Starlight Detectives

by Alan Hirshfeld

  Draper’s silvered-glass telescope was, by every optical measure, among the best reflectors in the world. The instrument magnified stars a thousand-fold without significant loss of definition. Draper could clearly distinguish the components of the binary star Gamma Andromedae, although separated at the time by only one ten-thousandth of a degree. The elusive celestial object Debilissima, in the constellation of Lyra, had been resolved into a pair of stars through John Herschel’s eighteen-inch reflector and into a triplet through Lord Rosse’s six-foot-wide Leviathan; Draper reported it to be, in fact, a quintuple system. Jupiter’s moons, seen through most telescopes as identical dots of light, showed a range of diameters in the eyepiece of the fifteen-and-a-half-inch telescope. Jupiter’s pastel-tinged atmospheric belts shone crisply from equator to pole. (Draper made three fifteen-and-a-half-inch mirrors and adopted the best. Recent tests of the extant mirror at the Hastings Historical Society revealed several optical flaws; it is likely not the reflector he used.)

  Henry Draper’s fifteen-and-a-half-inch reflector telescope, as depicted in his monograph On the Construction of a Silvered Glass Telescope, 1864.

  Although the visual acuity of his telescope was unmistakable, Draper focused on the instrument’s original intent. “This is the first observatory that has been erected in America expressly for celestial photography,” he had announced to the British Association in 1860, before the instrument and its shelter were completed. He carefully considered his initial photographic subject: the Moon. The Moon’s nightly movement across the sky is slightly askew from that of the stars: its own orbital velocity, combined with the inclination of its path through space relative to Earth’s equator, alter the Moon’s apparent motion in the sky. As Earth turns, a telescope’s clock drive (if sufficiently accurate) will keep a star centered in the eyepiece or fixed on the photographic plate; however, the same clock mechanism will slowly lose track of the Moon. Frequent manual adjustments must be made to keep the Moon stationary within the telescope’s field of view. Draper envisioned a stable, vibration-free telescope mount that could be moved with no more than a finger’s pressure—and without the observer having to step away from the camera. Preferably, the eyepiece or camera would remain fixed while the telescope merely reoriented itself to follow the Moon.

  When completed, Draper’s telescope mount looked like no other. Conventionally mounted telescopes pivot around a central fulcrum, where their axes of rotation intersect; a heavy counterweight or a long tube must be introduced to equalize the leveraging force of the offset lens or mirror. Draper’s design eliminated any such awkward counterbalancing scheme. The telescope’s twelve-foot-long tube was suspended inside a cage-like wooden cradle, whose internal counterpoise levers allowed the instrument to point, perfectly balanced, in any direction. The mount was a thing of engineering beauty, its cat’s-cradle web of pulleys, levers, and cables a three-dimensional précis of Newtonian mechanics. The mirror itself lay at the bottom of the telescope tube, surrounded by a black velvet curtain and resting like a pasha on an air cushion. Inflation of the rubber sac was controlled by squeezing a bulb near the eyepiece. The cushion counteracted flexure of the mirror under its own weight. (Computer-activated piston-arrays perform the same function in today’s large reflector telescopes.)

  For lunar photography, Draper brought the telescope to complete rest before exposing, then shifted the camera’s plate holder to track the gradual movement of the Moon. “[I]nstead of injuring the photograph by the tremors produced in moving the whole heavy mass of a telescope weighing a ton or more, it only necessitates the driving of an arrangement weighing scarcely an ounce,” he reported. During the seconds of exposure, the photographic plate was driven automatically by a clepsydra (water clock), connected to the plate holder by a cord.

  By December 1863, Draper had recorded fifteen hundred wet-plate photographs of the Moon. (He ceased operations for five months in 1862 while serving in the Civil War as a Union Army surgeon at Harper’s Ferry.) Prints from several of these negatives were regarded as the best lunar pictures ever taken. George Bond proclaimed them magnificent. In reputation, if not surface detail, they would be surpassed only by Lewis Rutherfurd’s celebrated shot of the quarter Moon from March 6, 1865. (John Draper reports that both he and Henry were present in Rutherfurd’s observatory that night.)

