Reflected Glory: How They Built Palomar
The great Hale Telescope, with its 200-inch mirror, is still the masterpiece of American astronomical engineering
When Halley’s comet returns to our quarter of the universe this year, the great 200-inch Hale Telescope, perched high on Palomar Mountain in California, will follow it across the sky. In fact, the 200-inch, the world’s largest telescope for a full three decades after its dedication in 1948, was the first telescope to detect the comet during its current return, back in 1982. We can always expect the phenomenal from the 200-inch. Its very construction, a major engineering feat in itself spanning more than twenty years, captured the attention and the imagination of the nation. Still enthralling us are the 200-inch’s contributions to our understanding of the nature and evolution of the universe. Even as new telescopes are launched into space and larger ones employing radically new technologies begin to be built on earth, the Hale Telescope’s usefulness to astronomers does not diminish. Long after Halley’s comet has departed once more, the 200-inch will continue to be an instrument of prime research value.
One man was centrally responsible for the Palomar 200-inch telescope and observatory and for several telescopes that preceded it as the world’s largest. He was also instrumental in founding the Huntington Library and the California Institute of Technology. And he was an eminent astronomer who invented important astronomical instruments, pioneered the spectroscopic study of the sun, and presided over the development of modern astrophysics in America. The 200-inch telescope appropriately bears his name: George Ellery Hale.
Born in Chicago in 1868, Hale was nearly a prodigy and always a whirlwind. His father, William Hale, acquired wealth devising the elevators that made tall buildings possible in Chicago and New York; Hale elevators threaded the Eiffel Tower until 1981. By age fourteen, George had purchased his first telescope, and by fifteen he owned a spectroscope and had begun his lifelong study of the sun. Before he was graduated from the Massachusetts Institute of Technology, he had invented a new scientific instrument, the spectroheliograph, which could minutely examine the sun’s surface.
Returning to Chicago after college, Hale built a private observatory in the backyard of his family’s home and began observation with his own 12-inch refractor, a gift from his father. A stream of papers went out from “Kenwood Observatory” to scientific and popular journals. Hale traveled to Europe and won the respect of astronomers abroad. When, in 1892, he learned of two 40-inch lens blanks going begging in Paris, he stumped Chicago looking for a donor. Hale had grown up among rich and powerful men and knew how to approach them. He found Charles Yerkes, a streetcar and land millionaire known as Yerkes the Boodler. “Build the observatory,” the Boodler pronounced. “Let it be the biggest in the world and send the bill to me.” Hale pursued him when he threatened to renege on his grandiose pledge, and became director of the new Yerkes Observatory, at Lake Geneva, in 1897. At twenty-nine Hale was master of the largest telescope in the world.
Hale ruined his health and suffered repeated nervous breakdowns, but one by one he got his telescopes.
Throughout his life Hale remained a modest, generous man. The engine of his ambition was obsessive curiosity, coupled with remarkable scientific foresight. While European theorists probed the most intriguing questions of cosmology, American astronomers remained drearily conservative, content to measure, photograph, and catalog. They were solidly observational, resisting the new science of astrophysics. Hale wanted American astronomy to break out, to unravel the mysteries of the universe by applying physics and chemistry to the stars. For that it needed larger and larger telescopes. Hale ruined his health, suffering prolonged, excruciating headaches and repeated nervous breakdowns, but one by one he got the telescopes.
The best telescopes had been refractors; their lenses collected and focused light. With the 40-inch Yerkes, refractors reached the limit of their usefulness to stellar astronomy. Larger lenses would distort under their own weight and absorb too much precious starlight. But a telescope that uses a mirror to collect and focus the light could avoid these problems. After the Yerkes refractor, which remains the largest in the world, Hale sought reflectors.
The first was a 60-inch, the mirror donated by Male’s father. Searching for better “seeing"—astronomy’s term for calm air—Hale led an expedition to Mount Wilson in 1903 and, with $30,000 of his own funds, established a solar laboratory there. Andrew Carnegie, who had made a fortune in the steel industry, founded the Carnegie Institution in 1904 with a $10 million grant. Hale contacted Carnegie and his new institution and persuaded them to back Mount Wilson; the 60-inch was operational by 1908.
