Bernhard Voldemar Schmidt extended both astronomy and astrophotography when he designed a special lens to correct a fundamental defect in images formed by spherical mirrors.
The first telescope, invented by a Dutch spectacle maker in the early seventeenth century, was a simple combination of two lenses in a tube. Because this type of telescope employs refraction, in which light obliquely entering glass from air undergoes a change in direction, it is often termed a refractor. Although Galileo did not invent the telescope, he pioneered its astronomical use, discovering sunspots, craters on Earth’s moon, the phases of Venus, and the four large moons of Jupiter, now called the Galilean moons in his honor.
Even today, many amateur astronomers use refractors; however, inescapable technical difficulties with such telescopes became evident as astronomy progressed. A lens, for example, can be supported only around its rim without obscuring the light path; therefore, large lenses may sag under their own weight and distort the image produced. Furthermore, a lens focuses red and blue lights at different places, a fault called chromatic aberration, causing colored rings around the images.
By 1897, when the Yerkes Observatory
Reflectors, on the other hand, are not without their own problems, and by the early twentieth century, one of the most troublesome of these problems was still unresolved: Would the optimum shape of the concave mirror be spherical or parabolic? A spherical one is easier to construct, but inherent in all such mirrors is a flaw called spherical aberration, in which light that does not strike the surface of the mirror along its axis of symmetry is not brought to an exact focus. Similarly, parabolic mirrors produce sharp images only for light entering from or nearly from the center of the telescope’s field of view. The image formed by all other light suffers from “coma,” in which fuzziness and cometlike tails appear. Stopping the aperture of either type of mirror to admit less light will sharpen the image but, unfortunately, prolongs photographic exposure times. In any case, the largest reflectors of the early twentieth century afforded a very narrow field of view, and a photographic survey of the entire sky with any of them would have required millions of separate pictures of small segments of the heavens.
The resolution to this impasse came from Bernhard Voldemar Schmidt, a highly skilled optical instrument maker. Born on the Estonian island of Naissaar in the Gulf of Finland, he was fascinated with scientific investigation in general and stars in particular. Despite the loss of much of his right arm to a boyhood experiment with gunpowder, Schmidt taught himself to grind lenses and mirrors. Hoping for better astronomical and astrophotographic equipment than he could afford, he wrote a letter in 1904 to Karl Schwarzschild, director of the Potsdam Astrophysical Observatory in Germany, offering to make a large mirror. Thus began ten years of successful cooperation in which Schmidt ground precise mirrors for Schwarzschild. The outbreak of World War I disrupted this collaboration; in addition, Schwarzschild’s brilliant career was aborted in 1916 by his death from service in the war.
In 1916, Schmidt contacted Richard Schorr, director of the Hamburg Observatory
At Hamburg Observatory, Schmidt met Walter Baade, a staff astronomer who reached prominence at Mount Wilson Observatory and Palomar Observatory. In 1929, Schmidt confided to Baade that he was confident he could produce a telescope with both a wide field of view and a large aperture for increased light collecting. From the details he gave Baade at the time, Schmidt apparently had been pondering this approach for some time, but he characteristically left no documentation of the process whereby he had attained the breakthrough.
Schmidt’s great contribution to optics was a marriage of lenses and mirrors, the prototype of catadioptric telescope designs, which include Schmidt-Cassegrain and Maksutov telescopes. In addition to a spherical mirror, Schmidt used a “corrector plate,” a lens thickest in the center, not so thick near the edges, and thinnest in between. It succeeded at its primary purpose: to correct almost completely the mirror’s spherical aberration by refracting the incoming light before it struck the mirror. (Strictly, the lens can exactly correct spherical aberration for only one wavelength of light.) The corrector also afforded several incidental benefits, one of which would be evident to anyone who has ever cleaned eyeglasses or contact lenses. Any optical surface open to the air will collect dust and require cleaning. Cleaning a large reflector, however, may jeopardize the thin metal coating on the mirror. By sealing the telescope tube, Schmidt’s corrector plate extended vastly the operational lifetime of its associated mirror.
On the other hand, there were drawbacks to Schmidt’s design. Although none has proved a serious obstacle to worldwide implementation of his idea, four might be mentioned. First, the corrector lens will introduce some small chromatic aberration (as the corrector plate is actually almost flat, this is relatively insignificant). Second, optical considerations dictate that a Schmidt telescope be longer than a comparable simple reflector. Third, the corrector plate is difficult to grind properly. Fourth, a photograph made by film inside a Schmidt telescope, thus functioning as a Schmidt camera, will be distorted unless the film itself is spherical.
With the encouragement of Baade and Schorr, Schmidt built the original Schmidt telescope in 1930; the Hamburg Observatory placed the first Schmidt camera in operation the following year. Revolutionary photographs quickly followed, but the skepticism of prospective customers, combined with international politicoeconomic considerations—including Adolf Hitler’s rise to power—prevented Schmidt from selling even one Schmidt camera outside of Hamburg before his death in 1935.
