 History of Photography in Astronomy
Niépce and Daguerre
 In 1824
Joseph "Nicéphore" Niépce (at left; 1765-1833) created the first semi-permanent images
using glass plates coated with a dispersion of silver salts in
bitumen. In the early 1830's Niépce's partner, Louis
Jacques Mandé Daguerre (at right; 1787-1851) accidently discovered a method for
creating a permanent image on a photographic
plate, which was simply a thin film of polished silver on a
copper base, sensitized by exposing the silver face to iodine vapors
to form a thin yellow layer of silver iodide on the surface of the
silver.
After a photograph was taken on the plate, it was developed by exposing
the plate to a current of magnesium vapor heated to a temperature of
75° Celsius. The vapor would adhere to the part of the plate which
was had been exposed to the light. The plate was then fixed by
immersing it into sodium thiosulfate, which was used to dissolve the
unused silver iodide, and finally rinsed in hot water to remove 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 image faint objects. The first daguerrotype of the moon was
made by American physiologist and chemist John William
Draper (at left; 1811-1882) in 1840, involving a full 20 minute exposure. The
first star was not recorded until the night of July 16-17, 1850, when William Cranch
Bond, the director of Harvard College Observatory,
and J. A. Whipple, a photographer associated with Massachusetts General
Hospital, took a daguerrotype of Vega. At right is an 1852 daguerrotype
of the Moon taken by Whipple.
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, however. They needed
a method to produce better quality images in less time. In 1851, Frederick
Scott-Archer (at left; 1813-1857) published an article describing
the wet collodion process,
although Gustave le Gray
(1820-1884) and Robert J. Bingham (1824-1870) earlier had suggested and
experimented with the technique. This process produced a plate which
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. Sulfuric acid and
potassium nitrate were reacted on a small quantity of cotton to create
guncotton (nitrocellulose). This guncotton was then dissolved in
alcohol and ether with iodides and bromides of cadmium, potassium, and
ammonium. The colloid which was produced was then spread on glass
plates and evaporated to leave a thin film of nitrocellulose impregnated
with bromides and iodides. When the plates were dry, they were dipped
into silver nitrate which was saturated with silver iodide, and this
transformed the iodide and bromide into salts of silver. This silver
halide coating was then sensitive to light, but the plate had to be used
immediately, or else the silver nitrate would crystallize. After the
image was taken, the plate was developed in a bath of iron sulfate,
 acetic acid, and alcohol which turned the exposed silver halide grains
into metallic silver. Sodium thiosulfate was used as a fixer to remove
the remaining (unexposed) silver halide grains, and the plate was then
washed to remove the chemicals. Finally a coat of varnish was applied to
protect the image.
Mizar and Alcor were photographed in March
1857 at Harvard College Observatory on wet collodion. The 1874 transit of
Venus was also widely photographed on collodion plates
as well as daguerrotypes. The collodion plate at right was taken in Japan by Jules Janssen (1824-1907), later director of the Meudon Observatory.
Silver Bromide Dry Emulsions
But again astronomers were inconvenienced by the fact that these wet
plates had to be used immediately after they were produced, and although
they had a higher sensitivity to light, the extra sensitivity often was
not made up for by the extra time and effort it took to have the plates
ready to go for the night's observing. The next phase of development,
then, 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 decade of the 1870's, there were several
dramatic technological breakthroughs in the field of
photography.
In 1871
Richard Leach
Maddox (at left; 1816-1902), a physician and photographer, produced
the first positive dry emulsion for physical development, using gelatin
(a transparent animal protein), and then in 1874, J. Johnston and
W. B. Bolton made the first negative emulsion for chemical development.
By 1878, Charles Bennett had discovered a method by which he could
increase the speed  (sensitivity to light) of gelatin-silver bromide emulsions
(at right) by aging them at 32°C 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. In 1879, George Eastman
(1854-1932) invented a machine to coat plates with emulsion, so that the
plates (at left) 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). A 51 minute exposure of the Orion Nebula was taken
in September 1880 by Henry Draper (at
right; 1837-1882), a doctor and prominent amateur scientist (and the son
of John William Draper), and two years later he took another lasting 137
minutes which 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
1872, the first spectrum of a star-Vega-was taken by Henry Draper. In
1882 Sir
William Huggins (who was the first to show that stellar spectral
lines could be identified with terrestrial elements, in 1864) took the
first spectrum of a nebula (the Orion Nebula), and in 1899 the first
 spectrum of a "spiral nebula" (now known as a
spiral galaxy and much more distant than anything else photographed
before) was taken, a 7½ hour exposure taken by Julius
Scheiner (at left; 1858-1913) with the Große
Refractor of the Astrophysical
Institute of Potsdam Observatory. 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 1882-1886, each photograph covering a
15°x15° area of the sky and reaching stars as faint 8th
magnitude.
Emulsion Grain Size and Color Sensitivity
A close look at any photograph, particularly one which has been blown
up, reveals a certain graininess. Because photographic emulsions are
made up of particles in suspension, this graininess can not 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 micrometer 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
Wilhelm Vogel (at right; 1834-1898), working in Berlin 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 back side 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
at all of the visible colors of light, and by just a year later, Sir William de
Wiveleslie Abney (1843-1920) was able to put together an entire
optical solar spectrum, from violet to infrared. During the first
couple of decades of the twentieth century, C. E. Kenneth Mees
(1882-1960) at Eastman-Kodak made outstanding improvements in emulsions
and spectral sensitivity. Mees 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, and insisted that Eastman-Kodak provide these plates to
astronomers at cost.
Eastman-Kodak and Hypersensitization
 During the
twentieth century, Eastman-Kodak
(George Eastman at right) was 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 which has come in at a quick rate versus light which 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 overcome this problem was
Fox Talbot (at
left; 1800-1877) in 1843, who discovered that heating emulsions prior to
exposing them increased their efficiency for short exposures. Fifty
years later, William 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
hypersensitizing 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 hypersensitizing their own photographic plates, and soon the
American Astronomical Society created a
Working Group on Photographic Materials to study the problem.
After years of research, it has been concluded
that different methods of hypering plates yield 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, they 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, as well. 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 hypersensitized, 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
Several techniques to obtain the most information from a photograph have
been developed David Malin
at the Anglo Australian Observatory.
 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, they could then superimpose or "stack" the
images and thus remove any irregularities which are not seen in all of
the images. This technique is displayed in the series of images of NGC 4672 by David Malin, at right.
 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.
Additional techniques, such as Photographic
Amplification and Unsharp Masking, have allowed some of the lowest
surface brightness objects to be discovered, including the giant low
surface brightness spiral galaxy Malin-1 (at left).
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