But 60 inches was only a beginning. Before the new telescope was even finished, Hale found a donor among wealthy Pasadena friends for a 100-inch mirror. When the Carnegie Institution balked at the added expense of building and operating a larger telescope, Andrew Carnegie himself interceded, doubling the institution’s endowment. He wanted that observatory built and built quickly. “I should like to be satisfied before I depart,” wrote the millionaire with regard to Mount Wilson, “that we are going to repay to the old land some part of the del [sic] we owe them by revealing more clearly than ever the new heavens.”
The 100-inch Hooker Telescope at Mount Wilson became an instrument of spectacular astronomical advance. Using it in the 1920s and early 1930s, a brilliant ex-Rhodes Scholar named Edwin P. Hubble and a former mule packer and observatory janitor named Milton Humason—Hubble’s patient assistant —determined that the universe was not static but expanding. Galaxies fly away from us in every direction; the greater their distance from the earth, the greater their velocity. Hubble’s work, wrote the distinguished American astronomer Allan Sandage a few years ago, “opened the last frontier of astronomy, and gave, for the first time, the correct conceptual view of the universe.” Male’s goal of bringing America into the modern era of astrophysics had been realized.
Even as Hubble was reporting his grand work, Hale was retreating before the battering of his recurring illnesses. In 1922 he resigned as director of the Mount Wilson Observatory, built a small solar laboratory in his Pasadena backyard, and between fierce headaches, studied the sun. But he wasn’t through with telescope building yet.
When headaches and depression kept him from scientific work, Hale had relaxed by writing popular articles for Scribner’s and Harper’s magazines. In 1927 Harper’s asked him to write once again about astronomy, and he responded with an article titled “The Possibilities of Large Telescopes.” He noted the enormous progress represented by the Yerkes and Mount Wilson telescopes, then fretted over their limitations. “Starlight is falling on every square mile of the earth’s surface,” he observed, “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 was certain he could do better. He addressed—and resolved—doubts that larger mirrors would compromise image quality, dramatically revealing that an ingenious arrangement at Mount Wilson already had demonstrated that greater gain was in fact possible. Francis Pease had attached an instrument known as an interferometer to the 100inch. The interferometer used two small mirrors and united the light they captured into a single image. The mirrors could be moved out as much as twenty feet apart. “By comparing these images,” wrote Hale, “with those observed when the mirrors are 100 inches or less apart, Pease concludes that an increase of aperture to 20 feet or more would be perfectly safe. For the first time, therefore, we can make such an increase without the uncertainties that have been unavoidable in the past.” He then explained the grand projects of galactic and extragalactic observation that a large mirror might serve. And he concluded: “I believe that a 200-inch or even a 300-inch telescope could now be built and used to the great advantage of astronomy.”
The primary challenge was to cast a good glass blank for the lens. McCauley worked up to it by stages.
Hale sent a galley proof of his Harper’s article—he was never one to waste time—to the Rockefeller Foundation. The foundation responded immediately and enthusiastically. A hitch developed in the proceedings when the Rockefeller board stipulated that not the Carnegie Institution but the California Institute of Technology should be responsible for the new telescope and observatory—some of the Carnegie people took the stipulation as a slur—but the intervention of Elihu Root, the former Secretary of State, a Carnegie board member and Hale patron and adviser, soothed hurt feelings, and soon the telescope was funded, $6 million fully pledged.
Funding was one thing, building the telescope quite another. In “The Possibilities of Large Telescopes,” Hale already had reviewed what he took to be the primary challenge the builders would face: casting a satisfactory blank of glass. If, under the stresses of fluctuating temperature and telescope position, the glass expanded and contracted unequally, the mirror coated on its surface would distort star images into useless blurs. What was needed was a type of glass with a low coefficient of expansion. The 100-inch was made of plate glass, but plate glass had far too high a coefficient of expansion for a 200-inch. Pure quartz would be the best material for the new mirror; Pyrex, a type of glass developed specifically to resist heat expansion, offered a good second choice.
