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Voyage Through the Universe
The Visible Universe recounts the evolution of optical telescopes, beginning with Galileo in the first chapter entitled "Harvesters of Light." Part of my edit appears below. We enter the story as it turns from early telescopes made with lenses to the rise of telescopes made with mirrors.

 

REFLECTIONS OF THE HEAVENS

In antiquity, mirrors were made from polished brass and other metals and, according to the Roman naturalist Pliny, from glass covered with a soft metal such as tin or silver. Mirrors held the same fascination as lenses for early investigators in the field of optics. The Arab physicist Alhazen paralleled his work in refraction—as did Kepler—with experiments on the reflecting properties of curved metal surfaces. And in Britain during the late sixteenth century, a few experimenters tried unsuccessfully to make a spyglass-like instrument from crude concave and convex mirrors. But no significant progress was made in the field of reflecting telescopes until sixty years after Galileo's earliest refractors. (Top)

Credit for the first reflecting telescope worthy of the name goes to Isaac Newton. In the course of his experiments in the mid-1660s with prisms and lenses, Newton acquired the erroneous notion that all types of glass bend light equally, and he logically concluded that refractors could never be made free of chromatic aberration. His solution to the problem was to avoid refraction altogether by using a mirror made of polished speculum, a metal alloy composed mostly of copper and tin. Reflected from the surface of the mirror and thus passed through nothing refractive—the light would not be split into its spectrum, and the characteristic halos of chromatic aberration would not form.

Newton brought all the necessary skills to the mirror-making job. Not only did he draw up the optical design of the telescope, but he also mixed his own highly reflective mirror alloy, using a recipe of six parts copper, two parts tin, and one part arsenic. He cast mirror blanks from the molten speculum metal; then, using techniques of his own invention, he ground and polished them with grains of abrasive mixed in putty. Newton designed a reflector consisting of a concave primary mirror—analogous to the refracting telescope's objective lens—that collected the light and focused it in a converging beam toward a small, flat secondary mirror placed on the centerline of the telescope tube, near the top. The secondary mirror intercepted the light from the primary and reflected it through a hole in the side of the tube, where Newton could view the image with an eyepiece. Although the diameter of the primary mirror in this first reflector was only about one and one-third inches, it worked well. In early 1669, Newton told a friend that the reflector magnified "about 40 times in Diameter" and that "I have seen with it Jupiter distinctly round and his Satellites." (Top)

An error contributed greatly to the telescope's success. Newton had planned to give his primary mirror a spherical curve, even though he was aware that by doing so he would introduce the spherical aberration that plagued lenses of the era. In the event, however, his polishing technique was flawed. It produced a parabolically curved surface that, like a hyperbolic one, focuses light originating near the optical axis at a single point. Like parabolic or hyperbolic lenses, mirrors having these kinds of curvatures focused off-center objects much less sharply than a spherical mirror.

In late 1671, he allowed his Cambridge professor, Isaac Barrow, to take the new reflector to London, where Christopher Wren and other prominent thinkers demonstrated it to Charles II. The instrument created a sensation. The Royal Society placed an order for two reflecting telescopes with a London optician and elected young Newton to their elite membership. 

A SLOW START
Although the greatest telescopes in the world today are all reflectors that can be traced to Newton's invention, the development of the technology was far from smooth. For example, a mirror made of speculum metal, with its high copper content, tarnished rapidly. Within a year or two—or considerably less time, depending on climate—a mirror became so dim that the faintest stars disappeared from view. To remove the tarnish, the mirror had to be repolished, a time-consuming process that risked spoiling the shape. For this reason and others, the mirror's lack of chromatic aberrations was not enough to win astronomers away from lenses, and progress in reflecting telescopes poked along for decades. (Top)

But in the early 1720s, when long-focal-length objective lenses were beginning to turn refracting telescopes into ungainly monsters, the idea of a reflecting telescope was reexamined by Englishman John Hadley. He constructed an instrument with a speculum mirror six inches in diameter. In competition against a tubeless refractor 123 feet long, the reflector acquitted itself well in its ability to resolve faint stars. Being only five feet long, the reflector was much easier to aim, and it had the further advantage that, because the mirrors were enclosed in a tube, extraneous light could not wash out the image.

