Starlight Detectives

by Alan Hirshfeld

  Wintertime observing was especially arduous, with subfreezing temperatures and towering snowdrifts between the living quarters and observatory. Taking up his shift at 1:30 a.m., staff astronomer Storrs Barrett donned “two sets of underwear and one pair of pants, one shirt, two coats, two pairs of socks, a collar (no tie), a neckscarf, a sealskin cap, a fur overcoat, a pair of shoes, a pair of arctics and two pairs of mittens”—only to have clouds roll in by the time he trudged over to the telescope. Edward Barnard was described by visiting Irish astronomer Robert Ball as a “moving cylinder of fur coats . . . running as briskly up and down as if he were playing football. Indeed, he had to be well clad, for that night [December 17, 1901] he worked from five in the evening till six in the morning.”

  Despite the pressing burdens of the directorship, constant tug of family, and recurrent insomnia and headaches, George Hale carried on his research. In the solar arena, the forty-inch was fixed up with a jumbo, Yerkes-built spectroheliograph that provided new insights into the stratification and movement of gas in the Sun’s upper atmosphere. Hale also delved into the extrasolar realm, obtaining the first-ever photographs of the spectra of faint red stars, a considerable achievement given the relative insensitivity of dry plates to these long wavelengths. By 1904, he had received the Janssen Medal from the Paris Academy of Sciences, the Rumford Prize from the American Academy of Arts & Sciences, the Henry Draper Medal from the National Academy of Sciences, and the Gold Medal of the Royal Astronomical Society.

  Astronomer George Van Biesbroeck inspecting the lens of the Yerkes forty-inch refractor in 1928.

  Although secure in his scientific reputation, if not in his own physical and emotional health, George Hale could not quell the inner fire that drove him to greater accomplishment. He knew all along that the Yerkes refractor was the child of opportunity: The glass disks became available just when he needed a successor to his paltry, twelve-inch home telescope. In Hale’s dawning estimation, the focal length of the forty-inch was too short for the brand of solar research he contemplated, and its largest-in-the-world aperture too small to capture the faint spectroscopic signatures of stars and nebulae at the fringe of the known universe. As grand as the Yerkes refractor had once seemed, it had been overswept by the observational challenges facing twentieth-century astrophysicists.

  Even as the foundation was being laid for the Yerkes Observatory, Hale conceived plans for a much larger instrument, this time a reflector. A mirror-based telescope reflects light from its silvery objective element; the light never enters the glass substrate and thus is relatively undiminished when it reaches the eye or the camera. The reflected rays converge to a common focus regardless of wavelength, mitigating the chromatic aberration that afflicts refractors. (As early astrophysics advocate Agnes Clerke put it, “None of the beams they collect are thrown away in colour-fringes, obnoxious in themselves and a waste of the chief object of the astrophysicist’s greed—light.”) And unlike an objective lens, which is supported solely along its circumference, an astronomical mirror nestles securely in a cradle, braced against gravity’s distorting influence. To a degree, the only limit on the size of a reflector telescope lies in the mechanics of the making—and in the aspirations of the maker.

  In 1894, fueled by his son’s latest vision—a telescope with twice the light-collecting area of the Yerkes refractor—William Hale ordered a sixty-inch mirror blank from the Saint-Gobain foundry in France. The eight-inch-thick, one-ton disk arrived two years later. But after grinding and polishing, it languished in the Yerkes basement while Hale sought funds to mount it. More than a decade later, the great mirror rose from its crypt in Wisconsin and, in 1908, found itself facing skyward on an airy mountaintop in California. The era of modern megareflectors was about to begin.

  Chapter 24

  THE UNIVERSE IN THE MIRROR

  It has often been asserted by prominent writers on the subject that a large reflector is necessarily inferior to a large refractor in many vital points, such as permanence of optical qualities, freedom from injurious flexure of optical parts, permanence of adjustments or collimation of optical parts, rigidity and stability of the mounting, and convenience in use. . . . Nothing could be further from the truth.