Hale first tried quartz. Between 1927 and 1931 engineers at General Electric in Lynn, Massachusetts, labored to cast a succession of fused quartz mirror blanks. Quartz is notoriously temperamental, and the work was plagued with difficulty. GE finally achieved a 60-inch disk but spent $639,000 to get it and estimated that a 200-inch disk might cost an additional million dollars or more. Hale wanted more rapid progress and less catastrophic expense. He thanked General Electric and turned to Corning Glass and Pyrex.
The man in charge of casting the great disk at Corning Glass was George V. McCauley, a practical, resourceful engineer. As GE had done before, McCauley worked up to the task by stages. He poured a solid 26-inch disk for practice, then began with a 30-inch—as large as Corning had ever cast before—and in consultation with Hale’s people designed it with a ribbed back to reduce its weight and increase its speed of response to temperature changes. To form the ribbing, McCauley had to design firebrick cores that stood in the mold like sawedoff columns. It was, he said, an “exercise in jig-saw puzzles in three dimensions, and we had to do it as if we’d known how all our lives.” The firebrick for the cores was made of puffed silica and could resist the terrible heat of the molten glass, but the cement McCauley used to anchor the cores to the mold could not: they bobbed to the surface during pouring, ruining the first attempt. He anchored them with firebrick dowels and got a 30-inch disk; the dowels gave way on a 60-inch. He anchored the cores with steel bolts and got a 60-inch and then a 120-inch and seemed to have the problem licked.
The battle of the cores was only one front in McCauley’s war. Pyrex was easier to handle than quartz but still tricky. Too hot, it lost its homogeneity to air bubbles. Too cool, it congealed and wouldn’t flow to fit the mold. McCauley resorted to domed ovens to keep the mold—and the Pyrex—hot during pouring. They looked like oversized igloos.
The practice efforts weren’t wasted. Along the way to the 200-inch, according to the physics writer Timothy Ferris, the “glassmakers produced a whole generation of mirrors for new telescopes. Two 60-inch blanks went to … Harvard, a 76-inch to Toronto, an 82-inch to McDonald Observatory in Texas, a 98inch to the University of Michigan, and a 120-inch, eventually, to Lick.”
Finally McCauley and his team were ready to attempt a 200-inch disk. Its mold had 114 cores anchored with steel bolts. A new “super-Pyrex,” with an even lower coefficient of expansion, had been developed in the interim, and this was melted for pouring in an oven that contained a tank fifty feet long and fifteen feet wide, holding sixty-five tons of molten glass. The oven could only heat four tons a day; the full charge required two weeks to prepare. McCauley was taking no chances with mechanical pouring systems. He had reverted to the oldfashioned system of pouring from ladles by hand. Each ladle, supported from the ceiling of the pouring room on a trolley, held 750 pounds of molten Pyrex heated to more than 2,700 degrees; 400 pounds would reach the mold as liquid, the rest sticking to the ladle to be returned to the heating tank.
Pouring began at 8:40 A.M. on Sunday, March 25, 1934, in Corning, New York. Scientists, reporters, and sightseers crowded the observation platform above the pouring room. Lowell Thomas described the occasion, with some hyperbole, as the “greatest item of interest to the civilized world in twenty-five years, not excluding the World War.” Every six minutes sweating craftsmen dumped another ladle of white-hot glass into the huge mold, retreated to the heating tank, dipped, and poured again. In the awful heat of casting, one ladleman fainted and had to be dragged away. Someone on the observation platform pushed forward for a better view, fell, and broke a rib. The pouring teams, McCauley with them, stopped for rest and lunch. McCauley was in his office when a man ran in—a core had broken loose. The engineer raced to the mold, peered through the blinding white heat—another core loose, bobbiqg to the surface—then another. “Break them up!” McCauley shouted. Workmen moved in with long steel rods and battered at the cores. The rods heated to red and then to white. McCauley feared they would melt and contaminate his pure yellow-green Pyrex. He called the rods out. The disk was probably ruined. Nothing for it but to pour on through the afternoon and learn how. Ten intense hours—105 ladles—42,000 pounds of glass. And finally it was done. The crowd dispersed, not realizing what had happened.