More important than Hadley's telescope to a revival of interest in reflectors was a method he introduced for optically inspecting the surface of a concave mirror as it was polished. Hadley illuminated the mirror with light shining through a pinhole placed above it at the center of curvature—that is, the center of the sphere of which the reflecting surface was a part. Looking at the mirror, he could plainly see patterns in the reflected light that indicated where more grinding and polishing were needed. Requiring no complicated instruments or gauges, Hadley's technique not only simplified the fabrication of telescope mirrors but also eliminated much of the risk to the shapes of mirrors inherent in repolishing them. (Top)

The premier mirror maker of the day was William Herschel, a German musician who moved to England as a young man and became an amateur astronomer in middle age. Because he could not afford to buy a large telescope from an optician, he taught himself to make mirrors and was soon building the best telescopes the world had seen, unsurpassed in their ability to resolve fine detail and reveal faint stars.

Herschel kept as trade secrets the details of how he produced mirrors but claimed that his training on the violin gave him skill at delicate polishing strokes. A perfectionist, he would test a mirror again and again by taking it from his optical workshop to the telescope, checking its performance, then returning it to the shop for more polishing. His first successful telescope, patterned on Newton's design, had a mirror six and a half inches in diameter and a tube seven feet long. While peering through this instrument one night in 1781, he spotted an object that was perceptibly larger than the stars around it. Convinced by the fuzziness of the image that he had found a comet, he changed eyepieces to increase magnification. Only after extended observation of his find, however, did Herschel realize that he had come upon not a comet but a new planet. He named it Georgium Sidus—George's Luminary—in honor of King George III; today, it is known as Uranus. (Top)

Following this momentous discovery, the king asked that Herschel's impressive telescope be taken to the Royal Observatory at Greenwich and tested against the instrument there. "We have compared our telescopes together," Herschel wrote home to his sister Caroline, "and mine was found very superior." George III, an astronomy enthusiast himself, granted Herschel an annual salary of £200.

During his studies of the "space-penetrating power" of telescopes, as he called their light-gathering ability, Herschel noticed that a mirror returned only part of the light that fell on it, absorbing the rest. In the layout for a Newtonian telescope, the light was reflected twice and dimmed to slightly more than half its original brilliance. So Herschel resolved to build telescopes having a single mirror. To eliminate the secondary mirror, he set the primary at a slight tilt in the base of the telescope so that it focused light directly into an eyepiece mounted just inside the top of the tube. (Top) 

A MAN OBSESSED
Making telescopes by day and observing with them by night, Herschel came to neglect his music pupils, clothes, and personal appearance. Sometimes the work of figuring a mirror engrossed him for sixteen hours at a stretch. Recalling such episodes, Caroline told of being "constantly obliged to feed him by putting the victuals by bits into his mouth." Herschel made fifty or so telescopes between 1782 and 1785. Some he built for himself; others he sold to supplement his stipend from the king. As he gained experience, he made several primary mirrors for most of his reflectors. When the surface of one mirror tarnished, he simply replaced it with another.

Before Herschel's telescopes, astronomy was limited largely to the study of the planets; stars served chiefly as reference points for charting planetary motions. But Herschel's superb reflectors permitted him to study many more double stars than had previously been visible. He also cataloged thousands of star clusters and objects called nebulae, thought to be clouds of interstellar gas and dust. Some nebulae were just that. others, however, viewed with a 19-inch reflector Herschel built, proved to contain vast numbers of stars. He concluded that "nebulae" of this kind are in fact galaxies, like the Milky Way –the only galaxy known at the time. (Top)

To gain a better idea of the dimensions of the Milky Way itself, Herschel invented the technique of star gauging. He counted all the stars that he could see through a given telescope aimed successively at many points in the sky. Herschel called out the star counts to Caroline, who recorded them in a log. He theorized that areas with a greater density of stars corresponded to directions in which the Milky Way extended farther along the line of sight. After much work, he concluded that the Milky Way is flat like a grindstone and that the Solar System is not at the center.

