Astronomical instruments can be divided into two major categories. The first category might include all of the instruments which are used in the actual process of observing celestial objects. Some of these, like the meridian transit, are designed for specific tasks such as the precise determination of an observer's position on the earth or a star's position in the sky; other observational instruments are principally collectors of the radiation emitted by stars, planets, nebulas, and galaxies. These latter, which are generally referred to as telescopes, enable objects invisible to the naked eye to be seen, photographed, or otherwise detected.
In the second category may be grouped the auxiliary instruments which are used to standardize, record, or analyze the data obtained by the observational equipment. Devices to provide an accurate standard of time, to determine the brightnesses of stars, to record their spectra, or to measure the positions of stars on photographic plates, are examples of instruments belonging to this second category.
   It should be mentioned at the outset that the radiation gathered from a celestial object by a conventional astronomical telescope lies in the visible and near visible region of the electromagnetic spectrum. Over the past few decades, however, an entirely different type of astronomical telescope has come into wide use. These instruments, known as radio telescopes, have been developed as the result of the discovery in 1928-1932 by Karl G. Jansky of the Bell Telephone Laboratories that the center of our own galaxy is a powerful emitter of electromagnetic radiation in the radio wavelength region. Since Jansky's initial discovery, many other celestial "radio sources" have been found. The operating principles  and the evolution of radio telescopes, as well as the significance and importance of the new field of radio astronomy which they have fostered, are treated under Radio Astronomy  and Radar Astronomy. Also described elsewhere are certain other electronic devices, such as "image intensifiers," which already belong, or soon will belong, to the growing list of techniques employed in modern astronomy. (See also Photometry.)
 

Early History

   The origins of astronomical observations go back to remotest antiquity. The necessity of keeping track of time for agricultural and civil purposes must have led primitive man to a serious study of the daily rotation of the heavens, and to the motions of the sun and moon relative to the stars. Certainly by 2500 b.c., both the Babylonians and the Egyptians had developed calendars in which approximations to our present day, month, and year appeared as basic units. Besides crude water and sand clocks, the time of day was estimated by the direction and length of shadows cast by vertical objects such as buildings, pyramids, stone columns, or simply stakes driven into the ground. Calibration of the shadow lengths and directions inevitably followed, and the forerunner of the present-day sundial was born. The earliest known sundial or shadow-clock is Egyptian and dates back to approximately 1000 b.c.(See also Sundial.)
   From 1000 b.c. to around 300 b.c., little was added to the science of observational astronomy. The practice of carrying out systematic astronomical observations, and the use of these in the formulation of theory, was revived in the third and second centuries b.c. by astronomers of the Alexandrian School, notably Aristyllus and Timocharis (third century b.c.), Aristarchus (c.220-c.150 b.c.), Eratosthenes (c.276-194 b.c.), and Hipparchus (c.190-c.125 b.c.).
 

The Gnomon

   The principal instruments of the day were the gnomon and the armillary sphere. The gnomon was simply a pointed vertical column of known height erected on a horizontal plane. In addition to telling time, this simple devise was used to yield a variety of fundamental data. From the direction of the shadow cast at noon, the north-south line was established enabling the azimuths (angular directions) of objects on the earth's surface to be estimated. From the known height of the gnomon, and the lengths of the shortest and longest noon shadows observed during the year, the angle of the ecliptic plane (the plane containing the apparent path of the sun) to the earth's equatorial plane and the latitude of the observer were calculated. The time interval between consecutive observations of the longest or shortest noon shadows gave the length of the tropical year.
 

The Armillary Sphere

   The armillary sphere was doubtless developed to increase the accuracy of the results obtained by the gnomon and to extend observations to the stars as well as the sun. One of the earliest armillaries, a solar instrument, consisted of two bronze concentric rings, several feet in diameter, mounted in the plane of the meridian. The inner ring turned within the outer ring and carried two small pegs mounted at opposite ends of a diameter. The inner ring was adjusted until the shadow cast by the upper peg fell on the lower peg. An angular scale on the outer ring, divided into degrees, indicated the meridian altitude of the sun. Eratosthenes may have used this type of "solstitial" armillary, instead of a gnomon, to determine the angle of the ecliptic plane. Another kind of solar armillary, with which Eratosthenes was probably familiar, consisted of a single ring mounted in the plane of the celestial equator. The times of the equinoxes, and hence the length of the tropical year, can be determined with such an instrument because the shadow of the upper half of the ring falls on the inner surface of the lower half when the sun is at either equinox.
   Armillaries designed for stellar observation, such as those used by Hipparchus, were much more complicated. They consisted of a number of rings, the largest of which was mounted on a stand and adjusted to lie in the plane of the meridian. Two pivot points on its inner rim, representing the north and south celestial poles, supported and allowed rotation of a slightly smaller second ring called the "solstitial colure." Permanently attached to the solstitial colure was a third ring of equal size, graduated into degrees, and lying in the plane of the ecliptic. The solstitial colure ring also had pivots on its inner rim, representing the ecliptic poles, on which a fourth and smaller ring could turn. Like the ecliptic ring, the fourth ring was graduated into degrees and, in addition, enclosed a fifth ring. Provision was made for the fifth ring, which carried diametrically opposite sights, to slide within the fourth ring. This ring thus remained in the plane of the fourth ring, from which the direction of sighting was read. With such a device, called a zodiacal armillary (Fig. 2), the differences between the celestial latitudes and longitudes of stars and planets could be measured.
 

The Quadrant

   The last Greek astronomer of antiquity who contributed to the development of astronomical instruments was Ptolemy (fl. second century a.d.). Ptolemy discussed in his astronomical writings three new instruments: the quadrant, the triquetrum, and the astrolabium, or astrolabe.
   The principle of the quadrant is illustrated in Fig. 3. As it was originally used, the plane of the quadrant was adjusted to lie in the plane of the meridian. Vertical alignment was indicated by a plumb-bob suspended from the quadrant's center. Pivoted from this center was one end of a movable rod approximately equal in length to the radius of the quadrant. Sights mounted on the rod enabled observations to be made of stars and planets as they crossed the observer's meridian, and an angular scale inscribed on the periphery of the quadrant denoted their meridian altitudes. It is not certain whether Ptolemy actually constructed such an instrument or not. The Arabians, however, subsequently adopted the idea of the quadrant and greatly improved upon its design  --  in particular, quadrants were developed which could rotate about a vertical axis.
 

The Triquetrum

   The triquetrum of Ptolemy performed the same function as the quadrant and was devised to overcome the difficulty of graduating arcs and circles. It consisted of a vertical post to which two intersecting rods or arms were hinged, the upper arm carrying sights. From a knowledge of the lengths of the arms and the distances between the hinges, the zenith distance (or, alternatively, the altitude) of a celestial object could be calculated.
 

The Astrolabe

   The astrolabe was one type of portable solstitial armillary, modified for stellar observation. Suspended by a small hook or "eye," the instrument consisted initially of a single ring which hung in a vertical plane. Pivoted at the center of the ring was a rod equal in length to the ring diameter, carrying sights at either end. When aligned on a star or planet, an angular scale inscribed on the armillary ring indicated the object's altitude.
 