That night, McCauley oversaw the transfer of the disk into its special annealing oven. He had calculated a slow, conservative rate of annealing and cooling but, since the disk was damaged anyway, he decided to increase the rate tenfold to test his calculations.
By June the thing was cool. A consulting scientist at Corning wrote Hale with some surprising news: “We were … overjoyed to find that notwithstanding the rapid cooling schedule and the somewhat unsymmetrical distribution of clay blocks, the disk remained completely intact. Accordingly, this is the assurance we give you that a 200” glass disk can be successfully made.”
In the meantime, McCauley solved the problem of the cores. For the next attempt at casting, each core would be anchored with bolts of chrome nickel steel fitted with individual cooling systems. He saw to the casting of the second disk on December 2,1934—only six hours of hot labor this time—and the cores held. The disk went into the annealer to soak at a steady temperature for two months and to cool for eight more.
McCauley’s troubles still weren’t over. In midsummer the nearby Chemung River flooded. Floodwaters reached the annealer’s electrical system, and, despite desperate sandbagging day and night, the power had to be shut off for seventy-two hours while the equipment was moved to higher ground. Later a small earthquake would break the windows in Corning, worrying the determined engineer. But when he finally had the disk removed from the annealer, set on its edge, and tested, it was perfect.
It would be ground in Mount Wilson’s optical shop in Pasadena. The New York Central built a special well car to carry it there. McCauley padded the disk with half-inch rubber sheets and packed it into a case made of heavy steel plate. This massive shipping crate was then lifted by crane into the well car, braced standing on its edge. Lloyd’s of London insured it for one hundred thousand dollars. Announced by national headlines, it left Corning on March 26,1936, amid crowds of onlookers, “just two years and a day,” McCauley noted, “since the casting of the first 200-inch blank.”
The railroads were determined to see the disk safely to its new home. They sent crews ahead of it to examine and chart the entire route from Corning to Pasadena, to clear obstacles, shore up bridges, and find branch lines past tunnels and around major cities. Traveling at no more than twenty-five miles an hour and only by daylight, and with curious Americans turning out to glimpse it at every town along the way, the special car progressed slowly across New York, Ohio, Indiana, and Illinois. The New York Central passed it to the Chicago, Burlington and Quincy, which passed it to the Atchison, Topeka and Santa Fe. It was stalled in Kansas City when frost heaving raised the rails enough to prevent it from passing under a bridge. Rerouted around the bridge, it advanced through Kansas, Texas, New Mexico, and Arizona, and finally into California. The disk arrived in Pasadena on April 10 at 8:25 A.M., intact.
The disk was coated with a layer of aluminum less than a thousand atoms thick. This was the true mirror.
Casting the disk was the hardest part, but engineering the horseshoe yoke that would support the mirror in its openframe tube wasn’t much easier. The tube had to be designed to standards of rigidity unheard of in so large a structure, the yoke to unusually close tolerances. Tube and yoke together weighed 500 tons. Westinghouse built them in Philadelphia and shipped them to San Diego in 1938 through the Panama Canal.
The optical shop began grinding the mirror soon after it arrived in Pasadena. The disk turned slowly on a mounting; above it, a grinding tool of Pyrex glass blocks turned more rapidly, making, between disk and tool, complex Lissajou figures like the filigree in the border of a one-dollar bill. Eight sizes of grit and two of aluminum oxide reduced the flat surface of the disk to a spherical hollow. After grinding, it would be shaped to a parabaloid by removing about 1/200 of an inch more glass with rouge and water.
Hale died on February 21,1938, while grinding was under way. He must have assumed that the telescope would be finished early in the next decade. It wasn’t. War interrupted the optical shop’s work; the technicians turned the disk up on its edge and set it aside. Mirror, tube, yoke, and dome waited for victory in Europe and in the Pacific.