To chart faint stars in the Milky Way and thereby (as he thought) see through the full extent of the galaxy, Herschel needed to build telescopes with still more light-gathering power. In 1785, he began work on a reflector with a 48-inch mirror. The telescope, which was financed by a grant of £4,000 from George III, had a tube fashioned from sheet iron that was almost 40 feet in length; a wooden structure was erected to support and steer it. Three primary mirrors were cast. The first was serviceable, although it was thinner at the center than intended. The second cracked. The third was close to perfect. On August 27, 1789, it was hoisted into position with a crane. On the night following the telescope's completion, Herschel, peering into the new reflector, discovered Enceladus, the sixth moon of Saturn. On September 17, he discovered the seventh, Mimas. (Top)

Despite early success, the telescope was doomed to failure by the size of its mirror; it was so huge that it sagged under its own weight, making clear images difficult to obtain. It was also inconvenient to use, with at least two assistants being required to help point it and record observations. And tarnishing took its toll: By 1801, Herschel found that the mirror was "much injured by time." In 1815, he abandoned the telescope.

The largest reflector built with a metal mirror—a 72-inch monster—entered service at Birr Castle, Ireland, thirty years later. Known as the Leviathan of Parsonstown, the huge Newtonian reflector was commissioned by William Parsons, the third earl of Rosse and an avid amateur astronomer. Its four-ton primary mirror enabled Lord Rosse to discern the spiral structure of some nearby galaxies and the filamentary shape of the Crab nebula, which he named. However, the Leviathan could rarely be used to full advantage because the sky in Parsonstown was so often overcast, and the telescope gradually fell into disuse  (Top)

THE DECLINE OF SPECULUM
A point of diminishing returns had now been reached with speculum metal mirrors. Not only did tarnishing rapidly weaken their light-gathering power, but also changes in temperature would cause them to expand and contract considerably. These stresses could alter the figure of the mirror, diminishing its resolution. Lord Rosse had rigged levers behind his mirror to compensate for such distortion, but the system did not work reliably. On some nights, he could adjust the mirror to produce extraordinarily sharp images; on others, the telescope's performance was a disappointment.

For these reasons, telescopes with lenses remained the instrument of choice among professional astronomers during the second half of the nineteenth century, even though the solution to all the reflector's problems was at hand: Around 1850, scientists in Europe had discovered how to deposit a layer of silver on a glass surface—even a concave one, as needed for the primary mirror of a telescope—by covering it with a solution of silver nitrate and sugar. After a time, fine granules of silver precipitated out of the solution and stuck to the glass. This reaction produced a thin, uniform, and highly reflective deposit. (Top)

The advantages of silver over speculum metal were manifold. The shiny metal returned a third more light and tarnished less rapidly than the alloy. Removing the discoloration was a relatively simple matter of dissolving the old silver and depositing a new coating, a process that did not demand reworking the concave figure of the mirror. Glass was cheaper than speculum metal—and lighter, too, so that a large mirror sagged less and could be more easily mounted and supported. Moreover, glass was less susceptible than metal to the distortions that could be caused by temperature changes.

Most prominent among those who experimented with glass telescope mirrors was the French physicist Jean Foucault. He made several small Newtonian reflectors during the late 1850s, the largest having a mirror less than eight inches in diameter. Noting that gravity caused even these mirrors to droop ever so slightly, Foucault remedied the situation by backing the mirror with a rubber bladder. While looking into the telescope, he could inflate or deflate the bladder to subtly change the shape of the mirror and thereby achieve resolution that approached the instrument's theoretical limit. (Top)

This boundary of telescope performance had been established in the 1830s by British astronomer George Airy, who discovered that the best resolution obtainable from any mirror depends on the ratio of its diameter to the wavelength of light falling on it. Thus, the wider the mirror, the better its resolution should be. In practice, however, the rule fails for mirrors larger than about eight inches in diameter. Distortion of light waves by Earth's atmosphere nullifies the potential gain in resolution from wider mirrors. Their chief value lies in their ability to gather more light, thereby revealing the presence of heavenly bodies invisible to lesser optics. In 1864, Foucault completed a mirror more than 30 inches wide. 0riginally installed at the imperial Observatory in Paris, it was soon moved to f Foucault's Marseilles. With more than ten times the light-gathering power of Foucault’s 8-inch reflector, the instrument enabled astronomer Edouard Stephan to catalog hundreds of nebulae. Despite this and other successes of the 30-inch mirror, Foucault's reflectors—and larger ones that followed before the turn of the century—were for the most part unproductive. Poor telescope design and underengineered mounts were at fault more often than the mirrors. A few such telescopes met their designers' expectations for image quality and utility, notably a French reflector of 47 inches made in 1877 and a 36-inch instrument installed at California's Lick Observatory in 1895. But a full realization of the potential of such devices awaited the energy and dedication of American astronomer George Ellery Hale. (Top)

