Starlight Detectives

by Alan Hirshfeld

  The most an astronomer can do to garner a better celestial picture is to mitigate the factors that tend to ruin a photograph. Of absolute necessity is a mechanical clock drive that turns the telescope to precisely counteract the movement caused by Earth’s rotation, movement that would otherwise smear out the image. This was a rare accessory for telescopes of the 1840s when celestial photography was inaugurated. And even if so equipped, the clock drive’s accuracy almost always fell short of the stringent demands of a long time-exposure. Witness the Harvard refractor’s original Munich drive, which was sufficient for visual astronomy, yet wholly inadequate for photographic astronomy. To achieve their breakthrough images, Whipple and Bond had to treat the drive like an uncooperative child: open the camera shutter while the drive is behaving, close it when it’s about to have a fit. They ultimately abandoned their photographic efforts until a better drive—and a better means of photography—could be put to use.

  From the very start of the photographic era, a number of practitioners tried to develop a process that would combine the clarity of the daguerreotype with the negative-to-positive print capability of the calotype. The most promising pathway appeared to be replacement of the calotype’s paper negative with a glass negative, whose transparency would make for finer-grained, daguerreotype-like prints. The problem was to develop a clear, nonreactive substance that would permit a dissolved photosensitive chemical to adhere to glass. (An early desperate attempt involved the slimy exudate of snails.) In 1839, noted English astronomer John Herschel exposed a glass plate on which multiple applications of a silver chloride solution had been allowed to dry; although successful, he judged the lengthy preparation phase to be impractical.

  In the late 1840s, John Adams Whipple and Claude Niépce de Saint-Victor, son of Nicephore Niépce’s cousin, independently produced glass negatives using albumen—egg whites—as the photographic substrate. The positive prints derived from these glass negatives were comparable in quality to daguerreotypes. But the long exposure times prevented their application to astronomy.

  The breakthrough came from an unlikely quarter: a reserved, pale-faced sculptor named Frederick Scott Archer, who had taken up calotype photography in 1847. Dissatisfied with the quality of prints produced from the calotype’s paper negatives, Archer tried albumenized glass. This, too, proved unsatisfactory; the albumen coating was hard to spread uniformly and extremely delicate when dry. Archer read of a recently discovered transparent, viscous substance called collodion, from the Greek “to adhere.” Collodion was developed independently in 1847 by Louis Menard of France, and John Parker Maynard, a medical student in Boston, who published the method of production in the American Journal of Medical Science. The substance, when dried, was useful as a flexible, adhesive cover for wounds.

  Collodion is made by dissolving guncotton—cotton treated with nitric acid—in a mixture of ether and alcohol. (Guncotton, a gunpowder alterative, was discovered accidentally when German–Swiss chemist Christian Friedrich Schönbein, working in his kitchen in 1846, wiped up some spilled nitric acid with his wife’s cotton apron; he set the apron to dry near the stove, where it abruptly ignited.) To the collodion, Archer added potassium iodide, then poured the syrupy liquid onto a glass plate and let it dry. When ready to take a picture, he immersed the prepared plate into a silver nitrate bath for a few seconds, the iodine and silver atoms combining to produce light-sensitive silver iodide within the collodion. The plate was exposed while wet. Once dry, it lost its photosensitivity and left a silver nitrate residue on the collodion’s surface; hence, the hard limit on the duration of time exposures. The plate, still wet, was developed in pyrogallic acid (which precipitates out the silver), then was fixed with hypo.

  The wet-collodion photographer had to be near a fully equipped darkroom—or bring one along—as exposure and development took place in rapid sequence. Civil War photographer Mathew Brady toted his equipment in a horse-drawn wagon train. In October 1863, a trio of enthusiasts from Philadelphia ventured into the Poconos for a week, bearing their photographic burden on their backs: cameras, tripods, glass plates, flannel tent, curtains, trays, bottles, rubber-sheet sink and drain hose, stool, plus a carryall box of chemicals. “We will not attempt to state all that the box contained,” they reported to the Philadelphia Photographer, “it would be easier to enumerate what it did not contain.” Or as photography historian Robert Taft puts it, “[T]o be an amateur in wet plate days required fondness for the art verging on fanaticism.”