Grinding and polishing resumed in 1946 and continued—more and more slowly as the disk was lifted from its mounting for more and more careful tests—through most of 1947. Early on the morning of November 18, 1947, the semifinished disk—it had not yet been completely trued or mirrored—went down from Pasadena on a special truck escorted by fifteen motorcycle police, halted overnight in Escondido, caravaned inland to Palomar, inched up the mountain in fog and hail and then sleet, and arrived at its shining aluminum dome, a dome the size and configuration of the Roman Pantheon, at 11:02 A.M. on November 19.
By January 1948 the disk had been mounted and vacuum-coated with a layer of aluminum less than a thousand atoms thick. This was the true mirror —one ounce of highly reflective, untarnishable aluminum that all the weight of glass and tube and yoke only served to aim and support. On the night of January 26 Edwin Hubble rode the elevator to the prime-focus cage in the upper end of the tube and took the first of several photographs, a fifteen-minute exposure of NGC 2261, a mass of gas in our galaxy shaped like a comet and illuminated by a star. “The first photographs,” he wrote that summer in Scientific American, “confirmed the most optimistic predictions of [the telescope’s] designers. They recorded nebulae at least four times as faint, and hence twice as far away, as had ever been photographed before. This early result was better than we had any right to expect, because the photographs were made at a time when further work still had to be done to bring out the full power of the mirror. When the mirror is adjusted … its range should surpass all advance expectations.”
In its permanent mounting within the observatory, the disk was polished to bring it within one-millionth of an inch of a true parabaloid and then recoated. The work of two decades was done.
Using the 200-inch Hale Telescope, astronomers have seen farther into space than ever before. By discovering optical quasars and calculating their speed of recession, scientists may have looked upon the very edge of creation. No quasars have been found more than about fourteen billion light-years away. This “cutoff” of quasars may mean that quasars are the first galaxies that formed after the Big Bang. “That we can, in principle, see the edge of the world is amazing,” remarks the astronomer Allan Sandage. “That we may have done so already would be unique.” Sandage believes we have done so—with the Hale Telescope.
Since it first became operational, the 200-inch giant has undergone many changes. For one, using devices developed since the late forties, Palomar astronomers have extended the range of the spectrum in which the Hale Telescope is effective. Where once the 200-inch detected only wavelengths in the visible portion of the spectrum, it now detects wavelengths in the near infrared portion. With the move into infrared astronomy have come new opportunities to study star-forming regions and the center of our own galaxy.
Electronic image intensifiers and light detectors have made it a vastly more powerful telescope.
Probably the most important changes in the Hale Telescope have occurred in the methods of light detection. Photographic plates, used almost exclusively in the telescope’s first two decades, gave way to electronic image intensifiers in the late 1960s. More recently, even more sophisticated equipment has been installed: silicon chips called chargecoupled devices that convert patterns of light into sequences of electrical signals; imaging pulse-counting systems; and even television cameras. In 1978 the new devices were equipped to give digital readouts, greatly facilitating computer processing of the data. Housed in an observing cage built in 1965, these detectors are estimated to be ten to twenty times more efficient than those used in the early days of the 200-inch. It all adds up to a vastly more powerful telescope.
But undermining the power of the telescope is a development that has astronomers all over the world worried—light pollution. At night the glow from lighted cities simply overpowers the dim light from space. Already the Carnegie Institution has announced that due to light pollution from Los Angeles, the 100-inch telescope at Mount Wilson will be mothballed in 1985. At Palomar, sky glow from Los Angeles and San Diego has lowered the effectiveness of the 200-inch to that of a 140-inch telescope pointed at a dark sky. Unless something is done, warns Palomar’s director, Gerry Neugebauer, light pollution will “prevent us from doing fundamental, frontier astronomy.” Cal Tech astronomers won two battles for the Hale Telescope last year. First, after intense lobbying, they persuaded the San Diego city council to install low-pressure sodiumvapor bulbs in city streetlights. This type of lighting system, unlike others commonly used, emits light that is easily filtered out by astronomers. Then, in December, a law was passed forbidding nonessential lighting after 11:00 RM. in all unincorporated areas of San Diego County. But observatories around the world remain threatened by urban sky glow, and given the very stringent siting criteria for good “seeing,” it’s not easy to run away from cities to new locations.