  Joseph Henry, Secretary of the Smithsonian Institution, visited the Hastings observatory during the spring of 1863. Impressed with Draper’s trove of practical knowledge about telescope construction, he persuaded his host to write a guidebook on the subject for the general audience. Published the following year, On the Construction of a Silvered Glass Telescope, Fifteen and a Half Inches in Aperture, and Its Use in Celestial Photography relates Henry Draper’s exhaustive, three-year investigation into glass-grinding machines (fully seven alternative models were tried), astronomical mirrors, optical testing, telescope mounts, clock drives, even the workings of a photographic laboratory. The optical performance of a professional refractor telescope could be achieved more simply, at lower cost, and to larger scale, with a silvered-glass reflector telescope. Draper hoped that his fifty-five-page treatise might spark a wave of amateur involvement in celestial exploration. From the very first page, he allies himself with the nation’s autodidact tinkerers: “[I] can see no reason why silvered glass instruments should not come into general use among amateurs. The future hopes of Astronomy lie in the multitude of observers, and in the concentration of many minds.”

  Wet-collodion photograph of the Moon by Henry Draper, 1863.

  On the Construction of a Silvered Glass Telescope was the ultimate how-to guide to an arcane subject, set down with an excruciating specificity that gave the neophyte telescope maker a fighting chance at success. Here the wisdom of experience was neither confided among high-toned attendees at scientific meetings nor abstracted in hard-to-acquire journals; it was available to anyone with the grit and spare cash to take up a most unusual hobby. For the first time, amateur astronomers had been given the means to construct instruments equal to those of their professional counterparts. There was no need for exhortation; Draper’s own example provided newcomers with the very model of persistence and high standards in the making of an astronomical mirror. Draper’s disciples would come to view the least defect in their silvery surfaces to be as offensive as a blemish on the chin of the Mona Lisa.

  After laying out virtually every technological particular he could think of, Draper concludes his manual with a triumphal vision of the future of the glass reflector telescope: “My experience in the matter . . . assures me that not only can the four and six feet telescopes of [William Herschel and Lord Rosse] be equaled, but even excelled. It is merely an affair of expense and patience.” Others astronomers, including England’s photographic specialist Warren De La Rue, disagreed, claiming that the silver overlay allowed too much photo-activating violet light to pass through. (Draper’s subsequent photographs would put this assertion to rest.)

  Among those energized by Draper’s example was John Brashear, a Pittsburgh laborer who would become one of America’s foremost commercial makers of telescopes. Brashear writes in his autobiography:

  I had become acquainted with [Henry Draper] through correspondence . . . and he was never too busy to answer my letters in such a way as to help me solve the problems which were troubling me in the work I loved. His letters found me a toiler in the rolling mill, and together with his work on ‘The Construction of a Silvered-Glass Telescope and Its Use in Celestial Photography,’ they opened a new world, a new heaven to me . . . indeed, his book was of almost inestimable value to hundreds who were enabled to make their own instruments.

  With obvious pride in both son and country, John Draper informed Henry from overseas that copies of his monograph were being circulated eagerly among amateur astronomers in England.

  While contemplating his next astronomical venture, Draper published a review of American advances in spectroscopy, highlighting his father’s contrib
utions and foreshadowing his own entry into the field. In October 1867, he married Anna Palmer, daughter of New York City real-estate magnate Courtlandt Palmer. A gregarious, auburn-haired beauty, Anna insisted from the start that she play an active role in her husband’s scientific inquiries. The day after she and Henry were married, they headed downtown on what Anna would later call “our wedding trip”: an outing to purchase glass for Henry’s next project—a twenty-eight-inch reflector telescope.

  The couple moved into the Palmer family mansion on Madison Avenue, between 39th and 40th Streets, near what was then the northern edge of the city. Here they entertained the nation’s scientific and political elite. With Anna’s money, Draper equipped an astrophysical laboratory, first in a pair of rooms on the third floor of their home, later in a cavernous space over the stables, behind the house. The laboratory featured a roof-mounted heliostat (solar-tracking mirror), gas-powered electrical generators, incandescent lighting, cameras, spectroscopes, induction coils, chemical apparatus, darkroom, machine shop—in short, Draper’s home-based facility rivaled those of the era’s best scientific institutions.

  Anna Palmer Draper.