Arabian Contributions

   Following Ptolemy, Greek astronomy rapidly declined and terminated with the Arabian conquest of Alexandria in a.d. 641. Although the magnificent library and museum were destroyed, the Arabs encouraged learning and for the next 800 years developed an important astronomical tradition of their own. Observatories were established at a number of cities including Damascus, Cairo, Baghdad, and Meragha. Quadrants of various types and sizes were constructed, culminating in an enormous masonry instrument, 180 feet (55 meters) high, erected in the 15th century by Ulug-Beg at Samarkand. Nasir al-Din al-Tusi built the first azimuth quadrant at Meragha about 1260. Actually, this instrument consisted of two quadrants that rotated on the same vertical axis over a common azimuth circle, enabling the altitudes and azimuths of a pair of stars to be measured simultaneously. It is probably fair to say that Arabian instruments were more distinguished for their craftsmanship than their originality. Angular scales on devices like quadrants and armillary spheres were subdivided to intervals of 10 minutes of arc (1/6 of a degree). The astrolabe was developed to a high degree of complexity and became an indispensable tool to surveyors and navigators. Its main use was in the determination of time. By auxiliary circles and scales, ingeniously incorporated into the astrolabe design, altitude measurements of the sun, planets, and stars could be converted to time directly without the use of separate tables.
 

Oriental Contributions

   The main stream of Greek and Arabian astronomical thought flowed to Europe by way of Spain after the conquest of that country by the Arabs in the 11th century. However, tributaries also flowed to India and China as the result of extensive caravan and trade routes. Both India and China were supposed to have developed elaborate and advanced astronomies many thousands of years before Christ. Unfortunately, no records have been found which substantiate these claims and the present view is that they are largely legendary. It is known that the Chinese were active in matters concerning the calendar and the prediction of eclipses from approximately 2300 b.c. until the fifth century b.c., at which time the study of astronomy seems to have been abandoned.          However, the gradual assimilation of Arabian ideas during the latter part of the first millennium a.d. led to a revival of astronomical inquiry in both China and India. Observatories were again established at many places. One of the most elaborate facilities for which reliable records exist was the observatory at Peking, founded sometime around a.d. 1260. Most of the Peking instruments were of Arabian design. The armillary spheres, however, differed from the Arabian ones in that the ecliptic ring was replaced by a ring lying in the plane of the celestial equator and a fixed horizon ring was added. Such an instrument, called an equatorial armillary, has a number of advantages over the zodiacal models of the Greeks and Arabs. In the first place, the equatorial armillaries tend to be simpler mechanically. Secondly, they are easier to operate because, at a given location, the fundamental reference plane (the celestial equator) does not change its orientation in the sky with time. Finally, unlike the zodiacal armillary, the equatorial instrument remains in balance regardless of the direction in which a sighting is made. While it is possible that the equatorial armillary might be of Arabian origin, most of the evidence supports the view that it was a Chinese invention.
 

Medieval Europe

   The initial development of astronomy in medieval Europe was slow. One of the first observational centers of note was established at Vienna around 1450 by George Purbach, who devised a variation of the triquetrum known as Purbach's Geometrical Square. Some time later Purbach's student, Johannes Müller, better known as Regiomontanus, erected an observatory at Nürnberg. In addition to several modified quadrants and a few of the first weight-driven clocks, the Nürnberg observatory possessed a torquetum. This rather unwieldy instrument, thought to have been introduced by Nasir al-Din al-Tusi at Meragha, consisted of a number of nonconcentric disks and rings, which permitted both the equatorial and ecliptic coordinates of a star or planet to be observed. Provision was also made for altitude measurements. Tycho Brahe (1546-1601) had a low opinion of the torquetum and, by the latter part of the 16th century, the instrument had fallen into general disuse.
 

Tycho Brahe's Instruments

   Pretelescopic astronomical instruments were brought to their ultimate level of refinement by Tycho Brahe. Under a royal charter from Frederick II, the King of Denmark, Tycho established in 1576 the famous observatory, Uraniborg, on the island of Hveen. Uraniborg surpassed all previous astronomical centers.
   To achieve maximum accuracy, Tycho made his instruments as large as possible without sacrificing mechanical rigidity. One of the largest was the great mural quadrant. This instrument, which was permanently mounted in the plane of the meridian, had an arc radius of slightly over 6 feet (1.8 meters). According to Tycho, the angular scale inscribed on the quadrant could be read to within 10 seconds of arc (1/360 of a degree). The sighting arrangement was unusual in that the individual sights were not connected by a common radial bar. One sight was fixed in position at the quadrant's center in an opening in the observatory wall; the other sight moved independently along the arc of the quadrant. As described below, the sights were also designed to reduce parallactic effects to a minimum.
   In addition to the mural quadrant, which measured only meridian altitudes, Tycho made extensive use of two other large quadrants of the azimuthal type. Both had circles approximately 6 feet (1.8 meters) in radius, angular scales divided to 10 seconds of arc, and parallax-free sights.
   Included among the major items of equipment at Uraniborg were several large equatorial armillary spheres. Al  though used in China at least as early as 1260, the equatorial armillary had been entirely unknown in Europe, and Tycho considered himself its inventor. One armillary consisted of three steel rings. The first ring was adjusted to lie in the meridian plane; the second ring was permanently attached to the meridian ring and lay in the plane of the celestial equator; the third ring, called the declination circle, rotated within the celestial equator about an axis representing the rotational axis of the earth. Both the equator and the declination circles were graduated and carried movable sights. A small cylinder mounted at the center and perpendicular to the polar axis served as the fixed objective sight.
   Tycho's largest equatorial armillary consisted of a graduated declination ring 9¹ feet (2.9 meters) in diameter that rotated about a polar axis, and a graduated semicircular ring representing that part of the celestial equator below the horizon. The latter was mounted separately on stone piers and was thus entirely free of the declination ring. The sighting system consisted of a single sliding sight on the equatorial circle and two declination sights mounted at the ends of separate radial arms of the declination circle. The radial arms pivoted around a small cylinder which served as the objective sight. Two independent declination measurements of the same object could be obtained by using first one declination sight, rotating the entire declination circle through 180°, and then using the other. Further, because the equatorial semicircular ring stood well clear of the sphere defined by rotating the declination circle, the declination of objects lying close to the celestial equator could be measured -- a feat not possible with the conventional equatorial armillary.
   The need for measuring the angular distance between any two objects in the sky prompted the development by Tycho of another class of instrument, which he called the sextant. Although similar in principle to the crude cross-staffs used previously, the sextant was capable of high precision. As shown in Fig. 4, the instrument consisted basically of a graduated arc representing the sixth part of a circle and a movable radius. Sights were mounted at the sextant's center, at the free end of the movable radius, and at one end of the arc. As originally designed by Tycho, a single observer adjusted the movable radius by a screw until the two objects under study were sighted simultaneously from the sextant's center. In later models, the roles of the sights were reversed, and two observers were required for an observation. A small cylinder at the "hinge" of the sextant became the fixed objective sight, and the sights at the end of the arc and movable radius became the viewing sights. The largest sextant at Uraniborg, of radius 5¹ feet (1.7 meters), was of this type.
   It was difficult to measure small angular distances with the sextant because of the tendency for the observers' heads to collide. One way in which Tycho overcame this problem was by the use of a special device called an Arcus Bipartitus. As shown in Fig. 5, this instrument consisted of two small cylindrical sights supported at the ends of a short crossbar. The crossbar in turn was attached to one end of a central rod some 5¹ feet (1.7 meters) long. Two 30° arcs, having the cylindrical sights as centers, were mounted at the opposite end of the central rod. Two traveling sights, one on each arc, completed the apparatus.
   The accuracy which Tycho achieved in his observations did not result solely from the large dimensions of his instruments. Equally important were the angular scales and sighting systems employed. Previously, the reading accuracy of an angular or linear scale could only be increased by adding more and more subdivisions. Tycho rejected this procedure as impractical and adopted instead the Method of Transversals to subdivide his angular scales. This idea, of unknown origin, is quite elegant in its simplicity and is shown in Fig. 6. In this drawing, the scale divisions correspond to 10 minutes of arc (1 minute of arc = 1/60 of a degree). A series of dots connects a given division mark on one side of the scale to the consecutive mark on the opposite side. In effect, the dots divide the scale interval into ten equal parts, thus increasing the reading accuracy of the scale from 10 minutes to 1 minute of arc. The angular scales on Tycho's largest instruments were divided directly into minutes; by the use of transversals, a reading accuracy of 10 seconds of arc was attained (1 second of arc = 1/3,600 of a degree).
   Before Tycho, sighting systems were rather primitive. Both the objective and the viewing sights usually consisted of a metal plate in which a small hole had been drilled. Alignment was supposedly achieved if the object under study appeared to be in the middle of the hole of the objective sight when seen through the hole of the viewing sight. Because of the finite sizes of the holes, however, the sighting procedure was rather uncertain -- in particular, the condition of alignment depended upon the actual position of the eye behind the viewing sight, an effect which Tycho called parallax. To utilize the inherent accuracy of his instruments, Tycho devised a simple, virtually parallax-free sighting system. Instead of a single hole, two parallel slits were cut in the viewing sight. The objective sight consisted of a small cylinder with axis parallel to the slits and diameter equal to the slit spacing. Correct alignment was achieved when a star or planet as viewed through either slit appeared tangent to the corresponding edge of the cylinder.
   It is interesting to note that, although Tycho's instruments were probably capable of accuracies within 10 seconds of arc, such accuracies were not, in fact, achieved. Tycho was apparently unaware that the human eye, without optical aid, has a limiting angular resolution of about 2 minutes of arc. So long as sightings are made by the naked eye, this value represents the highest accuracy attainable by any instrument regardless of size. Yet, Tycho's quest for precision was far from futile -- his ideas and contributions exerted a powerful influence long after his death in 1601.
 