Of course if they are above the sky, telescopes avoid sky glow altogether. Escaping light pollution is just one advantage of putting telescopes in orbit. Another is that space telescopes can detect types of electromagnetic radiation —ultraviolet light, X-ray emissions, and the far infrared—that cannot penetrate the earth’s atmosphere. The International Ultraviolet Explorer, launched in January 1978 by NASA, the European Space Agency, and the Science Research Council of the United Kingdom, studies hot objects in space, such as the interstellar gas at the edges of our own galaxy, that are bright in the ultraviolet section of the spectrum. NASA’s Einstein Observatory X-ray telescope, launched in November 1978, studies even hotter, explosive events—the violent death of a star, for example—in which X rays are emitted. The Infrared Astronomy Satellite (IRAS), a joint United States-United Kingdom-Netherlands project, scanned the far infrared sky for ten months in 1983, identifying 200,000 to 250,000 sources over 95 percent of the celestial sphere. When IRAS turned its eye on the star Vega, it detected what astronomers believe may be Vega’s planets, the first direct evidence that planets circle distant stars just as they do our own Sun.
Even in the range of wavelengths observable on the earth’s surface, astronomical observations made in space represent a tremendous improvement over those made with ground-based telescopes. The Space Telescope, a 94-inch conventional reflector due to be launched in 1986, will orbit above the atmospheric turbulence that blurs images of objects in space. As compared with the best ground-based telescope, its images will be ten times sharper and it will be able to detect celestial objects seven times more distant. Astronomers are lining up to use it. In June 1984 the agency responsible for administering the telescope, the Space Telescope Science Institute in Baltimore, reported that it had received three thousand proposals for experiments on the telescope—fifteen years’ worth.
Yet some of the most spectacular advances in astronomical observation have been made right here on the ground. Radio telescopes, which detect radio waves from space, were the first to detect quasars, pulsars, and the background radiation that supports the Big Bang theory. In New Mexico the Very Large Array radio telescope (actually twenty-seven radio antennas spread over miles) has been listening in on space since 1982.
And optical telescopes? Both the 200-inch and the Soviet Union’s 236-inch completed in 1978 (the only bigger telescope so far) solved the problem of all reflectors—how to keep the mirror surface accurate even as the temperature fluctuates and the telescope moves—with a thick and rigid mirror. To build larger reflectors that work on this principle would be too expensive, assuming it could even be done. But suppose astronomers took several mirrors—light, thin, and cheap—and put them on movable mounts controlled by a computer so as to make the many mirrors act as one? Then they would have an NTT, a New Technology Telescope. They would have what has been in existence since 1980 on an Arizona mountaintop—the Multiple Mirror Telescope designed to be the equivalent of a single 4.5-meter mirror (the Hale Telescope is a 5-meter). In the United States, three more NTTs are in the planning stages: a 7.6-meter, a 10-meter, and a 15-meter national NTT. Each of the six primary mirrors on the 15-meter NTT will be larger than McCauley’s 200-inch. To keep costs down, scientists are considering using an almost archaic material—Pyrex.
In light of these dramatic new developments, does Palomar have a dinosaur on its hands? Absolutely not, says Palomar’s assistant director Robert Brucato. Even in this era of space telescopes and NTTs, he explains, “there is no shortage of work for any ground-based telescope.” Too much remains to be done in astronomy. As it stands, “every major telescope in the world is oversubscribed by a factor of two to four.” Furthermore, because time on space telescopes and NTTs is so expensive, astronomers cannot afford to use these instruments as general-purpose telescopes. Space telescopes must be restricted to observations that can be made only above the atmosphere; use of the giant NTTs must be limited to study of the very faintest of celestial objects. That leaves a lot for conventional telescopes to do, and we can expect the 200-inch to long continue to be a leader in astronomical research.
The Hale Telescope achieved its initial fame because of its unprecedented size. No one had ever cast a glass disk of that dimension; no mirror that large had ever magnified the light from space. Two hundred inches. People were amazed. But where once they marveled at the Hale Telescope for the width of its aperture, now they marvel at it for the breadth of vision it has given. Unlike the great mirror, the progress in our understanding of the universe is immeasurable.