The application of photography to astronomy began in the 1840s. It would in time see the methods and goals of the science transformed by the photographic plate's ability to accumulate light over extended periods, and so reveal information on objects invisible to observers employing the naked eye at even the largest telescopes. The incorporation of photographic techniques into astronomical practice, however, was a lengthy process. It was marked by a series of successes as well as some spectacular flops, and an often subtle interplay of technical, scientific, social, economic, and cultural factors.
The decade of the 1840s witnessed a number of firsts, including daguerreotypes of the sun, the solar spectrum, and the moon. At the end of the decade, William C. (1789-1859) and George P. Bond (1826-65), based at Harvard and employing a large refracting telescope, collaborated with commercial photographers to obtain still better-quality lunar images.
There were further experiments during the 1850s, but the only protracted efforts came from the Bonds (who particularly investigated the photographic images of stars), and Warren de la Rue (1815-89), a successful British businessman with a wide range of scientific and technical interests. In 1851 he saw daguerreotypes of the moon by the Bonds on show at London's Great Exhibition. A major problem that pioneering astronomical photographers faced was that refracting telescopes (which form their images by means of lenses) were corrected for visual (yellow-green) light, whereas early photographic plates were most sensitive in the blue region of the spectrum. Hence the focus for the eye was not the same as for a photographic plate. Reflecting telescopes, however, did not suffer this problem as they use a mirror to bring all colours of light to a common focus. In the 1850s de la Rue, armed with a reflector and the newly developed wet-collodion process, photographed the moon. He also acquired a telescope drive that let him accurately track objects as they shifted across the sky, a key requirement for successful astronomical photographs as the earth's daily rotation causes an object to move 15 degrees per hour (as a rough measure of comparison, the apparent diameter of the moon is about half a degree). De la Rue was also responsible for the first result of astronomical significance to be achieved by photography when he compared his plates of the 1860 eclipse of the sun with other photographs taken a few hundred miles away. Through this comparison, he established that the prominences detected at earlier eclipses were real phenomena and not optical effects, and belong to the sun, not the moon.
During the 1860s, perhaps the two most significant researchers in addition to de la Rue were the Americans Lewis M. Rutherfurd (1816-92)—who fashioned the first refracting telescope designed specifically for photography in the USA—and Henry Draper (1837-82). By 1872, Draper had photographed dark absorption lines in the spectrum of the star Vega, which yielded information on the star's chemical composition. From the 1870s, the amateurs Sir William Huggins (1824-1910) and his wife Margaret (1848-1915), already an experienced photographer before she turned to astronomy, also obtained photographs of stellar spectra.
The 1874 transit of Venus (i.e. the passage of the planet across he face of the sun as viewed from the earth) was an event long anticipated and planned for by astronomers in many nations, as accurately timed observations of such a transit provide a means to determine the ‘solar parallax’, the value of which would provide a fundamental constant known as the astronomical unit. Photographic techniques were central to the arrangements of many expeditions. The results, however, were disappointing. When astronomers examined their plates under even low magnifications they found it extremely difficult to identify the exact position of the edge of either the sun or of Venus because of the blurring of image edges due to light scatter within the emulsion. Hence when a conference was convened in Paris to plan for the observations to be made at the next transit in 1882, the delegates from fourteen nations recommended that photographic techniques should not be employed.
Despite this failure, some astronomers reckoned that exciting possibilities were opening up, with the new dry plates beginning to supersede the slow, coarse-grained wet-collodion plates that allowed only relatively short exposures. The investigations of W. de W. Abney, a prominent British photographer, were especially important. Drawn to astronomical photography when the became involved in the plans for the 1874 transit of Venus, Abney had much improved the albumen dry-plate process in preparation for the British observations.
Increased sensitivity and superior photographic materials were not the only factors in the growing acceptance of photography as a research technology in the 1880s. Important too was the fact that by this time more individuals trained in physics were entering astronomy, and their chief interests were in the physical composition of the sun and stars, not the positions of these objects in the sky. For such researchers, often known as astrophysicists, photography was a crucial aid. Also, impressive photographs of a range of astronomical objects were secured during the 1880s which demonstrated that photographs could reveal far more detail than could the human eye. Especially influential in this respect were the photographs of nebulae and star clusters taken by a number of amateurs, most notably the British sanitary engineer A. A. Common. At first, defects in his telescope's drive mechanism led to poor images. But he improved the drive and also devised a special plate holder that could be moved during exposures to help compensate for any remaining irregularities.
The great majority of the early pioneers of astronomical photography were amateurs. Free of institutional demands and usually backed by substantial amounts of their own money, men such as Common had been able to experiment, take risks, and strike out in new directions. But as photographic techniques improved, and as some professionals showed what could be done with their aid, so photography became more widely accepted even in professional circles. Here Sir David Gill (1843-1914) played a key role. In 1882, he had photographed a comet, and in so doing registered images of background stars that persuaded him of their enormous potential for positional astronomy. Gill, director of the Royal Observatory at the Cape of Good Hope, now resolved to produce a photographic catalogue of the stars visible in the southern hemisphere. This project became known as the ‘Cape Photographic Durchmusterung’ (CPD; Cape Photographic Review), the end product of which comprised over 450, 000 stars. The CPD was produced in collaboration with Jacobus C. Kapteyn (1851-1922) of the University of Groningen, who established a special laboratory to measure and reduce the plates exposed at the Cape.
A less successful large-scale photographic mapping project was the Carte du Ciel, the aim of which was to construct a photographic chart of the entire sky. Established by the international Astrographic Congress of 1887 in Paris, the project involved the collaboration of many observatories. It proved to be overly ambitious and was not completed until 1964.
