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In the early part of the nineteenth century, Louis J. M. Daguerre created the first photographic plate, which was simply a thin film of polished silver on a copper base. The plate was sensitized by exposing the silver, face down, into a vessel that contained a few particles of iodine. The plate was then fixed by immersing it into a solution to dissolve the unused silver iodide, and then rinsed in hot water to get rid of any remaining chemicals.
The importance that photography could have in the field of astronomy was immediately realized. It would allow an accurate and easy recording of brightness, positions, spectra, and physical aspects of celestial bodies. However, these early photographic plates were not sensitive enough to capture images of faint objects. The first daguerrotype of the moon was made by American physiologist J.W. Draper in 1840, involving a full 20-minute exposure. The first star was not recorded until 1850, when American director of Harvard Observatory W.C. Bond and Boston photographer J.A. Whipple took a daguerrotype of Vega.
Wet Collodion Process
Astronomers were not thrilled with the prospect of waiting hours and hours to get an image of a single star or nebula. They needed a method to produce better quality images in less time. In 1851, Frenchman G. Le Gray invented the wet collodion process. This process produced a plate that had a much higher sensitivity than the early daguerrotypes, but it needed to be used as soon as it was made. Furthermore, the process for producing such plates was much more complicated. After the image was taken, the plate was developed in a bath of iron sulfate, acetic acid, and alcohol.
Silver Bromide Dry Emulsions
Once again astronomers were inconvenienced by the fact that wet plates had to be used immediately after they were produced, and although they had a higher sensitivity to light, the extra sensitivity came at a cost: the extra time and effort it took to have the plates ready to go for the night's observing. The next phase of development was to create a plate which was highly sensitive to light, but which had a dry rather than wet surface, so it did not need to be used immediately. During the 1870's, there were several dramatic technological breakthroughs in the field of photography.
In 1871, R.L. Maddox produced the first positive dry emulsion for physical development, and then in 1874, J. Johnston and W.B. Bolton made the first negative emulsion for chemical development. By 1878, C.E. Bennet had discovered a method by which he could increase the speed of emulsions by aging them in a neutral medium. This was a most important development for the field of astronomy, since the universe is filled with very faint objects, and astronomers wanted to be able to photograph them without waiting for days and days to get an image on a photographic plate. And in 1879, George Eastman built a machine to coat plates with emulsion, so that the plates could be produced in mass numbers, relatively quickly and cheaply.
Utilizing the new silver bromide dry emulsion plates, the first good photographs of Jupiter and Saturn were made in 1879-1886, and of comets in 1881 (Tebbutt's comet). Henry Draper did a 51-minute exposure of the Orion Nebula in 1880, and two years later he took another lasting 137 minutes that revealed the entire nebula and the faintest stars in it. The study of spectra could also be undertaken with the new plates, since they were so much more sensitive to light than those previously. In 1876, the first spectrum of a star-Vega-was done by English amateur astronomer and spectroscopist W. Huggins, and the first spectrum of a "spiral nebula" (now known as a spiral galaxy and much more distant than anything else photographed before) was done in 1899. The new kind of plates also brought along with it the era of sky surveying, systematically photographing large expanses of sky. The first sky surveys were done at Harvard during the period of 1882-1886, each photograph covering 15 degree squares of sky and reaching as faint as 8th magnitude stars.
Emulsion Grain Size and Color Sensitivity
A close look at any photograph, particularly one that has been blown up, reveals certain graininess. Because photographic emulsions are made up of particles in suspension, this graininess cannot be completely eliminated and so at some level there will always be a loss of detail in taking a photograph. The first emulsions, which were developed, had grain sizes of about 10 micrometers in diameter. Although this seems tiny relative to most things that we know, such large grains took much of the detail out of a photograph, particularly in astronomy where small details are of utmost importance. Today, emulsions generally have grain sizes about ten times smaller than the earliest ones, or about one micrometer, and this allows for much more detailed photographs to be taken.
