 Brief History
of Astronomical Photography
Niépce and Daguerre
 In 1824
Joseph
"Nicéphore" Niépce (at left;
1765-1833) created the first semi-permanent images using glass
plates coated with bitumen dissolved in lavender oil. 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 mercury vapor heated to a
temperature of 75° Celsius. The vapor would adhere to the part of
the plate which 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 micrometers in diameter.
Although this seems tiny relative to most things that we know, such
large grains could result in a loss of detail in certain circumstances
(excellent seeing and resolution). More recently, finer grain
emulsions became available (with typical grain sizes of about 1
micrometer) which can be used in order to exploit excellent observing
conditions to produce more images with more fine detail. However, the
smaller grain size results in a drastically reduced sensitivity, since
the amount of light striking an individual grain has now decreased
when compared to larger grained emulsions. Exposure times are
significantly longer for fine grained emulsion, and hypersensitization
techniques are often employeed (see next section).
 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 during the exposure 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. For instance, the method of
pre-exposure involves flashing a light on a plate before
the actual exposure is taken for the purpose of reducing 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 during exposure, as discovered by
Talbot, works by increasing the lifetime of the silver atoms liberated
(by the incoming photons) from the silver halide crystals. These
silver atoms then survive for long enough to aggregate to make
developable latent-image specks. Plates also are baked in
nitrogen, oxygen, or just air before exposure. The result is a
reduction of the reciprocity inefficiencies (the best improvements are
seen for the nitrogen bake, with the least seen from baking in air).
Another technique involves soaking a plate in nitrogen or
hydrogen gas at room temperature. This helps to drive out the oxygen and water
present in the gelatin.
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 also be hypersensitized, in this case 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 remove bromide or iodine ions in solution in the gelatin. The
removal with a silver niteate solution will dramatically increase the
sensitivity of the IR emulsion.
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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