Even so, the Carte signalled the further acceptance of photographic methods. The use of photographic images of stars for detailed and accurate information on stellar positions and brightnesses, however, still posed severe difficulties. While Frank Schlesinger (1871-1943) at Yale proved particularly adept at solving those problems to do with determining a star's yearly shifts in position as the earth revolved around the sun (information which could be exploited to measure the distance of the star), photographic photometry proved even more vexing. Measuring the brightness of a star image required astronomers to take into account a range of systematic errors in their instruments and photographic plates, as well as deciding upon standardized procedures. Tackling these issues and producing consensus among the international astronomical community took some decades.
Around the turn of the century, James Keeler (1857-1900) at the Lick Observatory in California (using Common's former 91.5 cm (36 in) telescope) and Max Wolf (1863-1932) at Heidelberg underlined how effective large reflectors and photography could be in revealing new information on the heavens. The leading exponent of this sort of astronomy in the 1900s, however, was the American George. W. Ritchey (1864-1945). His first major success was in designing and fashioning at the Yerkes Observatory of the University of Chicago a very fine 61 cm (24 in) reflector for astronomical photography. Through careful attention to all aspects of its design, Ritchey ensured the telescope was capable of exposures of several hours. When the Mount Wilson Solar Observatory was founded in California in 1904, Ritchey joined the staff, and played a critical role in the construction of a 152.4 cm (60 in) reflector (1908) and a 254 cm (100 in) reflector (1919), the latter easily the most powerful telescope in the world at the time. In so doing, he and his colleagues had made the big reflecting telescope designed for photography into an essential tool for astrophysical research, and major research observatories aspired to own such an instrument.
In the 1920s and 1930s there were particular efforts to improve photographic plates so that they could capture more of the light falling upon them. In these researches, C. E. K. Mees of Eastman Kodak played a major part. Some investigators would also seek to increase the sensitivity of emulsions by various techniques, including ‘hypersensitization’ and the superposition of individual plates. A new sort of photographic telescope, the Schmidt telescope, also came into use c. 1930 to photograph large areas of the sky.
Before the Second World War the idea of telescopes in space seemed totally impractical. The crude state of existing rocket technology implied that, at best, such instruments lay decades in the future. But the war transformed this situation. The incentive to build weapons quickened the development of rocket technology, with the largest advances being made by German engineers and scientists with their V-2 rockets. Large numbers of captured V-2 components were transported to the USA after the war, where completed rockets were soon being used for research. An early scientific success came in 1946 when the US Naval Research Laboratory flew a spectrograph in a V-2. When the exposed photographic film was recovered after the flight it showed that the further the rocket travelled through the earth's ozone layer, the more of the sun's ultraviolet spectrum was recorded. Such rocket flights, however, were very limited, and enabled observations to be made for only a matter of minutes before the instruments arched back into the atmosphere under the pull of gravity. Satellites, however, offered the prospect of making observations for far longer periods, and the advent of satellite astronomy came close on the heels of the launch of the first man-made satellite, Sputnik I, in 1957. By this time astronomers were also seeking to intensify the light entering their telescopes via various electronic devices, or to replace the photographic plate with another sort of light detector. Experiments with television cameras for these purposes began in the 1950s, for example. The effort to replace photographic plates was further heightened by the development of astronomy from space. Astronomers much preferred to avoid relying on astronauts to exchange plates from orbiting telescopes and so were attracted by automated space observatories equipped with electronic detectors.
During the 1960s, American and Soviet space missions also gave a huge boost to studies of the solar system, most spectacularly with the flights of spacecraft to the moon, planetary fly-bys, and the landing of spacecraft on other worlds. The photographs taken by the Apollo astronauts on their journeys in the late 1960s and early 1970s were significant scientifically as well as in evoking wider popular responses. Images of the earth from space were also often credited with having heightened environmental concern for the planet.
The first craft to soft-land on another planet was the Soviet Venera 7 on Venus in 1970. A later version, Venera 13, soft-landed on Venus in 1982 and returned a colour picture. In mid-1975 the USA launched two craft to Mars, Viking 1 and Viking 2. Each was a kind of double spacecraft. Both carried a ‘lander’ and an ‘orbiter’, the lander (which carried a television camera) to touch down on the Martian surface, and the orbiter (which carried a telescope and vidicon tube) to orbit the planet and return data, including images of the surface. Both the orbiters and landers returned thousands of images, and these would play an important part in changing ideas about the planet.
As spectacular and scientifically fruitful in their own way as the images from Viking were those secured by the flights of NASA's two Voyager spacecraft—both of which carried vidicons—to the outer solar system. Both craft were launched in 1977 and reached Jupiter in 1979. Jupiter's moon Io, for example, exhibited a remarkable array of light and dark patches against a vivid orange background, as well as faint plumes that were rapidly interpreted as evidence of volcanic activity. Such pictures stirred enormous public interest.
During the 1970s charge-coupled devices (CCDs) began to become the leading light detector for astronomy and to displace photographic plates from various (although not all) applications. The main camera for the Hubble Space Telescope (HST), an orbiting observatory planned since the 1970s and launched in 1990, carried CCDs, and the many remarkable images they have secured have been the main means by which the public has become aware of the telescope's activities, as well as providing the principal body of its scientific results. CCDs were also incorporated in the camera for the Galileo mission to Jupiter, one of the most important planetary spacecraft of the late 1990s and early 2000s. A further milestone was reached early in 2004, when two NASA landers began to transmit high-quality pictures of the surface of Mars from digital stereoscopic cameras.
— Robert Smith
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