  Having scaled up the various grinding and polishing machines he had used to produce his fifteen-and-a-half-inch mirrors, Draper started work on the twenty-eight-inch reflector. There were no more treadmills to trudge; the new appliances were machine-powered. For the next year and a half, he would grind and polish the big mirror forty-one times before accepting its form. The fifteen-and-a-half-inch reflector, though it displayed stars and planets with remarkable clarity, was expressly designed to function as a giant lunar-tracking camera. The gaping aperture of the twenty-eight-inch telescope would be better suited to the deep-space realm, gathering up feeble rays from distant stars.

  Once again John Draper scouted out the landscape of astronomical research in England and advised his son in 1870, “From what I see here your proper course is to use your telescope first in getting some good lunar photographs. . . . [T]hat done, try your hand at the stellar spectra.” With its severe limit on time exposures, the wet-collodion process made it difficult enough to photograph a faint star with tolerable distinctness. John Draper proposed that Henry pass the starlight first through a prism, which diffuses the star’s pinpoint gleam into a pale, almost indiscernible spectrum—and to photograph that. At the time, no one had succeeded in recording the spectrum of any star other than the Sun. (Joseph Fraunhofer had eyeballed features in stellar spectra through a telescope as early as 1823; England’s William Huggins and William Allen Miller had tried—and failed—to photograph the spectrum of Sirius in 1863.) It’s no surprise that Henry Draper would have been guided by his father in his choice of research: John Draper was second—after Edmond Becquerel—to photograph the solar spectrum and was a pioneer in diffraction-grating spectroscopy. (A diffraction grating is a finely ruled plate that, like a prism, disperses light into a spectrum.) The two Drapers were, in a sense, sequential collaborators, the younger avidly carrying on the work of the elder.

  In fact, the idea for Henry Draper’s eventual pursuit of stellar spectroscopy had been percolating for a while. In 1860, word reached the United States from Germany that chemist Robert Bunsen and physicist Gustav Kirchhoff had performed a remote chemical analysis of the Sun’s atmosphere based on visual inspection of features in the solar spectrum. If the elemental composition of our nearest star could be deduced from its light, the same spectroscopic process might reveal the makeup of its distant peers. With Draper’s photographic expertise and his completion of the nation’s largest reflector telescope, the opportunity to obtain and compare a picture of a star’s spectrum with that of the Sun’s must have seemed irresistible.

  In August 1869, the twenty-eight-inch telescope was installed in a large dome adjacent to the existing observatory at Hastings. (Draper had offered to place the instrument on Great Hill in New York’s Central Park, near 105th Street, but nothing came of the proposal.) To avoid perching atop a ladder to reach the eyepiece, as in the common Newtonian-style reflector, Draper reconfigured the optics to the more convenient Cassegrain layout: light striking the primary mirror converges onto a small secondary mirror, which in turn reflects it through a central hole bored through the primary. This places the eyepiece—or camera—at the back of the telescope, where it is more easily accessed.

  Henry Draper’s observatory at Hastings-on-Hudson, New York.

  Two more years of testing, alignment, and adjustment followed, during which Draper hand-built—and discarded—six clock drives. In early 1871, having boasted about the still-gestating telescope before a dinner meeting of the Royal Astronomical Society, John Draper pressed his son to finish: “By a little pushing you might have it all in readiness by the time I get back early in April. You will have nothing to do at the University as I will take charge there and so might get to work without interruption or delay. Push the thing a little and you can do it.”

  On August 1, 1871, Draper began a series of test photographs of the Moon through the twenty-eight-inch. Whereas the fifteen-and-a-half-inch telescope projected an inch-wide lunar image into the camera, the beam of the new instrument was fully five inches across. Yet the photographs were dispiriting: even with all the care lavished on its construction, the twenty-eight-inch telescope proved inferior in definition to its smaller cousin. Draper promptly removed the mirror, took it back to the city, and returned it to the polisher. Finally, in June 1872, now with a seventh clock drive installed in its mount, the twenty-eight-inch telescope was complete. Saturn could be now viewed profitably at magnifications up to two thousand. And the latest driving clock, in the opinion of visiting astronomer Charles A. Young, “was as good as any in existence, keeping a star [centered] . . . for an hour at a time.”

