Bernhard Voldemar Schmidt

Estonian-born German optician

• Born: March 30, 1879
• Birthplace: Island of Naissaar, Estonia
• Died: December 1, 1935
• Place of death: Hamburg, Germany

In 1930, Schmidt invented an optical system that revolutionized astronomy by significantly widening the field of vision of the largest telescopes then in use. The Schmidt photographic telescope used a spherical mirror in combination with a glass plate to capture celestial images on photographic plates. For the first time, wide areas of the sky could be photographed with sharp definition across the entire field, edge to edge.

Early Life

Bernhard Voldemar Schmidt (BEHRN-hahrt VOHL-deh-mahr shmiht) was born on Naissaar, an island in Estonia. His family was poor, and he had little formal schooling. However, he had an early interest in science and often conducted simple experiments. One such experiment with gunpowder resulted in a disastrous explosion that cost him part of his right arm. Despite that handicap, his curiosity was undeterred, and he continued to study physical science and eventually demonstrated a strong aptitude for optical engineering, aided by an intuitive understanding of the physical nature of optical structures. While still a youth, he made his first telescope lens by cutting the bottom from a glass bottle and grinding it with sand. Schmidt was as resourceful as he was curious and conducted fairly complicated optical experiments with extremely crude instruments.

In 1900, Schmidt entered the Institute of Technology at Göteborg, Sweden, where he studied engineering. He left Göteborg to study in Mittweida, Germany, where he became a skilled crafter of mirrors and lenses for telescopes. He remained in Mittweida after graduation and created a top-quality, fifteen-inch mirror for an observatory at Potsdam, Germany.

From the earliest days of his life, Schmidt was a loner. He was moody, difficult to approach, and often unpredictable. He worked alone at a variety of jobs until 1926 when, at the age of forty-seven, he joined the staff of the Hamburg Observatory. He was considered an eccentric by fellow astronomers. He refused to accept a conventional position with the observatory staff, preferring to continue working alone as a crafter of optical instruments. He had few, if any, friends and labored with crude equipment. Occasionally he would disappear on drinking binges that would often last for days. Despite his contrary disposition and antisocial nature, Schmidt was considered a genius in the field of optics, wherein he made the discovery that revolutionized celestial photography.

Life’s Work

Schmidt devoted his life to the development of mirrors and optic lenses for use in astronomical observatories. Late in life, he developed the optical system for which he became famous, one that was later modified and enhanced to improve significantly the field of observational astronomy. Ironically, the basis for that system came from an idea proposed by Sir Isaac Newton in the early eighteenth century, but which had been long abandoned by astronomers. Newton, better known for his study of the laws of gravity than for his lifelong work in the field of optics, recognized the need for a telescope lens that could take in a significantly larger field of view than was possible with the telescopes of his day. He designed such a lens using spherical curvature. However, telescope-makers found the concept to be unworkable because different areas of lens received light beams of varying length, thus causing aberrations in the focus of captured images. Schmidt overcame the problem by placing an error-correcting glass plate as far ahead of the focal point of the telescope as the spherical mirror lay behind it. Since the telescope was designed to function with a relatively short focus, the length of the tube required to house the optics was also relatively short. A curved film was placed in front of the mirror, and for the first time ever a clear, sharp image of an area of the sky greater than 20 degrees of arc could be photographed. This allowed scientists to study galactic star clusters in several wavelength ranges, while searching for young stars, the study of which is useful in understanding the structure of the universe. Before 1930, most telescopes were of the reflector or refractor type. Each was useful in the study of stars and star clusters in distant space but had the inherent disadvantage of being able to see only a small portion of the sky. Interestingly, in 1910, Kellner invented the optical components that became the basic technology incorporated in the Schmidt telescope, but it was Schmidt who actually constructed the first one.

The theory behind the Schmidt telescope, often referred to as a camera, relates to the physical properties of reflected light. Parallel light rays reflected at the edge of a spherical mirror travel shorter distances to a medium placed above it than does light reflected from the center of the sphere. That causes the focus to be in one region of the medium, or film, rather than over its entire area. Schmidt’s glass-correcting plate compensated for those variant distances, thus achieving sharp images over the full range of the area being viewed through the telescope. In later versions of the Schmidt telescope, the correcting plate was ground so thinly that very little chromatic distortion occurred at that point, making the image even sharper.