  Mobile darkrooms and wet-plate requirements notwithstanding, Archer published a detailed report on his use of collodion in the March 1851 issue of The Chemist. His instruction manual appeared in England, France, and the United States in 1852. By year’s end, a core of professionals had judged Archer’s process to be straightforward and relatively inexpensive, yielding daguerreotype-like detail and calotype-like reproducibility. Collodion plates treated with cadmium bromide (to produce photosensitive silver bromide) were some ten times as fast as daguerreotype plates. The switchover among studio photographers from the daguerreotype to Archer’s method accelerated through 1853, as wholesale collodion manufacturers appeared. At the 1856 annual exhibit of the Photographic Society of London, only three of the six hundred displayed images were daguerreotypes.

  John Whipple was one of the early adopters of Archer’s process, replacing his daguerreotypes and albumenized glass plates with collodion. At the 1853 World’s Fair in New York, Whipple received the highest award in photography for his collodion-derived “crystalotype” prints on paper. Samples of his prints were bound that year into issues of the Photographic Art-Journal. He became a national authority on collodion photography, teaching it to all comers—for a fifty-dollar fee. With his newfound experience, it was only a matter of time before his attentions were drawn back to the celestial realm.

  Archer chose not to patent the collodion process, even though he was first to test and publish its practical aspects. After several years as a landscape and architectural photographer, he died in poverty in 1857 at age forty-four. His wife died a year later. Following Archer’s death, the satirical magazine Punch commented, “The inventor of Collodion died, leaving his invention, unpatented, to enrich thousands, and his family unapportioned, to the battle of life.” In recognition of Archer’s contribution to art and the economy, the government awarded his three orphaned children an annual pension of fifty-five pounds.

  As a trained chemist, Warren De La Rue would have understood the particulars of the daguerreotype process. And, charmed as he was with Harvard’s lunar daguerreotype, he would have grasped the defects of the method when applied to astronomy. His telescope, despite its optical refinement, lacked a clock drive; to track the movement of celestial objects across the sky, it had to be repeatedly nudged by hand. A satisfactory time-exposure photograph would be a challenge with such an instrument, if not a virtual guarantee of failure. At least with the excruciatingly slow daguerreotype technology. De La Rue does not say whether he saw Frederick Scott Archer’s wet-collodion photographs at the Crystal Palace in 1851. In any case, he soon learned of the new technique and immersed himself in its astronomical possibilities.

  From the start, De La Rue was an ardent advocate for the use of reflector telescopes in celestial photography. Not only is a reflector telescope cheaper to make than a refractor telescope of the same aperture, it focuses all colors of light onto the same plane. That the reflector’s visual focus is coincident with its chemical focus obviates the need to guess the proper placement of a photographic plate; wherever the image appears in sharpest focus on a ground glass screen, that’s where the photographic plate should go. It was this aspect of the reflector telescope to which De La Rue attributed much of his eventual success in astronomical photography.

  De La Rue’s original telescope was a thirteen-inch Newtonian style reflector: its solid-metal objective mirror sat at the base of a ten-foot-long wooden tube and converged incident light back up the tube onto an angled secondary mir
ror, or diagonal, which in turn deflected the light straight out the side of the tube. Here the image was either inspected through an eyepiece or introduced to a camera. The telescope was equatorially mounted—tilted such that one axis is parallel to the axis of Earth’s rotation—which allowed it to track stars with a uniform rotation of a single axis. (An alt-azimuth telescope, mounted like a cannon, requires two simultaneous rotations to track celestial objects.)

  Warren De La Rue’s upgraded thirteen-inch reflector telescope, as depicted in the British Journal of Photography, 1868.

