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Astrophotographers rely on an unusual assortment of techniques to get the most
out of their deep-sky photographs. To improve its response to faint light,
film can be cooled during an exposure, soaked beforehand in various liquids or
gases (hypersensitizing), baked in a controlled environment, or briefly exposed
to weak light (preflashing).
Photographers can also use color filters to enhance the appearance of
certain features of the sky. For example, a red filter will improve the
contrast between the sky and red-emitting gas clouds, such as the North America
nebula in Cygnus. While this technique has been used for years with
black-and-white films, I had never heard of anyone trying it with color
emulsions. So I performed my own tests.
With color film, using a red filter during the entire exposure, which is
the standard black-and-white practice, would cause everything to appear red on
the final image. I wonder what would happen if the filter were used only part
of the time. My guess was that stars and brighter objects would be recorded
during the unfiltered portion of the exposure, and red nebulae would be
highlighted during the filtered portion. This hunch paid off as the photographs
here demonstrate.
As a test field I chose an area around Orion that contains a large, very
faint cloud of glowing hydrogen. This cloud is known as Barnard's Loop, named
for the pioneer astrophotographer E.E. Barnard who photographed it in the late
19th century. The few color photographs I had seen of this object all show the
loop to be disappointingly faint with very little contrast between it and the
background sky.
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When I used a red filter for the entire exposure, Barnard's Loop was
barely visible against the background sky. Increasing the exposure time only
washed out the image without improving the nebula. However, using the filter
during only half of the exposure enhanced the nebula and kept the overall color
quite acceptable.
I had slightly better results when I used the filter during the first
half of the exposure rather than the second. The color density of the filter is
also important: a deep-red filter works better than a light-red one.
Thus far I have only tried this technique with normal and wide-angle
lenses and the camera mounted piggyback on a equatorially driven telescope.
Care must be taken when attaching or removing the filter to insure that the
camera is not shifted or the focus disturbed. For this reasons I prefer to use
the Cokin filter system rather than screw-in types. The Cokin system has a
bracket that attaches to the front of the lens, and the filters slip into the
bracket. Cokin filters are also less expensive than those made of glass.
This type of photography is wide open to experimentation. Different
films, filters, and lenses may all yield different results. I am interested in
hearing from others who try the technique.
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