REFLECTORS REASCENDANT
Born in 1868, Hale was the son of a wealthy Chicago businessman who manufactured hydraulically actuated elevators. He studied physics as an undergraduate at the Massachusetts Institute of Technology, where he demonstrated a knack for instrumentation. He also proved to be a talented organizer who possessed remarkable powers of persuasion. Here was a man who could talk turkey with industrial tycoons and at the same time make sense to scientists.

These skills, rarely combined in one individual, enabled Hale to become the dynamo behind some of the most important telescopes ever built. The Yerkes Observatory, home of the world's largest refractor, was his idea. With money that he solicited from a Chicago streetcar magnate while still in his twenties, he had been the one to commission Alvan Clark and his sons to grind the 40-inch lens that made the telescope there famous. (Top)

Realizing that refractors had no further potential, Hale persuaded his father in 1896 to buy him a blank for a 60-inch mirror. The Carnegie Foundation, a philanthropic organization started by business tycoon Andrew Carnegie, put up the money for a telescope equipped with the mirror. To provide a home for the instrument, Hale founded the Mount Wilson Observatory in 1904, using his own funds as seed money. By the year 1908, the 60-inch reflector was at work.

Mount Wilson, in the San Bernardino Mountains east of Los Angeles, was an excellent site for the telescope. Situated more than a mile above sea level, the facility rose above much of the star-obscuring dust and smoke in Earth's atmosphere—and higher than any other observatory of its day. By reducing atmospheric distortion, the altitude contributed to the telescope's unprecedented performance. It was, for example, able to resolve stars in the Andromeda nebula that, through the Yerkes 40-inch refractor, had appeared as so much cosmic dust. (Top)

In 1907, Hale persuaded Los Angeles hardware entrepreneur John D. Hooker to pay for a 100-inch reflector that would join the 60-inch reflector on Mount Wilson and bear its benefactor's name. When funds for the Hooker Telescope ran out, Hale talked the Carnegie philanthropists into making up the difference. By 1918, the Hooker Telescope was peering into the universe, helping to establish—far more accurately than Herschel had been able to—the size of the Milky Way and the number of stars it contains.

Hale believed that the future of astronomy lay in the new field of astrophysics—the application of physical laws studied in the laboratory to celestial objects investigated in the observatory. This discipline uses spectroscopy to study stars through analysis of the electromagnetic radiation produced by the internal nuclear reactions that power them. All elements emit distinctive wavelengths of light—their spectra—by which they can be identified. Furthermore, the phenomenon known as the Doppler effect makes light appear to be of slightly shorter wavelengths when it comes from an object moving toward the observer and of slightly longer wavelengths when it is receding. Since the magnitude of this Doppler shift varies with speed, spectroscopy provides the key to charting the courses of stars and galaxies as they transit the cosmos. (Top)

Hale made sure that each of the Mount Wilson reflectors was equipped with a spectrograph—the special instrument needed to make such studies. The device was attached at the base of the telescope, behind the primary mirror. Such a setup was possible only because Hale had abandoned the Newtonian design common to most earlier reflectors, using instead an arrangement of mirrors proposed in 1672 by Guillaume Cassegrain, an astronomically inclined Frenchman of uncertain occupation. (He is thought to have been either a sculptor or a university professor.) Cassegrain's scheme substituted a convex mirror for the flat secondary reflector of Newton's telescope. This mirror, instead of directing the image to the side of the telescope tube, focused it back through a hole in the center of the primary mirror (page 39). The design, though difficult to execute because it required the grinding of two curved mirrors, more than halved the length of the tube required for a reflector of a given focal length.