  De La Rue’s initial target was the Moon, in part because it was the brightest of nighttime celestial objects, but also because he had a benchmark against which he—and his astronomer colleagues—could gauge his success: the Whipple-Bond lunar daguerreotypes. A major motivation for photographic surveillance of the Moon, beyond humanity’s deeply rooted imperative to make maps, was to check for changes in the lunar surface. It was not known at the time whether the Moon had geologically sputtered out or was active, like Earth. Were there intermittent volcanoes, shifting faults, or new impact craters that might show themselves in direct comparison of photographs taken at different times?

  Precise hand-guiding of De La Rue’s driverless telescope was essential if photographic images of the Moon were to come out sharp. At first, De La Rue used a small guide refractor, complete with crosshairs, like a rifle sight to keep the telescope centered on a target lunar crater. When that proved insufficient, he devised a special plate holder with its own eyepiece and guided by looking directly through the back of the glass photographic plate itself (something that would have been impossible with an opaque daguerreotype plate). However, either of these schemes required the concerted efforts of two people: one to remove the black merino wool cover from the mouth of the telescope—the camera itself had no shutter—and the other to guide the telescope. De La Rue laments, “[I]t was not easy to find a friend always disposed to wait up for hours, night after night, probably without obtaining any result.”

  In 1852, De La Rue enlisted the steady aid of his collodion supplier, William Henry Thomthwaite, who ran an optical shop in London. By year’s end, he had encountered “numberless impediments sufficient to damp the ardor of the most enthusiastic.” But he also secured a number of high-quality collodion images of the Moon that he exhibited to the Royal Astronomical Society. Exposure times were typically less than thirty seconds, compared to some twenty minutes or more for lunar daguerreotypes. Even with the improvement in speed, De La Rue had already exhausted the capability of his telescope, as Whipple and the Bonds had done with theirs. There was nothing to do now but join his compatriots across the Atlantic, waiting until their respective telescopes were fitted with precise clock drives.

  Chapter 6

  THE EVANGELISTS

  The wonderful exactness of the photographic record may perhaps best be characterized by saying that it has revealed the deficiencies of all our other astronomical apparatus—object-glasses and prisms, clocks, even the observer himself.

  —Herbert Hall Turner, “Some Reflections Suggested by the Application of Photography to Astronomical Research,” 1905

  IN MARCH 1857, JOHN WHIPPLE once again marched up Summer House Hill in Cambridge, camera equipment in tow, to Harvard College Observatory. Much had changed during the five years since he taken the final daguerreotype of the crescent Moon through the Great Refractor. Whipple had won international acclaim for his photographic innovations, notably the improvement of paper printing from glass negatives. He was now partnered with James Wallace Black, a former house painter turned photographer who shared Whipple’s zeal for invention. Their shop on Washington Street in Boston was a mecca for students eager to learn the latest techniques.

  William Cranch Bond, ailing but still the observatory’s director, continued his work in precision astronomy and his relentless promotion of Harvard’s astronomy facility. Money was always needed for equipment, personnel, and publication of research reports. As his health declined, the elder Bond had increasingly handed off administrative and observing duties to his son George, who had risen to prominence in his own right.

  Astronomy at Harvard in the 1850s was a family affair. George Bond had married the daughter of Harvard’s librarian in 1853, and now four years later, was the father of two girls. To elder daughter Elizabeth, the observatory grounds were something of a nature park—a meadow-dotted, bird-filled island a million miles from the city. On one side of the director’s residence stood the majestic observatory dome, on the other a homely barn with a cow, horses, pigs, and chickens. The family gathered on summer evenings around a large stone behind the observatory to watch the setting Sun. Of her doting father, Elizabeth Bond writes, “Most patiently he taught us the names and the positions of many of the stars and the constellations, and we were always shown anything of special interest in the skies. When a mere baby, not more than three years old, I can remember being held out of an open window in my father’s arms . . . to see an eclipse of the moon.”

  During this period, George Bond worked with his father to complete and publish the first in a series of catalogs listing precise sky coordinates of stars. He himself carried out a long-term visual study of Saturn’s rings, which convinced him—erroneously, as it happens—that they were fluid in makeup, not solid as widely believed. (In 1857, Scottish mathematician James Clerk Maxwell proved through a theoretical analysis that the rings must consist of numerous small particles. Astronomer Royal George Biddell Airy described Maxwell’s paper as “one of the most remarkable applications of mathematics to physics that I have ever seen.”)