 

OPTICAL TELESCOPES
 

The Invention of the Telescope

   Although glass had been known to the Egyptians as early as 3800 b.c., and the Phoenicians became expert in its manufacture, its optical applications were not fully appreciated until medieval times. Roger Bacon (1214-1294) was one of the first to investigate the properties of lenses and mirrors. The introduction of spectacles took place in Italy around 1300 and, by the early part of the 16th century, optical centers had also been established in Germany and Holland. The first telescopes appeared in Holland in 1608. Some doubt exists as to the identity of the inventor. Both James Metius and Zacharias Jansen claimed the honor, but Hans Lippershey appears to have been the first to use lenses in combination. Galileo first heard of the Dutch invention in the spring of 1609. Lacking a detailed description, he set about to discover the principle of the telescope himself and, within a matter of weeks, had produced his first instrument which he immediately directed to the heavens. There thus began a new and exciting era of observational astronomy, undreamt of by Tycho, which has continued to the present day.
   Galileo made a number of telescopes ranging up to 5 centimeters in aperture, 170 centimeters in focal length, and having magnifying powers from approximately 8 to 30. All were basically of the same design, as shown in Fig. 7, and consisted of a plano-convex or double-convex objective lens and a plano-concave or double-concave eyepiece. The Galilean system gives bright and erect images but has a comparatively small angular field. In addition, it suffers from the same major defects as any other simple lens system, namely, spherical and chromatic aberration.
   Spherical aberration arises from the fact that different radial zones of a lens, possessing spherical surfaces, have effectively different focal lengths. Rays of light which pass through the edge of a lens, for example, are refracted to a different focus than those rays which pass through its center, resulting in a blurred image. The effect is illustrated in Fig. 8. Chromatic aberration arises because the index of refraction of glass varies with wavelength. This means that a simple lens cannot bring all colors of light to the same focus; the image of a white object, like a star or planet, appears to be surrounded by a number of colored concentric rings. Chromatic aberration is illustrated in Fig. 9.
 

The Development of the Telescope

   The development of astronomical instruments from Galileo's time up to the present is a long, complicated, and fascinating story, involving the skills and talents of scores of individuals. It is impossible to include here a complete account of the endless experiments in optical techniques that were attempted, of the individual advances that were made, or of the hundreds of telescopes, each theoretically possessing some unique advantage over its predecessors, that were actually constructed. Only the broadest outlines of the subject can be described.
   The aberrations present in Galileo's telescopes were recognized and subsequent efforts were devoted to their suppression or elimination. The task was all the more difficult because the nature of light itself and the cause of chromatic aberration in particular were imperfectly understood. René Descartes, in 1637, suggested the use of hyperbolic lens surfaces instead of spherical, but attempts to make such lenses failed. Marin Mersenne in 1636, improving upon a suggestion made twenty years earlier by the Jesuit, Niccolo Zucchi, proposed a telescope consisting of two parabolic mirrors. Again, the problem of making parabolic surfaces was considered insurmountable and the idea was abandoned.
 

Early Refracting Telescopes

   Some improvements in the performance of telescopes were made later in the 17th century when it was realized that the effects of spherical and chromatic aberration decreased with increasing focal length. Johannes Hevelius of Danzig and the Huygens brothers, Christiaan and Constantine, were among the first to build long telescopes. Hevelius' largest instrument was a monster, with a focal length of 150 feet (45 meters). The tube consisted of an open V-shaped wooden trough, braced with diaphragm stops, which carried the objective  lens at its upper end and the eyepiece at its lower end. The whole structure was suspended from a mast 90 feet (27 meters) high and was manipulated by a complicated system of ropes and pulleys. Christiaan Huygens dispensed with the telescope tube altogether and simply mounted the objective lens on a platform which could slide up and down a vertical pole. Observations were made from the ground with the observer steadying the eyepiece on a wooden stand. A length of thread connected the objective lens and eyepiece -- when drawn taut, the lenses could be aligned and the position of focus found.
   Refracting telescopes of long focal length, ranging from 20 to 300 feet (6-90 meters), continued to be built during the latter part of the 17th and into the 18th centuries. Various attempts, all more or less unsuccessful, were made to overcome the formidable difficulties in operating such unwieldy instruments. One of the most ingenious proposals was made by the Englishman, Robert Hooke, in 1668. As shown in Fig. 10, Hooke intended to reflect the light rays gathered by the objective lens back and forth between plane mirrors before they entered the observer's eye. A drastic reduction in tube length was therefore gained without sacrificing the advantages of long focus. Unfortunately, Hooke's idea was never put into practice because of the difficulty in his day of making optically flat surfaces for the mirrors.
   About 1663, Isaac Newton began his famous experiments on the dispersion and refraction of light. Among other things, he was the first to differentiate clearly between spherical and chromatic aberration. Curiously, Newton held the view that all substances possessed the same dispersive power and that it was therefore impossible to eliminate or suppress chromatic aberration in any optical system consisting of lenses. While this erroneous conclusion no doubt delayed the invention of the achromatic lens (which, by using two different types of glass, corrects chromatic aberration), it had a salutary effect on the development of the reflecting telescope, since mirrors were known to be inherently free of chromatic aberration.
 