  —George W. Ritchey, “The Two-Foot Reflecting Telescope of the Yerkes Observatory,” 1901

  AS THE NINETEENTH CENTURY DREW TO A CLOSE, the near-universe was coming into focus, yielding to the allied technologies of the New Astronomy: the camera and the spectrograph. Practitioners of the maturing science of observational astrophysics sought to redefine astronomical research by demonstrating to their visual counterparts the potential of these new devices for cosmic analysis and discovery. Already they had teased out the chemical recipes of the Sun and several bright stars and nebulae, accumulated essential data about stellar motions through space, and imaged celestial structures that had eluded sharp-eyed observers and their exquisite telescopes. But what of the deeper reaches of space beyond the ragged borders of the Milky Way? Here, in the realm of the mysterious spiral nebulae, the camera and the spectrograph faltered because of the sheer dimness of the cosmic glow reaching Earth. To render these technologies effective with such miserly bits of light required a radical transformation—a scaling up—of that most fundamental astronomical instrument: the telescope.

  The premier telescopes of the late 1800s, both in the United States and abroad, were refractors. A centuries-long evolution had made these instruments synonymous with optical and mechanical refinement. Yet their upsizing raised concerns about the deleterious effects of increased aperture: A wider lens is necessarily thicker and heavier; it more substantially absorbs incident photons and might sag under its own weight. By contrast, an ideal reflector telescope transmits light relatively undiminished and is free of the refractor’s chromatic aberration. With proper support from behind, even the bulkiest mirror might retain its critical concave form as the telescope swings from one position to the next.

  As in celestial photography and spectroscopy, the development of reflector telescopes during the nineteenth century was driven in large measure by amateur astronomers, who took the conceptual and methodological risks their professional counterparts would not. Only toward the end of that period, once cornered by the limits of their existing instruments, did institutional astronomers fully comprehend the scientific imperative of larger aperture. Their introduction of sizable, yet refined, instruments paralleled an irrevocable change in the mode of astronomical research: from the lone observer operating a modest-sized, out-of-pocket telescope to the salaried, institutional team wrestling with a million-dollar machine on a remote mountaintop.

  Isaac Newton is credited with the first functional reflector telescope, having developed in 1668 the hallmark design that now bears his name. (A small plane mirror deflects the gathered light through an eyepiece at the side of the tube.) His diminutive instrument—just over six inches long and one-and-a-third inches in aperture—magnified about thirty-five times. The mirror was formed of bell metal, or speculum metal, an exceedingly brittle alloy of copper and tin, to which Newton added a dash of arsenic for increased whiteness; still, it reflected only 16 percent of incident light and tarnished easily. Newton’s telescope reportedly imaged the Moon about as distinctly as a typical, small-aperture refractor of his day.

  In 1672, Guillaume Cassegrain in France produced an instrument with a secondary mirror that reflected light back through a hole in the center of the primary mirror, instead of out the side of the tube as in a Newtonian. (Englishman James Gregory had experimented unsuccessfully with a variant of this design several years before Newton constructed his telescope.) In the 1700s, English telescope makers John Hadley and James Short each conducted a brisk business selling metal-mirror telescopes with apertures up to eighteen inches—and wildly inflated magnification claims.

  A one-man crusade for greater aperture began in 1781 with the discovery of the planet Uranus by German émigré William Herschel, a conductor and music teacher in B
ath, England. Herschel sought to develop large telescopes with which he could survey the night sky for unseen planets, stars, and nebulae. Although high-precision refractors were ascendant at the time, he found reflectors to be more easily and economically scalable in size.

  Disks of ever-larger diameter were cast from molten metal poured into molds of loam, charcoal, or compressed horse dung; each disk was then laboriously abraded with pulverized emery and rouge until its cross section had a parabolic shape. (Only a parabolic mirror converges all incoming light to a single focus.) Between 1773 and 1795, Herschel produced some 430 telescope mirrors, most sold for profit, the best kept for his own use.

  Herschel’s involvement in astronomy was all-consuming. His sister Caroline, who became an astronomer in her own right, complained in one diary entry that almost every room of the house had been turned into a workshop. She read to her brother and placed morsels of food in his mouth while he polished mirrors. On most clear nights, Caroline served as his amanuensis, sitting next to an open, second-story window while William called out his observations from atop a ladder beside the telescope.

