses the light, which is diverted to the eyepiece by a small plane mirror within the telescope. There are also various other reflecting telescope designs. A radio telescope (D) uses a similar principle; a paraboidal “dish” aerial focuses radio waves onto a receiver.

Radio telescopes

The curved dish aerial of a radio telescope acts in a similar way to the curved mirror of a reflecting optical telescope. It concentrates radio waves from space onto a receiver supported at the focus of the dish.

The fact that radio waves come from beyond the earth was first proposed by Karl Jansky in 1931. But systematic study of the phenomenon did not begin until nearly a decade later. Since then, however, it has developed into what is probably the most important area of astronomical study. This is because the amount of information that can be obtained from radio telescopes (particularly about the more distant reaches of the universe) is so much greater than anything we can learn from the study of radiations in the visible and near-visible range. Furthermore, although such great advances have been made through the study of radio waves, investigations by similar means of extremely short wavelength radiation promises to extend the boundaries of our understanding of distant stars and galaxies. These investigations are made possible by the siting of equipment above the earth’s atmosphere, which blocks X rays and gamma rays.
Although the equipment for detecting radio waves can take various forms (for example, a series of aerials extended in line) the main type of radio telescope resembles a radar dish. Like the parabolic reflector in an optical telescope, it collects the electromagnetic waves and focuses them at a point, in this case called the receiver. Radio telescopes of this type can be extremely large, although beyond a certain size they cannot be moved.
Radio waves focused at the telescope’s receiver are recorded, then represented for analysis in one of several ways. They may be plotted graphically, on a revolving drum; converted by a computer into a contour map showing the intensity of emissions; shown as a three-dimensional shape, with “peaks” of increased radio activity; or represented as a multicolored image in which the colors signify wavelengths or intensities of signal received.

The two chief mounts for astronomical telescopes are the altitude-azimuth (alt-AZ) mounting (A) and the equatorial mounting (B). An alt-AZ mount has a vertical axis so that the telescope rotates with respect to the horizon. A star s position can be given by its altitude h and azimuth a. In an equatorial mounting (B), the equivalent axis is aligned with the celestial pole (and so depends on the telescope’s latitude), and the instrument rotates with respect to the celestial equator. A star’s position is given in terms of its declination x and right ascension y.