The Schmidt telescope, as historian of science Barbara Land has noted, “sees far and wide at the same time.” Furthermore, it sees quickly. To appreciate why in each case, one must understand the concepts of angular size and f-number. The width of a fist at the end of an extended arm covers about 10 degrees of the observer’s sky. In order to see faint objects, astronomers need large telescopes for their large light-gathering power. On the other hand, the field of view of the largest reflectors is minuscule. As Richard Learner has observed, the 100-inch (254-centimeter) Hooker telescope at the Mount Wilson Observatory would take at least fifty plates to photograph the full Moon, which is about half a degree across but “covers an area that is only a few millionths of the whole sky. A complete survey with such a telescope would take about 10,000 years.” In contrast, the first Schmidt telescope had a field of view more than 15 degrees wide.
The focal length of a lens or mirror is the distance from the component to the point where incoming parallel light is focused. A stronger component thus has a smaller focal length. The f-number of a lens or mirror is its focal length divided by its diameter; the smaller the f-number, the shorter the time required for a given amount of light to be collected, as for a photograph. Schmidt’s work can be viewed as inventing a succession of lower and lower f-numbers, culminating in f/1.75 for the 1931 Schmidt telescope. The Schmidt telescope is therefore well suited for either visual or photographic sky surveys, because it produces a wide and sensitive view of the sky with minimal distortion even out to the edges of the image or photograph.
During his life, Schmidt had jealously guarded details of the corrector plate and the curved film. In 1936, the year after Schmidt’s death, Schorr revealed these details to the world. In only six years, more than twenty Schmidt telescopes with mirror diameters of more than 24 centimeters (about 9.5 inches) were produced. A mere three years after Schmidt’s death, Baade and others persuaded the Rockefeller Foundation to fund an instrument that became known as the “Big Schmidt.”
With a field of view of more than three hundred times that of the Hale telescope, the Big Schmidt became the ultimate “spotter scope.” Far more important, however, is its epochal survey of the northern two thirds of the sky during the 1950’s, capturing objects down to one-half-millionth as bright as the dimmest naked-eye star. Each photographic plate covers an area approximately 6 degrees square and may show up to a million stars. Some two decades later, two Schmidt cameras (at the United Kingdom Siding Spring Observatory in Australia and the European Southern Observatory in Chile) cooperatively surveyed the southern third of the heavens.
Astronomical progress is ongoing, and better surveys are certain to be completed as technology improves. With these pioneering surveys, however, Schmidt cameras have already provided invaluable information in recording conditions at the time of the survey, furnishing an inventory of objects surveyed, and allowing better identification of previously observed objects.
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Asimov, Isaac. Eyes on the Universe: A History of the Telescope. Boston: Houghton Mifflin, 1975. Provides helpful informal background on observational astronomy, starting in prehistory. Briefly discusses Schmidt and his telescopes. -
Capaccioli, Massimo, ed. Astronomy with Schmidt-Type Telescopes. Boston: D. Reidel, 1984. Presents the proceedings of a colloquium of the International Astronomical Union. Many contributions are highly technical, but interested readers will find this a useful overview of the diversity of applications of Schmidt’s work by major-league astronomers. -
King, Henry C. The History of the Telescope. 1955. Reprint. Mineola, N.Y.: Dover, 2003. Presents drawings, diagrams, and technical details on the mounting and optics of various kinds of telescopes. Includes photographs. -
Land, Barbara. The Telescope Makers: From Galileo to the Space Age. New York: Thomas Y. Crowell, 1968. Simple and lucid account of the history of astrophotography up to Schmidt’s time. Chapter 8 is devoted to Schmidt. -
Miczaika, G. R., and William M. Sinton. Tools of the Astronomer. Cambridge, Mass.: Harvard University Press, 1961. Presents a moderately technical discussion of telescope optics. Serves as an excellent primer for most general readers. -
Silverman, Milton. “The Eye That Exposes Secrets.” Saturday Evening Post 222 (April 22, 1950): 28-29. Article on the Big Schmidt at the outset of its sky survey presents a delightful interview with Baade and tantalizing views of other applications of Schmidt’s corrector plate. Highlights Schmidt’s personality. -
Wachmann, A. A. “From the Life of Bernhard Schmidt.” Sky and Telescope 15 (November, 1955): 4-9. Excellent source of primary information on Schmidt and his work, written by a colleague of Schmidt on the occasion of the installation of a new Schmidt telescope at the Hamburg Observatory. Provides priceless glimpses of Schmidt in both prose and photographs. -
Watson, Fred. Stargazer: The Life and Times of the Telescope. New York: Da Capo Press, 2005. History of the telescope’s development includes discussion of the impacts on society of the discoveries the instrument has made possible. Presents the stories of the astronomers and other scientists responsible for advances in telescope technology, including Schmidt. -
Zirker, J. B. An Acre of Glass: A History and Forecast of the Telescope. Baltimore: The Johns Hopkins University Press, 2005. Describes the building of telescopes in the past and present and speculates about how they will be built in the future. Includes glossary and index.
Hale Establishes Mount Wilson Observatory
Hooker Telescope Is Installed on Mount Wilson