The largest Schmidt photographic telescope was installed on Mount Palomar in California. Its intended function was to do the first full-scale photographic sky mapping. It was equipped with a forty-eight-inch correcting lens and a seventy-two-inch reflecting mirror with a radius of curvature of twenty feet. The focal length was ten feet. Photographs were taken in both red and blue light, with all stars displaying luminescence down to a magnitude of 21.1 appearing on the blue plates and those down to a magnitude of 20 appearing on the red plate. More than seventeen hundred plates were required for each photographic image to accommodate both colors. The first photographs taken by the Schmidt camera were published in the 1950’s.

It is important to note that the Schmidt telescope was used only as a camera. Photographically, the f-ratio, or focal length divided by the aperture, of the Schmidt telescope was similar to that of a fast photographic lens. That fact, combined with developments in photographic image processing, resulted in modifications to the Schmidt telescope that made the instrument more efficient and led to new estimates of the distances of galaxies far from Earth.

In spite of the breakthrough for astronomers that was embodied in the Schmidt telescope, one problem resulted from the curved photographic plates required to achieve wide-area coverage: They made it awkward for making precise determinations of distances among stellar objects. Nevertheless, the Schmidt photographic telescope became extremely popular, primarily because it was able to accomplish significantly more mapping at those infrequent times when weather conditions were ideal. Schmidt died in an institution for the insane in Hamburg in 1935. His personal life was troubled, but his contribution to astronomy was great.

Significance

Schmidt was a technician, not an astronomer. He was concerned with developing the technology of astronomy rather than with studying the universe. However, his optical system has earned for him a place among the great astrophysicists of his time. Over the centuries prior to 1930, the study of astronomy had been carried on in much the same way. While lenses and other telescopic components had been enlarged and refined over time, the techniques of astronomical observation were much the same. The Schmidt telescope represented a major step forward, providing a means for expanding the field of celestial observation. Over a seven-year period, a giant Schmidt telescope at Mount Palomar photographed the entire sky that could be viewed from that location in California. Schmidt telescopes were later installed in other regions of the world to photograph celestial objects visible only from those regions, adding significantly to the body of knowledge about the structure of the universe.

Bibliography

Hoyle, Fred. Astronomy. Garden City, N.Y.: Doubleday, 1962. A general book on astronomy that also contains several good photographs of the Schmidt telescope. Index, appendix.

Marx, Siegfried, and Werner Pfau. Observatories of the World. New York: Van Nostrand Reinhold, 1982. An excellent description of several of the important astronomical observatories of the world, including Schmidt’s home observatory at Hamburg, Germany. Includes photographs and an index.

Moore, Patrick. Men of the Stars. New York: Gallery Books, 1986. Contains a brief biographical sketch and picture of Schmidt. This highly illustrated book is particularly useful to the young reader looking for a fine introduction to astronomy.

Richardson, Robert S. The Star Lovers. Toronto, Ont.: Macmillan, 1967. A survey of the work of many of the best-known personalities in astronomy. Tracks the evolution of astronomy through the early 1960’s and contains many useful photographs, including several Schmidt photographic telescopes, such as the one at Mount Palomar. Includes a brief passage on Schmidt’s professional relationship with Walter Baade, one of the great astronomers of the early twentieth century. Bibliography, index.

Ronan, Colin A. Changing Views of the Universe. New York: Macmillan, 1961. An overview of developments in astronomy over the centuries as viewed from the social perspectives that prevailed at the time. Contains a passage on the significance of the Schmidt camera from a social and political context.

Rousseau, Pierre. Man’s Conquest of the Stars. New York: W. W. Norton, 1961. A highly readable survey of the evolution of astronomy for the layperson. Contains no complicated, technical descriptions. Includes an index.

Shapley, Harlow, ed. Source Book in Astronomy, 1900-1950. Cambridge, Mass.: Harvard University Press, 1960. Contains a discussion of the instrumentation used during the first half of the twentieth century to study the solar system and the position and motion of stars.

Sidgwick, J. B. Amateur Astronomer’s Handbook. London: Faber & Faber, 1961. Contains a good discussion of the technical components of astronomy: telescopic function, optics, oculars, mountings, and several modifications to the Schmidt camera, including the thick mirror Schmidt, solid Schmidt, folded solid Schmidt, off-axis Schmidt, all reflector Schmidt, spectroscopic Schmidt, and others. Bibliography and index.

Watson, Fred. Stargazer: The Life and Times of the Telescope. Cambridge, Mass.: Da Capo Press, 2005. Chapter 15, “Silver and Glass,” contains several pages of discussion about Schmidt and his telescopes.