Early Reflecting Telescopes

   The reflecting systems of Zucchi and Mersenne have already been mentioned. About 1664, James Gregory proposed the design illustrated in Fig. 11. The primary mirror was a concave paraboloid with a hole at its center, and the secondary mirror a concave ellipsoid. In addition to freedom from chromatic aberration, Gregory's design is noteworthy in that it is also free of spherical aberration as a result of the nonspherical mirror surfaces employed. Gregory's attempts to construct this telescope ended in failure because of practical difficulties encountered in casting and polishing the mirrors.
   The first person to devise successful methods for casting mirrors and polishing them to the correct form ("figuring") appears to have been Newton himself. In 1668, Newton produced his first reflector, a scientific toy only 6¸ inches (16 cm) long and having a mirror diameter of 1¸ inches (3.1 cm). As shown in Fig. 12, Newton replaced Gregory's ellipsoidal secondary mirror by an optical flat, which simply diverted the converging cone of rays from the primary mirror to one side of the telescope tube. Like the Gregorian, the Newtonian reflector is free of both spherical and chromatic aberration. Newton made his mirrors of speculum metal, a shiny alloy of tin and copper somewhat resembling silver in appearance. The copper in the alloy caused the mirror to tarnish after a time, and frequent repolishings were necessary. It is interesting to note that speculum mirrors were the only kind available for astronomical use until approximately 1850 when the silver-on-glass coating process was discovered.
   Another design for a reflecting telescope was suggested about 1672 by a Frenchman, Guillaume N. Cassegrain, about whom very little is known. Although Newton was harsh in his criticism of the system, it has come into wide use today. The primary mirror is a concave paraboloid and the secondary mirror a convex hyperboloid. As shown in Fig. 13, light rays reflected from the secondary pass through a hole in the primary and come to focus behind the primary. In an alternate scheme, called a modified Cassegrain, a small optical flat placed immediately in front of the primary brings the light out to the side of the telescope tube and eliminates the necessity of a perforated primary. Like the Gregorian and Newtonian telescopes, the paraboloid-hyperboloid combination of the Cassegrain is free of spherical aberration.
   Efforts to improve the techniques of casting and figuring speculum mirrors continued during the sixty-year interval following Newton's death in 1727. John Hadley was the first to devise a laboratory method for testing the parabolic figure of a mirror. He placed a tiny illuminated pinhole at  the mirror's center of curvature and examined the reflected cone of light in the vicinity of the image. From the appearance of this cone, Hadley could infer the state of the mirror's surface and was thus able to pass, by successive polishings, from a spherical to a paraboloidal figure.
   Two other 18th-century opticians who contributed to the initial development of the reflector were James Short and John Herschel. Short produced a large number of excellent Gregorian-type telescopes. Herschel was equally prolific in making Newtonian telescopes, although he is perhaps best known for his prodigious attempts to cast large mirrors. In 1789, he completed the largest telescope that had ever been built -- a Newtonian with a focal length of 40 feet (12 meters) and a mirror diameter of 48 inches (1.2 meters).
   A major event bearing upon the future of the refractor occurred in 1729 when Chester Moor Hall, a barrister by profession, designed the first achromatic lens. Consisting of a concave component of flint glass and a convex component of crown glass, the "doublet" possessed far better color correction than any of the simple long-focal-length lenses used previously. Hall never submitted a claim for his invention nor did he attempt to publicize it. It was not until 1760 that John Dollond, who knew of Hall's work, took out a patent in his own name and began manufacturing achromatic lenses commercially. Owing to casting problems peculiar to flint glass, the first English achromatic lenses were marred by internal striations, were never larger than four or five inches (10 or 13 cm) in diameter, and (from the standpoint of light-gathering ability) were unable to compete with the mirrors being made by Short and Herschel.
   This situation, unfavorable to the refracting telescope, was vastly improved by Pierre Louis Guinand of Lac Brenet, near Geneva, Switzerland. After many experiments conducted during the years 1784 to 1790, Guinand succeeded in casting 5-inch and 6-inch (13-cm and 15-cm) flint blanks of a quality never before achieved and in the 1820's produced blanks 12 inches (30 cm) in diameter. One of Guinand's closely guarded secrets was his discovery that a far greater homogeneity in the glass melt was obtained if the stirring paddles used were made of fire clay instead of wood.
 

The 19th Century

   From 1806 to 1814 Guinand worked in Germany and, while there, was understudied by a young man named Joseph von Fraunhofer. Fraunhofer quickly became one of the most skilled glassmakers and lens designers in the entire history of German optics. His greatest telescope was probably the 9¹-inch (24-cm) Dorpat refractor, which was installed at the Pulkovo Observatory in Russia around 1825. Besides possessing a superb objective lens, this telescope was unique in that it also possessed the first equatorial mounting in the modern sense.
   Basically the German, or Fraunhofer, equatorial mounting consists of a polar axis which is accurately aligned with the  rotational axis of the earth. A declination axis, mounted at the upper end and perpendicular to the polar axis, carries the telescope tube at one of its extremities and counter-weights at the other. The two rotational degrees of freedom about the declination and polar axes permit the telescope to be directed to any part of the sky. A great advantage of the equatorial mounting is the fact that the polar axis can be continuously driven at the sidereal rate (the rate at which stars appear to move in the sky) by a clock mechanism, thus counteracting the daily rotation of the heavens and enabling the telescope to track a star automatically. Fraunhofer incorporated such a drive in the Dorpat refractor. Visual observations could be made with greater ease than ever  before. When photography was introduced into astronomy in the latter part of the 19th century and long time exposures became commonplace, the clock-driven equatorial mounting became an absolute necessity.
   Due largely to Fraunhofer and his successors, by 1850 the refracting telescope had become the principal observational tool at the majority of observatories. Most professionals of the day felt that the difficulties inherent in casting large speculum mirrors would discourage further development of the reflecting telescope and prevent its widespread adoption. Although one dissenter from this general opinion, William Parsons, third Earl of Rosse, succeeded in producing several 36-inch (91-cm) Newtonians and later, in 1845, a colossus 72 inches (1.8 meters) in diameter, the making of speculum mirrors remained a tricky and uncertain business. A major breakthrough occurred around 1853 when Justus von Liebig perfected a method for precipitating metallic silver out of a solution and depositing it as a thin reflecting film upon a glass surface. In 1856, Carl August von Steinheil and Léon Foucault, a French physicist, independently applied Liebig's idea to astronomical mirrors. From this point onward, with a few exceptions, mirrors were made exclusively of glass, a much lighter substance than speculum metal and far easier to cast, grind, and polish. Besides these advantages, silver-on-glass mirrors possess a higher reflectivity in the visible range than speculum mirrors.      Furthermore, after tarnishing, the original luster of a silver-on-glass mirror can easily be restored by dissolving the old silver coating and depositing a new one. In the case of a speculum mirror, however, the optical surface itself had to be repolished.
   As valuable as Foucault's efforts were in perfecting the silver-on-glass process, his most important contribution to the development of the reflector was a simple technique for determining the exact figure of a mirror. Like John Hadley some 200 years earlier, Foucault placed a pinhole source at the mirror's center of curvature and arranged the image to be formed alongside the source. However, unlike Hadley, Foucault examined the rays converging to a focus by placing his eye behind a knife-edge, which he then slowly introduced into the image. If the surface of the mirror darkened uniformly, Foucault knew the mirror was spherical; if it did not darken uniformly, Foucault was able to deduce where and by how much the mirror surface deviated from sphericity. This technique, called the Foucault knife-test, is incredibly sensitive and is very much in use today -- bulges or hollows in a mirror surface with a relief as little as one millionth of an inch are easily detectable. Armed with his knife-edge, Foucault was able to produce mirrors with an accuracy of figure never before achieved.
 