  The lure of the observatory was powerful, and Draper’s energy seemed inexhaustible. On clear nights when the university was in session, he sometimes traveled the twenty-mile round-trip to Hastings at the close of the workday. During the summers, when he and Anna were at their country home in Dobbs Ferry, two miles from Hastings, the two of them would head over to the observatory together. “So great was [Anna Draper’s] interest,” notes Harvard astronomer Annie Cannon, “that he never went to the observatory without her, and in the days of the wet plate, she herself always coated the glass with the collodion. Mrs. Draper told how sometimes after they had been to the observatory and returned to Dobbs Ferry on account of clouds, they would find the sky clearing, and would drive back again two miles to the observatory and recommence work.” (In 1878, Anna accompanied her husband—and Thomas Edison—on an expedition to Rawlins, Wyoming, to photograph a solar eclipse. During the minutes of totality, Anna called out the seconds from a clock while sitting in a tent, so as not to be distracted by the heavenly spectacle.)

  On August 1, 1872, Draper pointed the twenty-eight-inch telescope, equipped with a camera, toward the bright star Vega. He inserted a small quartz prism into the light path and took an exposure. The recorded spectrum was a hazy slash of light, a mere half inch long and one-thirty-second-inch wide. Microscopic examination revealed the presence of four dark gaps, like those Robert Bunsen and Gustav Kirchhoff had found in the solar spectrum—gaps they had identified with chemical elements found on Earth. On the glass plate was objective, hold-in-your-hand confirmation of what visual spectrum studies had found: atoms in the atmospheres of remote stars are no different than those that constitute the Sun or our own bodies. Frederick Barnard, president of Columbia University, characterized the achievement as “probably the most difficult and costly experiment in celestial chemistry ever made.” (The twenty-eight-inch mirror survives in the History of Science Collection at Harvard University.)

  Now at the pinnacle of American astronomy, Draper was drafted to coordinate the country’s effort to photograph the 1874 transit of Venus across the solar disk. Accurate timing measurements of this twice-a-century event were critical to a geometric determination of the Sun’s distance. Photography prom
ised to improve the distance estimate gleaned from visual studies of previous transits in 1761 and 1769. Historian Agnes Clerke summed up astronomers’ expectations:

  Observations made by its means would have the advantages of impartiality, multitude, and permanence. Peculiarities of vision and bias of judgment would be eliminated; the slow progress of the phenomenon would permit an indefinite number of pictures to be taken, their epochs fixed to a fraction of a second; while subsequent leisurely comparison and measurement could hardly fail, it was thought, to educe approximate truth from the mass of accumulated evidence.

  Scientific teams were dispatched from the United States, Germany, Britain, and France, each using different equipment and methods. The Americans adopted the wet-collodion plate, the British and Germans the dry-collodion plate, and the French (unsurprisingly) the daguerreotype. The results were unrelievedly dismal. Given the indistinctness of the Sun’s limb, it proved impossible to establish the precise time of contact between the solar and planetary disks. In the end, the Sun’s distance was recomputed based on visual observations; the photographs were ignored.

  Afterward, an astronomical congress of fourteen nations summarily rejected the use of photography for the Venus transit of 1882. In a statement to the St. Petersburg Academy of Sciences in 1886, Pulkova Observatory director Otto W. Struve summed up the feelings of classical astronomers: “God forbid that astronomy should be carried away by a fascination with novelty.” In the realm of the professional observer, the human eye still reigned supreme.

  Following the Venus transit debacle, Henry Draper ramped up his spectroscopic examinations of chemical elements in the laboratory and in solar light. He startled the astronomical community by announcing the discovery of oxygen in the Sun’s atmosphere, not by its expected dark-line spectral signature, but as an array of bright lines. His explanation posed an alternative theory of solar chemistry, which was met with wide condemnation overseas. (The bright lines proved to be spurious.) In the Hastings observatory, Draper compared the performance of his twenty-eight-inch reflector to a state-of-the-art, twelve-inch visual refractor he had purchased from Alvan Clark and Sons, who had succeeded Henry Fitz as the nation’s premier maker of lens-based telescopes. Mounted side-by-side, the two telescopes proved virtually equivalent in their photographic capabilities: the refractor’s rigid, unitized construction offset the increased light-grasp of its bigger, but more vibration-prone, rival.