  There were two circumstances that brought John Whipple back to Harvard in 1857. The first was the long-awaited replacement of the Great Refractor’s vexatious Munich clock drive. No more lurches and lags from its out-of-round friction wheel, afflicting the time exposures he and the Bonds had taken during their earlier foray into celestial photography. Now the motion of the telescope was governed by a rigidly precise mechanism, hand-assembled by the Massachusetts firm of Alvan Clark and Sons from William Bond’s specifications. The second development that resurrected photography at Harvard arose in Whipple’s own realm: the displacement of the daguerreotype by the wet-collodion plate.

  The advantage of the new technology was evident from the start. “On a fine night,” George Bond told amateur astronomer William Mitchell in 1857, “the amount of work which can be accomplished, with entire exemption from the trouble, vexation and fatigue that seldom fail to attend upon ordinary observations, is astonishing.”

  Obligatory pictures were taken of the Moon, mostly for public consumption and promotional purposes. Indeed, they were a significant improvement, both in aesthetic and practical terms, over the lunar daguerreotypes of the early 1850s: exposure times were considerably reduced, and prints could be generated from the wet-collodion negatives. Yet to George Bond, the Moon was only a stepping stone to a more ambitious agenda: to identify and explore quantitative applications of celestial photography. Bond’s belief in the scientific potential of the technology required a rapid accumulation of high-quality photographic images that could be subjected to critical scrutiny. Whipple and Black now joined Bond on a regular basis at the observatory, sometimes taking dozens of pictures in the course of several hours. William Bond confided in July 1857 to astronomer Maria Mitchell that “George is, and has been for months, almost hidden from the ken of us mortals in the clouds of Photography.”

  One of the most promising technical applications of telescopic photography was measurement of the separation and orientation of double stars: pairs of stars that appear close together in the sky. Their adjacency might be spurious—a chance alignment of a star with a faraway counterpart, or it might stem from true proximity to each other—a binary star system bound together by gravity. Compiling a decades-long record of the changing separation and orientation of a double star often reveals whether the double star is an apparent pair—the stars maintain their separation or
gradually drift apart—or whether they are a real pair—the stars trace out a mutual orbit.

  The first double-star target was the Mizar-Alcor system, a pair of moderately bright stars in the handle of the Big Dipper. Mizar itself has a fainter companion, discovered telescopically in 1617 by Benedetto Castelli, a friend of Galileo’s. Bond found that his photographic measures of the separation and position angle between Mizar and Alcor, as well as between Mizar and its companion, agreed with values obtained by direct visual observation. A critical difference, he reminded colleagues, is that he was able to measure the photographs during the day, in the comfort of his office, instead of acquiring the data at the telescope in the often bone-chilling conditions of the observatory. He added that photographs form a permanent record of an otherwise fleeting observation that can be re-analyzed, if needed, at a later date.

  Of great concern to professional astronomers was the propensity of their newly minted colleagues to introduce errors into visual measurements of stellar positions. (To prevent contamination of the database, each astronomer was characterized by a unique personal equation, a mathematical algorithm expressing the degree to which one’s measurements deviate from the communal average.) Bond suggested that extraction of stellar position data from photographs—conceivably by means of a semiautomatic plate-measuring machine—would be less prone to error than subjective eye-estimates of a star’s passage across the meridian.

  The sensitivity of the new wet-collodion plates was remarkable. A photograph of Vega, which in 1850 had required an exposure of a minute and a half, could now be accomplished in a matter of seconds. Bond and Whipple’s daguerreotypes from the early 1850s had recorded only a handful of the very brightest stars, no matter how long the duration of the exposure. Even Polaris, the North Star, had remained frustratingly out of reach. Now dozens of stars appeared on the long-exposure plates, some of them so dim they are invisible to the naked eye. In Bond’s opinion, there was no reason to doubt that further technological refinements of the camera and the chemistry would reveal a multitude of even fainter stars never before seen.