Modern Refracting Telescopes

   Present-day refractors have changed little from the instruments of Joseph von Fraunhofer's time. Possibly the most important advances made over the past 75 years have been in the field of glassmaking. New types of glass have become available which permit the execution of advanced lens designs, characterized by smaller and smaller aberrations. However, even today, chromatic aberration cannot be entirely eliminated from a lens system. Refracting telescopes, for example, are designed at the outset with their intended use in mind. If the instrument is to be employed visually, the objective lens is highly corrected in the colors to which the eye is most sensitive, namely, yellow and green. If photography is intended, the telescope is normally corrected for the blue and ultraviolet. The two largest refractors in existence, both visual instruments, are the 40-inch (1-meter) telescope at the Yerkes Observatory in Williams Bay, Wis., and the 36-inch (91-cm) instrument at the Lick Observatory on Mount Hamilton, Calif. Both instruments date back to roughly 1890. Their equatorial mountings are of the Fraunhofer type and were manufactured by the Warner and Swasey Company of Cleveland, Ohio. The enormous objective lenses were cast in France and were figured by the famous American optical firm of Alvan Clark and Sons.
   So long as lenses are made of glass, the 40-inch (1-meter) at Yerkes represents the practical limit in size of refractors. Although glass blanks larger than 40 inches in diameter have been cast during the past 50 years, they are seldom, if ever, sufficiently free of internal defects to make satisfactory lenses. Even if an acceptable blank were obtained, the resulting lens, supported only by its edge, would distort so badly from its own weight as to be useless optically.
 

Modern Reflecting Telescopes

   Large mirrors do not suffer from these disadvantages. In the first place, light is reflected only from the front surface of a mirror and, therefore, the quality of the glass in its interior is immaterial. Secondly, a mirror can be supported from the back as well as the sides, thus reducing the problem of distortion. Further relief from distortion can be obtained by honeycombing the back of a mirror, thereby decreasing its mass. Finally, in all reflectors, the mirror is located at the bottom of the telescope, close to the supporting axes of the mounting. It is therefore generally easier to counterbalance and to design against mechanical flexure in a reflector than in a refractor where a massive lens must be carried at the upper end of a long slender tube. For these reasons, plus the ever increasing demand for greater light-gathering ability, it is not surprising that the large reflector has been vigorously exploited.
   Many reflecting telescopes, exceeding 50 inches (1.3 meters) in aperture (useful mirror diameter), are in use in both the northern and southern hemispheres. Most of these instru  ments are Newtonian-Cassegrains in the sense that the secondary mirrors are interchangeable. In some, the Newtonian form is dispensed with altogether, and observations are carried out at the prime focus directly. A few of the largest reflectors can operate in yet a third form, called a coudé. The coudé is similar to the modified Cassegrain except that the optical flat immediately in front of the primary mirror diverts the light rays down an opening in the polar axis. The advantage of the coudé is that its focus is located at the bottom of the polar axis and remains fixed in position regardless of the orientation of the telescope. Elaborate spectroscopic or photoelectric equipment can be housed separately. Fig. 14 is a schematic drawing of the coudé arrangement.
   From 1919 to 1948, the 100-inch (2.5-meter) Hooker telescope, on Mount Wilson in California, was the largest reflector in the world. The primary mirror was cast by the St. Gobain glassworks in France and was figured by G. W. Ritchey between 1910 and 1915. The main tube is mounted in a rectangular steel frame that forms the major part of the polar axis. One disadvantage of this type of mounting is that stars near the north celestial pole are inaccessible. Because of high operating costs and newer technology, the Hooker telescope was closed in 1985.
   In 1948, the 200-inch (5-meter) Hale reflector, until 1974 the largest telescope in existence, was put into operation on Mount Palomar, California. This enormous instrument is the realization of the dream of George Ellery Hale, a leader in the development of modern astronomical instruments. The horseshoe-shaped northern bearing of the polar axis carries a large fraction of the 540-ton weight of the telescope and is floated on a thin film of oil that is forced between the bearing surfaces at a pressure of about 20 atmospheres. A cage mounted inside the telescope tube at its upper end enables an observer to ride with the telescope and work directly at the prime focus 55 feet (17 meters) above the primary mirror. The Hale telescope can also operate as a Cassegrain or a coudé with focal lengths of 267 and 500 feet (81 and 152 meters), respectively.
   The 200-inch mirror is made of Pyrex, a glass having a low coefficient of expansion. The blank was cast in December 1934 at the Corning Glass Works in New York State and took some ten months to cool and anneal. The back of the blank was deeply ribbed, or honeycombed, to reduce weight and to provide cells or pockets for the complicated system designed to support the finished mirror.
   The largest telescope at present is the 236-inch (6-meter) reflector of the Academy of Sciences of the U.S.S.R. It is situated near Zelenchukskaya, in the Caucasus Mountains, at an altitude of 7,120 feet (2,170 meters). The 925-ton telescope was designed by B. K. Ioannisiani and manufactured in Leningrad. It was put into operation in 1974, and a new mirror was installed in 1979. The mounting is computer controlled and is altazimuth rather than the usual equatorial. The focal length is 79 feet (24 meters).
The large reflectors in the United States include the 158-inch (4-meter) instrument of the Kitt Peak National Observatory near Tucson, Arizona, installed in 1970, and the 120-inch telescope of the Lick Observatory in California, completed in 1959. To observe the southern sky, several large telescopes were constructed in the southern hemisphere in the 1970's. The 158-inch reflector of the Inter-American Observatory at Cerro Tololo, Chile, is operated by the Kitt Peak National Observatory. Another large reflector is the 150-inch (3.8-meter) telescope of the European Southern Observatory at La Serena, Chile. In Australia, the 158-inch Anglo-Australian Telescope (with an unusually wide field of view), was inaugurated at Siding Spring, near Dubbo in New South Wales, in 1974. A slightly smaller telescope on Mount Stromlo, near Canberra, was already in operation.
   No account of the evolution of the reflecting telescope would be complete without some mention of the vacuum technique developed during the 1930's by R. C. Williams and by J. D. Strong and C. H. Cartwright for coating astronomical mirrors. Up to this time, mirrors had always been silvered by the chemical-precipitation process dating back to Foucault. In the modern method, the mirror is placed inside a chamber from which the air is evacuated. Small loops of aluminum wire are suspended from tungsten coils installed in the roof of the vacuum chamber. After a vacuum of the order of 10-8 atmosphere is attained, a pulse of current is passed through the tungsten coils, causing the aluminum to vaporize and to deposit as a thin uniform film over the mirror surface below. Apart from the convenience and reliability of the method, aluminized mirrors possess two major advantages over silvered mirrors. In the first place, aluminum reflects better than silver in the visible and ultraviolet range. Secondly, an aluminized mirror never tarnishes -- immediately upon exposure to air, a tough, colorless protective layer of aluminum oxide that is only one molecule thick forms over the metallic surface. Needless to say, aluminized mirrors are used almost exclusively today.
 