Mount Wilson astronomer Edwin P. Hubble and coworker Milton Humason made good use of the spectrographs hitched to the two reflectors there. After devoting six years to analyzing Humason's many spectrograms of faint galaxies, Hubble announced in 1929 that all but a few nearby galaxies are receding from the Milky Way at rates proportional to their distance. That is, the more distant the galaxy, the faster it is moving away. The discovery revolutionized cosmology. Heretofore, most astronomers had thought the universe to be static, neither increasing nor decreasing in its dimensions. The Mount Wilson astronomers proved that it is expanding. (Top)

By this time, Hale had officially retired. For much of his life, he had suffered from painful headaches, vivid hallucinations, and nervous breakdowns. These infirmities were aggravated by responsibilities that reached far beyond the narrow world of astronomy. At his urging, for example, the National Research Council had been formed to advise President Woodrow Wilson's administration on scientific matters during World War 1. He had also joined the board of trustees of Throop Polytechnic Institute, a 550-student engineering college in Pasadena, California. In 1920, acting on a Hale proposal to make Throop the MIT of the west, the board renamed the school the California Institute of Technology.

A MAN FATIGUED
At age fifty-four, heeding the advice of his physician, Hale went abroad for rest and recuperation. But upon returning home, he set out on yet another campaign: to provide astronomers with a more powerful telescope than the Hooker. Abandoning a concept for a 300-inch reflector as impractical (the primary mirror for such a behemoth would have been too large to ship from manufacturer to observatory, he thought), Hale lowered his sights and settled on a 200-inch mirror. In an article written for the April 28, 1928, issue of Harper's Weekly, he once again framed an eloquent appeal to the worlds of money and science, acknowledging the "princes and potentates, political or industrial" who had financed his earlier telescopes and bemoaning the fact that "starlight is falling on every square mile of the earth's surface, and the best we can do at present is to gather up and concentrate the rays that strike an area 100 inches in diameter." Hale's plea was heard by the Rockefeller Foundation, which agreed to fund not only the telescope but also an observatory on California's Mount Palomar to shelter it. Altogether, a little more than $6.5 million would be spent. (Top)

Corning Glass Works of Corning, New York, produced the blank for the mirror of Pyrex, the trademark name of a new type of glass that was much less prone to change shape in response to variations in temperature than were earlier formulations. For stiffness, the casting was made two feet thick, but not of solid glass. A ribbed back (page 60) kept the weight of the casting to 29,000 pounds. The ribs would also enable the finished mirror to match the temperature of the night air more rapidly than could a solid slab of glass —and thus diminish relatively quickly the disruptive convection currents rising from its surface.

The blank was transported by rail from New York to Pasadena, where Caltech opticians ground and polished the glass surface into a nearly perfect parabola. Begun in 1936 and interrupted for several years by World War II, the process was not completed until 1947. By that time, Hale was nine years dead, victim of a variety of ailments, and Max Mason—a professor of mathematics and physics, as well as a director of the Rockefeller Foundation—had taken over the project. (Top)

The precipitation process for applying a reflective coating to a polished blank had been made obsolete in 1932 by the invention of a superior technique. The brainchild of American physicist John Strong, the new method called for placing the blank in a vacuum chamber that also contained thin wires connected to a powerful source of electricity. When sufficient current was passed through the wires, they exploded, vaporizing onto the glass a microscopically thin layer of metal from the wires that exactly mimicked the carefully figured surface of the mirror. No polishing or buffing was necessary. Instead of silver for the wires, Strong used aluminum to coat the 200-inch mirror, the lighter metal being more reflective and tarnishing less readily. (Top)

The finished mirror was hauled to the summit of Palomar Mountain aboard a flatbed truck, then hoisted into its huge equatorial mount three days before Christmas, 1947. The completed instrument, weighing some 530 tons and able to see twice as far into space as the 100-inch reflector at Mount Wilson, remains a colossal achievement of optical engineering.

At a dedication ceremony six months later, the new telescope, which until then had been referred to simply as "the 200-inch," was named after the driving force behind it—George Ellery Hale. After an extensive shakedown period, the Hale reflector went into regular service in 1949. Among numerous other contributions, the telescope played a part nearly fifteen years later in the enumeration of the quasars that currently define the visible edge of the universe. (Top)

 

Voyage through the universe: The Visible Universe.
Copyright Time-Life Books, 1990

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