Hermann Vogel in 1873 accidentally discovered a way to make photographic emulsions sensitive to colors of light other than blue. At the time, green dye was used to soak up reflections off the backside of the glass in a photographic plate. Sometimes this green dye got into the emulsion along the plate edges, and Vogel noticed that the plate in this area was more sensitive to light of a longer wavelength or redder color. This observation was quickly exploited in making new kinds of emulsions which were sensitive to all visible colors of light, and by just a year later, W. Abney was able to put together an entire optical solar spectrum, from violet to infrared. During the first couple of decades of the twentieth century, Kenneth Mees at Eastman-Kodak made outstanding improvements in emulsions and spectral sensitivity. In addition, he grew particularly interested in the astronomical applications of these new emulsions and so he formed a partnership with several observatories in developing new ways to satisfy their needs.
Eastman-Kodak and Hyper sensitization
Since the early part of the twentieth century, Eastman-Kodak (George Eastman at right) has been the leading producer of new, faster emulsions. One of the major problems with photography of very faint objects, as is often the case in astronomy, is that the emulsions may react with the incoming light, but the emulsions react differently with light that has come in at a quick rate versus light that slowly filters in. For example, if a plate receives, say, 100 photons all at once, it will have no trouble reacting with them, but if the plate receives those same 100 photons over a period of an hour, it will probably not detect the light. And since astronomical light often filters in rather slowly, over a longer period of time, the emulsions do not usually detect it as well. This phenomenon is known as reciprocity failure. The first person to determine a way to partially repair this problem was Fox Talbot in 1843, which discovered that heating emulsions prior to exposing them increased their efficiency for short exposures. Fifty years later, Abney and King found that chilling emulsions made them more efficient for long exposures. It was not until the mid-twentieth century that scientists at Eastman-Kodak and elsewhere put together true scientific studies of why these different techniques worked and what other techniques might work even better for hyper sensitizing the emulsions. I.S. Brown and L.T. Clark in 1940 published results of their tests of water bathing, pre-exposure, ammoniating, mercury-vapor treatment, and high temperature baking for several different emulsions. This study then inspired many astronomers to attempt hyper sensitizing their own photographic plates, and soon later the American Astronomical Society created a Working Group on Photographic Materials to study the problem.
After years of research that is still ongoing, it has been concluded that different methods of hypering plates results in different results. Depending on what result the astronomer prefers, whether it be fewer impurities, increased chemical sensitization, better stability of images, or more light sensitivity, he or she should choose a different technique. For instance, the method of pre-exposure involves flashing a light on a plate before the actual exposure is taken for the purpose of raising the total exposure time of the plate. Thus, image specks will form more quickly and be more stable against decay, so subsequent light is absorbed efficiently. Cooling a plate before exposure, as discovered by Talbot, works by keeping the silver ions still in the plate and thus the final image is more stable. Plates also are baked in nitrogen, oxygen, or just air before exposure. The result is a gain in speed of light absorption and better sensitivity, best for the nitrogen bake and worse for the air bake. Another technique involves soaking a plate in nitrogen or hydrogen gas at room temperature. This helps to remove any impurities and to stabilize the emulsion.
Emulsions to absorb infrared light have also been developed, but they are much more sensitive to heat and so much more delicate. However, they can be hyper sensitized, as well, by placing them in a high humidity, oxygen-free environment. For example, they are usually hypered in a bath of distilled water, which results in a gain in speed, or else a bath of ammonia or silver nitrate solution, which helps to eliminate impurities and increase the sensitivity of the plate.
Newer Photographic Techniques
All photographs suffer from some degree of granulation due to effects in our own atmosphere and also from irregularities in the emulsion itself. A technique for removing these imperfections was invented in the middle part of the twentieth century. If an astronomer can take several images almost simultaneously, each of which presumably would have slightly different granulations, he or she could then superimpose or "stack" the images and thus remove any irregularities, which are not seen in all of the images.
A method was also developed for detecting very faint and extended objects such as nebulae, which are often not noticed in traditional photographs because they blend into the background light. However, by superimposing the glass photographic negative onto a positive print which was made from light of a different color, astronomers can easily see, for example, blue stars as black spots with white halos around them and red stars as white stars with black halos around them. This contrast more easily allows astronomers to detect nebulae and other faint objects.
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