Schmidt-Type Telescopes

   In general, the area in the sky which can be photographed by a conventional reflector is rather small. This circumstance arises from two factors.  First, the long focal lengths of most reflectors set initially a narrow limit on the attainable field. Secondly, all conventional reflectors suffer to one degree or another from inherent aberrations known as coma and astigmatism. These two aberrations conspire to destroy the quality of stellar images formed at an angle to the optical axis of the telescope. To put it another way, the quality of stellar images on a photograph taken by a reflector deteriorates rapidly with increasing distance from the photograph's center. In the case of the Hale 200-inch reflector, for example, the area at the prime focus in critical definition is only as large as a postage stamp, which corresponds to an angular region in the sky measuring approximately 2.5 by 2.5 minutes of arc. Special lens systems to correct coma can be designed and placed immediately in front of the photographic plate, effectively enlarging the usable field by a factor of 10 or 15. Even so, the conventional reflector remains a narrow-field instrument.
   The need for a wide-angle high-speed reflector-type camera possessing excellent image quality over its entire field was satisfied by Bernhard Schmidt of the Hamburg Observatory in 1932. Schmidt devised the basic optical system shown in Fig. 15, which consists of a spherical primary mirror and a thin nonspherical correcting plate, located at the mirror's center of curvature. The spherical primary assures the absence of coma and astigmatism; the correcting plate is designed to eliminate the spherical aberration of the mirror.
   Although the correcting plate is extraordinarily difficult to make and the focal plane of the instrument is curved, many Schmidt-type telescopes have been constructed over the past 35 years. Their high photographic speed and large angular field have been employed with great effect in studies of the aurora, meteors, and artificial satellites. The largest Schmidt in existence is located at the Karl Schwarzschild Observatory overlooking Tautenberg, near Jena, Germany. Completed in 1960, this instrument can also be used as a conventional telescope with a quasi-Cassegrain and coudé foci. When used as a Schmidt telescope, the light-gathering element is a 79-inch (200-cm) spherical mirror with a focal length of 157.5 inches (400-cm). The 52.8-inch diameter Schmidt correcting plate provides a 4.7 by 4.7 degree angular field.
   The second largest Schmidt telescope is that located at Mount Palomar. This instrument accommodates a photographic plate 14 inches square (90 cm sq), corresponding to an angular field in the sky measuring 6 by 6 degrees. The primary mirror is 72 inches (183 cm) in diameter and the correcting plate, which was figured by Hendrix, has a clear aperture of 48 inches (122 cm). The National Geographic Palomar Sky Survey, the most complete ever undertaken, was compiled from plates taken by the 48-inch Schmidt.
 

The Maksutov Telescope

   Other forms of wide-angle high-speed cameras have been suggested by various workers. One of these schemes was proposed by the Russian D. D. Maksutov around 1941 (see Fig. 16). Like the Schmidt, the Maksutov camera employs a spherical primary mirror. However, unlike the Schmidt, spherical aberration is removed by a meniscus lens instead of a complex nonspherical correcting plate. Since a meniscus lens is relatively easy to make, the Maksutov system is attractive from a practical point of view, and its performance is high.
 

Stellar Spectroscope

   The simplest spectroscope consists of a glass or quartz prism or, more commonly today, a diffraction grating placed in front of the objective lens of the telescope. Light collected by the telescope passes through the prism or grating, where it is split into its components, producing a characteristic spectrum.
   The use of spectroscopes and spectrographs in conjunction with modern astronomical telescopes enables the astronomer to obtain more detailed information about various celestial bodies. By measuring the displacement of the spectral lines (the Doppler shift), he can determine the velocity of a star moving toward or away from the earth. This displacement of spectral lines also provides considerable data on spectroscopic binaries, or double stars, which cannot be resolved by a telescope alone. From the intensities and profiles of the spectral lines, information about temperature, pressure, stratification, and chemical composition of stars is obtained.
Because of the opacity of the earth's atmosphere, only a very narrow range of electromagnetic wavelengths can be studied spectroscopically from the earth's surface. High-altitude balloons, research rockets, and satellites carrying spectroscopic equipment have extended the range to the infrared and ultraviolet portions of the spectrum and even to the X-ray and gamma-ray regions. Radio telescopes using techniques of microwave and radio-frequency spectroscopy are employed for wavelengths ranging from a few millimeters to about 15 meters.
 
 

SOLAR INSTRUMENTS
 

   Professional observations of the sun in white light are not normally carried out with conventional telescopes. The large reflectors gather so much solar radiation that heating of the optical components becomes a serious problem. On the other hand, most refractors are too short to form sufficiently large solar images.
   The main requirement for a solar telescope is therefore that of great focal length, just like the refractors of Hevelius and Huygens. The difficulties of maneuverability encountered with these earlier instruments can be avoided by the use of a device called a coelostat which produces a beam of sunlight whose direction in space remains constant. The concept of the coelostat is not new. It was first suggested in Hevelius' time. Later, from 1830 to approximately 1900, the device was brought to its present level of development by a variety of workers, including Hale.
 