  In 1783, Herschel completed what would become his bread-and-butter research instrument for the next three decades: a reflector, nineteen inches in aperture and twenty feet long, suspended within a rotatable wooden frame. Herschel’s next project, completed in 1789, was unprecedented: an instrument with a mirror forty-eight inches in diameter. American writer Oliver Wendell Holmes, father of the U.S. Supreme Court justice, recalled his first glimpse of Herschel’s telescope: “It was a mighty bewilderment of slanted masts, spars and ladders and ropes from the midst of which a vast tube . . . lifted its mighty muzzle defiantly towards the sky.”

  William Herschel, depicted in an 1807 engraving of a painting by J. Russell.

  Despite its yawning aperture, the forty-eight-inch never lived up to its potential. The massive metal mirror warped under its own weight and took hours to settle into equilibrium with the cool night air. On all but a few occasions, Herschel sacrificed size for the utility of his mainstay nineteen-inch telescope. Either way, he was in the enviable position of being able to see celestial objects invisible to the rest of the world’s astronomers. The potential of large-aperture reflectors was underscored by Herschel’s prodigious record of astronomical accomplishments, including descriptions of thousands of deep-sky objects, discovery of new moons around Saturn and Uranus, and from tedious star counts in different parts of the sky, the determination that our Milky Way system is shaped like a disk. Nevertheless, the troublesome forty-eight-inch telescope shouted a warning to any future enthusiast who sought to surpass it: this was a path reserved for those with single-minded devotion—and deep pockets.

  William Herschel’s forty-eight-inch-aperture reflector telescope, completed in 1789 at Slough, England. Commonly known as the forty-foot telescope for its focal length.

  Lord Rosse’s seventy-two-inch Leviathan reflector, constructed in Ireland during the 1840s, was a worthy successor to Herschel’s optical giants. But its utility was compromised by its restrictive support walls, which hemmed in its view of the night sky. The engineering challenge remained: how to make a massive telescope’s mechanism of movement more conducive to the demands of astronomical observation. (The Leviathan was restored in the 1990s with a modern, aluminum-coated glass mirror.)

  In 1845, inspired by Herschel’s and Rosse’s examples, William Lassell, a wealthy brewer in Liverpool, built the world’s largest free-swinging, equatorially mounted telescope, incorporating a twenty-four-inch speculum-metal mirror. (An equatorial mount tracks the diurnal movement of celestial objects by rotating the telescope around one axis instead of two.) The 370-pound mirror, like that of Rosse’s Leviathan, had been shaped by a steam-driven device that mimicked the epicyclic grinding-strokes of the artisan. Lassell incorporated a multipoint mirror support system, brainchild of noted Dublin optician Thomas Grubb, which prevented distortion of the reflector as its orientation changed. This model, adapted in various forms, promoted the growth of astronomical mirrors during the early twentieth century.

  From his aptly named private observatory, Starfield, just outside Liverpool, Lassell discovered Neptune’s satellite Triton in 1846, and two years later, the satellite Hyperion circling Saturn. (William Bond observed Hyperion at the same time through Harvard’s fifteen-inch refractor.) At every turn, Lassell was hamstrung by Starfield’s routinely poor observing conditions. His friend, the famed double-star observer William Rutter Dawes, dubbed the troubled site “Cloudfield.” Lassell’s frustration with the English climate permeates his scientific correspondence and foretells the sentiments of astronomers like George Hale and James Keeler in the 1890s: “I was never more struck with the conviction how necessary a pure tranquil sky is to the just performance of a very large telescope.”

  In 1852, Lassell took the dramatic step of disassembling his twenty-four-inch telescope and shipping it—and himself—to the more astronomically friendly climate of Malta. Over the next year and a half, he delighted in the clear night sky (and British naval protection) of his Mediterranean outpost. “The nights are as remarkable for their tranquility as transparency,” he enthused to fellow astronomer Warren De La Rue in November 1852, “and I have not encountered one of those nights so frequent at Starfield, on which I opened the Observatory to close it in disgust.” Lassell focused his studies on planets and their satellites, often taking advantage of the clear, steady air to ramp up his telescopic magnification to more than a thousand.

  Lassell returned to England in 1854, relocated to a new estate farther from Liverpool’s smudgy skies, and commenced work on his next project: a forty-eight-inch reflector. The new telescope saw “first light” at Liverpool in 1859 and under Maltese skies in 1861. Twin metal mirrors were cast, each weighing a ton, one placed in service while the other was repolished. This instrument, too, featured an equatorial mount, its iron-lattice tube jutting skyward between beefy conical tines. Like a star-struck Juliet, Lassell peered into his lofty eyepiece from the balcony of a rail-borne tower. On the ground below, an assistant turned a hand-crank to slew the telescope’s eight-ton bulk in a manner sufficient for visual observing.