The Coelostat

   The modern coelostat consists of two optically flat mirrors whose centers normally lie in the meridian plane. As shown in Fig. 17, one mirror is mounted above the other. The lower mirror is attached to a polar axis which is clock-driven at one half the solar rate. The upper mirror is fixed. Light rays from the sun fall on the lower mirror and are reflected to the upper mirror which, for the case illustrated, diverts the rays into a parallel horizontal beam. This beam then passes through a stationary objective lens, which forms an image of the sun in its focal plane. The size of the solar image depends upon the focal length of the lens. The fact that the lower mirror is clock-driven enables the coelostat to "track" the sun automatically. Having the centers of both mirrors lie in the meridian plane prevents rotation of the solar image.
   One difficulty with the horizontal solar telescope is that air turbulence along the optical path tends to impair the quality of the final solar image. Most of the major installations are therefore built vertically to minimize effects of turbulence in the air. The largest solar telescope is the McMath Telescope at Kitt Peak in Arizona. An 80-inch (2-meter) mirror at the top of a tower 100 feet (33 meters) high reflects sunlight 450 feet (140 meters) down a diagonal tunnel to a 60-inch (1.5-meter) image-forming mirror near the bottom of the tunnel. This mirror reflects the light 300 feet (90 meters) back up the tunnel, where a 48-inch (1.2-meter) mirror reflects the image of the sun into an observing room 25 feet (7.5 meters)  below ground level. Under the observing room a shaft 72 feet (22 meters) deep houses several powerful spectrographs, including one 55 feet (17 meters) long.
 

The Coronagraph

   Another solar instrument, called a "coronagraph," was invented by Bernard Lyot around 1930. This device enables the corona of the sun to be studied at any time without waiting for the occurrence of a solar eclipse. Basically, the coronagraph consists of a high-quality refractor in whose focal plane a small disk occults the image of the sun. The diameter of the disk exactly equals the diameter of the solar image, so that an artificial eclipse is produced, and only the faint light from the sun's corona reaches the camera mounted at the end of the coronagraph. For good results, the objective lens must be superbly polished and must be entirely free of any internal defects such as striations or bubbles. Extraordinary precautions are taken to reduce scattered light by mounting a series of diaphragm stops inside the coronagraph tube. Observations can only be carried out under the most favorable atmospheric conditions -- clean air is absolutely essential. For this reason, coronagraphs are normally installed at high-altitude stations.
 

The Spectroheliograph and Spectrohelioscope

   The solar telescope and coronagraph are instruments which operate in the "integrated" or white light of the sun. Between 1891 and 1895 George Ellery Hale developed a monochromatic solar instrument called a "spectroheliograph" or a "spectrohelioscope," depending upon whether the instrument is used photographically or visually. Monochromatic studies of the sun reveal far more detail on the solar surface than is possible to detect in white light.
Hale's device consisted of a coelostat which formed a small image of the sun on the small slit of a high-dispersion spectrograph. A second slit, located immediately below the first slit and lying in the spectrograph's focal plane, isolated a narrow band of wavelengths from the solar continuum. If now the solar image is made to traverse the first slit, and a photographic plate behind the second slit is moved at the same rate, a monochromatic photograph of the sun is obtained. By placing the eye instead of a photographic plate behind the second slit, and then vibrating both slits in unison through a small amplitude, a monochromatic image of part of the solar disk can be seen. The image appears steady because of the persistence of vision.
   A more convenient viewing arrangement, far simpler mechanically, was suggested by J. A. Anderson. In Anderson's modification, the slits remain stationary and two square prisms mounted on a common vertical shaft, one in front of each slit, are rotated at high speed. The combined effects of refraction in the prisms, plus rotation, result essentially in a scanning of part of the solar disk. When the prisms are rotated rapidly, the successive scans are blended by the eye and the monochromatic image again appears steady.
   Another technique for observing the sun in monochromatic light was introduced by Lyot in 1933 and has since received development at the hands of a number of other workers. Based upon an idea conceived by R. W. Wood in 1914, the device, called a monochromator, is placed in the light path of a refracting telescope. The monochromator is an extremely complicated optical filter consisting of a series of polaroid screens separated by quartz plates. The filter functions on the principle of the interference of light and can be designed to pass an extremely narrow band of wavelengths. Because of this fact, the monochromator gives  somewhat better monochromatic definition of the solar image than the spectrohelioscope. However, one complication in the use of the monochromator is the fact that its performance is highly temperature-sensitive, and thermostatic control to within 0.1°C. is necessary.
 
 

ANGLE-MEASURING INSTRUMENTS
 

   Improvements in the Quadrant. The evolution of divided instruments, like the quadrant and sextant, from Tycho Brahe's time to the present is almost as complicated a history as the history of the telescope. It is therefore impractical to record here each successive improvement in instrument design that occurred, or the accompanying advances made in machining techniques. As with the telescope, only broad lines of development can be followed.
 

The Telescopic Sight

   A significant advance in the field of divided instruments was made in the era immediately following Tycho. In 1640 William Gascoigne, an amateur astronomer from Middleton, England, installed the first crosshairs made of spiderweb in the focal plane of a refractor. By using a positive eyepiece, such as the simple double convex lens suggested 20 years earlier by Kepler, Gascoigne was able to focus on the image of a star or planet and the crosshairs simultaneously. In short, Gascoigne invented the telescopic sight, and for the first time it became possible to point a telescope at an object with an accuracy of alignment far greater than the angular resolution of the naked eye. Following Gascoigne, the telescopic sight was adopted on virtually all divided instruments.
 

The Vernier Scale

   Another important development in the direction of greater accuracy took place in 1631 when Pierre Vernier invented an ingenious technique, more sophisticated than the method of transversals, for reading a linear or angular scale. The basic principle is illustrated in Fig. 18. Vernier replaced the fixed zero point by a small auxiliary scale. In the case illustrated, 10 divisions of the so-called vernier scale V equal 9 divisions of the main scale S. It follows that the interval between divisions on V equals 0.9 times the interval between divisions on S. When the zero points of V and S are aligned, as shown in Fig. 18(a), the 1 mark on V will lie to the right of the 1 mark on S by 0.1 times the main-scale interval; similarly, the 2 mark on V will lie to the right of the 2 mark on S by 0.2 times the main-scale interval, and so on. Suppose now the main scale S is moved to the right of the fixed vernier V by an arbitrary distance, say, 0.6 times the main-scale interval as shown in Fig. 18(b). The zero of the vernier will lie between the zero and 1 marks on the main scale and, further, the 6 mark on V will be aligned with the 6 mark on S. In other words, the numerical value of the vernier mark which coincides with a mark on the main scale indicates the decimal part to be added to the main-scale reading.
 

Micrometer Scales

   Angular scales on modern instruments may also be read by using micrometers, in which the decimal part of the main-scale reading is indicated by the fractional revolution of a precision screw of known pitch.
   As a result of the advances described above, plus improvements in mechanical design and execution, the azimuth quadrant of Tycho's time has evolved over the past 250 years into an instrument now called the theodolite, a more accurate version of the familiar engineer's transit. A portable, electronic digital theodolite is manufactured by the Nikon Company. The altitude and azimuth scales are engraved on glass plates, approximately 3 inches (8 cm) in diameter, which are totally enclosed within the instrument and are viewed by an internal optical system. By means of a micrometer arrangement, these scales can be read to an accuracy of approximately ± 0.4 seconds of arc.
 

The Sextant

   The evolution of the ancient astrolabe is most interesting. As far back as the latter part of the 17th century, Newton proposed a device that he called an octant, which performed the same function as the astrolabe. It is not certain whether Newton actually constructed an octant or not. However, in 1731, an American, Thomas Godfrey, and, in 1732, the Englishman Hadley had produced working models. This basic system, illustrated in Fig. 19, is the underlying principle of the modern marine sextant. The sea horizon is viewed through a telescope and through the clear part of a half-aluminized mirror called the horizon glass. A second mirror, called the index glass, is rotated about an axis until light rays from the object under study are reflected by the index glass to the aluminized part of the  horizon glass and into the viewing telescope. When properly adjusted, the observer sees the celestial object tangent to the sea horizon. A radial arm permanently attached to the index glass travels over a circular arc, having the rotational axis of the index glass as center, and indicates the altitude of the object directly. Special sextants employing artificial bubble horizons have been developed for the navigation of aircraft.
 

The Meridian Transit

   It remains to describe the development of the great mural quadrants. Just as Tycho regarded his mural quadrant as the primary standard of precision at Uraniborg, so today, instruments of the meridian type play the most fundamental role in positional astronomy. The telescopic sight replaced naked-eye sights on mural quadrants after 1640. However, with the greater sighting accuracy that resulted, it was soon found that mechanical flexure in mural quadrants of conventional design introduced observational errors that could not be neglected. In 1684, Olaus Roemer circumvented this difficulty by constructing the first meridian transit. Basically, this instrument consists of a refracting telescope, equipped with crosshairs, which is mounted on a horizontal axis oriented in the east-west direction. The ends of the axis are supported above the ground by separate piers on either side of the telescope. Rotation about the horizontal axis causes the telescope to move in the meridian plane. Meridian angles were originally measured by a long pointer moving over an associated angular scale. Roemer's transit possessed greater mechanical rigidity than any other mural quadrant of that time. Later, in 1704, Roemer produced an improved version of the transit instrument, called a meridian circle. The awkward pointer was replaced by a large circular angular scale mounted concentrically on the rotational axis. The scale rotated in the meridian plane with the telescope, and meridian angles were read opposite a fixed zero point.
   Around 1730 the last basically new meridian-type instrument was developed by George Graham. This device consisted of a fixed telescope pointing to the zenith and, because of the virtual absence of motion of its parts, was capable of extremely high accuracy. James Bradley discovered the effects known as the "aberration of starlight" and "nutation" from observations taken with a zenith telescope.
   Over the past two hundred years, meridian-type instruments have been developed to an incredible level of reliability and accuracy -- angles as small as 0.01 seconds of arc can now be measured. Such instruments are usually found in the major governmental observatories of the world where they are used to establish, among other things, the precise positions of fundamental stars and the basic unit of time.
 
 

OTHER INSTRUMENTS AND ACCESSORIES
 

Clocks

   Clocks are an essential feature of an astronomical observatory, where two kinds of time are kept -- mean solar and sidereal. The mean-time clocks are simply high-precision clocks running on normal standard time. They are used for general purposes, including the timing of observations. Sidereal clocks measure time by the apparent motion of the stars. Aside from the fundamental determination of time itself, the sidereal clock is used for finding latitude and longitude and for various other purposes. If the local sidereal time is known, together with the right ascension and declination of an invisible object, a telescope may be pointed at the object. An observatory clock is by far the most accurate clock made; it is quite large, usually beats seconds, and has a 24-hour dial. Often it has an electric connection to be used with a chronograph or with other instruments.
 

Chronograph

   A chronograph is an excellent means of recording astronomical observations accurately and permanently. A revolving drum covered with a sheet of paper causes a pen to trace a continuous line on the sheet. The main observatory clock is connected with this device, and an electric contact marks off each second. Any series of observations involving time, such as star transits, may be run onto the chronograph, and such signals are recorded as breaks in the otherwise continuous record of time on the sheet.
Spectrograph. The spectrograph is an exceedingly valuable instrument; indeed, some observatories specialize in research in stellar spectroscopy. Photographs made with this device reveal, in the arrangement of spectral lines, the elements composing the various stars. (Each element always gives lines spaced at definite intervals in the band of electromagnetic radiation.) Such photographs may be made in a few minutes or in several hours, depending upon the star's brightness.
   In the spectrograph, either a prism, a train of prisms, or a grating of thousands of fine lines very close to one another is used to separate the star's light into its spectral colors. The light enters the instrument as it emerges from an optical telescope through a slit only a few thousandths of a millimeter wide, passes through a collimater lens where the rays become parallel, then through one or more prisms, and finally through another lens which focuses the spectra on a photographic plate. If the photographic plate is replaced by an eyepiece so that the spectra may be viewed directly, the instrument becomes a spectroscope.
   As used on large reflecting telescopes, the spectrograph is a large and complicated apparatus. It has special lenses and prisms that are transparent to ultraviolet radiation. The spectral colors are not of special importance, for it is only the positions of the vertical lines crossing the spectral band that are significant. On each side of a stellar spectrum, a comparison spectrum made from a laboratory arc light is recorded for positive identification of the lines from the star. The arc spectra are frequently made under special conditions, such as pressure, increased to provide astronomers with a clue to the physical condition of the star. (See also Spectra; Spectroscopy.)
 

Comparators

   The comparator, or blink microscope, is used for the examination of photographic plates taken of the same region of the sky at different times. Two plates are exposed to the eye in rapid succession by the movement of a lever. Changes in the configuration of the objects on the plates, such as the movement of a suspected planet, are detectable by their relative motion, or new objects become apparent by their alternate appearance and disappearance. The device essentially consists of a microscope, prisms, lenses, standards to hold the plates, and illumination for the plates. Other instruments are the stereocomparator, to measure rectangular coordinates; the spectrocomparator, to measure the displacement of spectral lines; and the coordinate measuring machine.
 

Photometers

   Photometers are light-measuring instruments with which to measure stellar magnitudes or the difference of magnitude between two stars. The wedge photometer is the simplest type used in visual photometry. A wedge of dark glass is inserted at the focal plane of the telescope and adjusted until the intensity of the star equals that of a previously selected standard. The wedge is calibrated, and readings may be made from it directly. In the polarizing photometer, the apparent brightness of a star and that of a standard are equalized by passing their light through a polarizing-prism system.
   In photographic photometry, the brighter the star image on a plate the greater the stellar magnitude. One method of obtaining magnitudes is to compare the image of the star of unknown magnitude with images of nearby stars of known magnitude. Graduated scales of star images are also used. The photoelectric photometer measures objectively and records photographically the blackening of the photographic plate. (See also Photometry.)
 

Thermocouple

   The thermocouple is an extremely sensitive instrument used to measure the heat radiated from a celestial body. It utilizes the junction of small pieces of unlike metals, such as platinum and bismuth, which are connected to a galvanometer. The thermocouple is placed at the focus of a large reflector, and the heat from the star or planet causes a small electric current to be produced. The latter is proportional to the intensity of heat from the celestial source. The device has to be vacuum-enclosed to prevent the escape of heat. (See also Thermoelectricity.)