  Ungainly as its mechanics might have been, the instrument’s extraordinary light-grasp was evident from the start: Lassell likened the bright star Sirius to an incandescent diamond and the Orion Nebula’s hazy swirls to luminous masses of wool. He tracked planetary satellites and cataloged six hundred previously unseen nebulae before leaving Malta for England in 1865, where he resumed observations with his more wieldy twenty-four-inch telescope. Dismantled, the forty-eight-inch lay in storage until shortly before Lassell’s death in 1880, when it was sold for scrap. “I was not without a pang or two,” he informed the Royal Astronomical Society, “on hearing the heavy blows of the sledge-hammers necessary to overcome the firmness of the alloy.”

  William Lassell’s forty-eight-inch, equatorial-mount reflector telescope at Malta.

  William Lassell was a stalwart observer who gleaned factual information about the cosmos, duly reported it, then let others speculate as to its meaning. He possessed neither the broad scientific ambitions nor the research résumé of his antecedent William Herschel. Yet his forty-eight-inch instrument surpassed Herschel’s, Rosse’s, and every other sizable reflector up to that time in functionality and mechanical sophistication. Although primitive compared to its sleek refractor cousins, it was the forerunner of a coming generation of large-aperture, fully movable reflector telescopes. Except for one glaring deficiency: its weighty metal mirror.

  The era of jumbo metal mirrors came to a crashing end in the 1870s, following a pledge by the citizens of gold-enriched Victoria, Australia, to build a Southern Hemisphere telescope of supreme optical power. The planning committee politely rejected William Lassell’s offer of his twenty-four-inch reflector (too small) and subsequently his forty-eight-inch (too cumbersome) and also a proposal from Joseph Fraunhofer’s successor, George Merz, in Munich, for a
thirty-inch precision refractor (too costly—and, well, not British enough). Instead the committee accepted a bid from Thomas Grubb’s firm in Dublin for a more modestly priced, forty-eight-inch metal-mirror instrument of the Cassegrain design. This optical arrangement, invented in 1672, employs a convex secondary reflector that channels light through a central hole in the concave primary reflector, and from there through an eyepiece. As a result, the elevated viewing position of a Newtonian telescope drops conveniently to the base of a Cassegrain’s tube.

  The Great Melbourne Telescope, as it was known, was put into service in 1869 and became one of the biggest flops in the annals of instrumental astronomy. The mounting was unstable, the tube vibrated in the wind (the telescope was unsheltered in use), and the tarnish-prone metal mirror required frequent repolishing, a delicate process for which no one in Australia had been trained. Nor had the civil-service astronomers the experience or commitment to properly operate the eight-ton opti-mechanical beast.

  The telescope’s primary mission was to search for physical changes in Southern Hemisphere nebulae cataloged from South Africa during the 1830s by William Herschel’s son John. The project was doomed from the beginning, given the inherent differences in human perception and artistry and in the telescopes themselves. The effective focal length of the Melbourne telescope was fully forty-one times its aperture, endowing it with a Lilliputian field of view that hampered the visual study of nebulae. (A shorter focal length would have increased the diameter and weight of the secondary mirror at the tube’s mouth.)

  The Great Melbourne Telescope in an imagined setting.

  In terms of popular sentiment, the death blow came in the autumn of 1877. Despite repeated attempts, the Melbourne observers could not see the recently discovered satellites of Mars, even though the pair were clearly visible in telescopes in the United States and England. (The imperfectly shaped mirror bled light from the Martian disk into an obscuring halo.) Even in the realm of deep-sky observation, where aperture reigns, the Great Melbourne Telescope was about to be eclipsed. In 1883, Andrew Common displayed his dry-plate photographs of the Orion Nebula, to which the Royal Astronomical Society added its imprimatur by awarding him its Gold Medal. Astrophotography was the coming wave, and the Melbourne reflector was ill adapted to the new technology. (A consortium of government and civic organizations is restoring the Great Melbourne Telescope for public viewing—this time, with a modern glass mirror.)