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Tri-Color CCD Imaging at the Grasslands Observatory
By Tim Hunter and James McGaha
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Color astrophotography with a CCD camera is possible if three
black and white images of an object are taken through red,
green, and blue filters and then combined into a final color
image. The production of good color pictures with correct color
balance, sharp focus, and little star trailing is not easy. It
requires an excellent CCD camera, a telescope with a superb
tracking, appropriate image processing software, and software to
combine separate monochromatic images into a final color image.
Color astrophotography has only been practical since the mid
1960's. Since then, there has been a revolution in the quantity
and quality of color films available. This resulted in color
astrophotography becoming a routine occurrence with thousands of
gorgeous astronomical photographs published. Color
astrophotography is relatively inexpensive and readily
available. Until recently, fast color films had fairly large
grain. Also, most films suffer from reciprocity failure, and
quite long exposures are required for photography of faint
objects, especially with slow, very fine grain film. To obtain
the finest quality images, one also has to have access to a
color darkroom or pay for professional photolab work. CCD
cameras have revolutionized professional and amateur astronomy.
They have incredible speed compared to film, and they have a
linear response from nearly the lowest light levels to the
highest light levels. Because of their linear response and their
digital data format, CCD images are readily amenable to image
processing, and they can easily produce direct numeric data.
The days of film astrophotography are rapidly coming toward a
close for most application. Professional black & white
astronomical films have been discontinued, and fine grain black
and white films are becoming uncommon with Kodak discontinuing
its Technical Pan 2415 film, long a favorite of amateur and
professional astronomers. Digital single lens reflex cameras (DSLR)
and web cameras have increasingly replaced film photography for
telephoto images of the night sky and for Solar, Lunar, and
Solar System imaging. High quality CCD cameras are the mainstay
for professional astronomical work and for imaging of faint
objects.CCD cameras do suffer from several disadvantages.
They are expensive, and they require the use of a computer to
run the camera and to process the data. CCD cameras only produce
black and white images. Their spectral response and sensitivity
vary greatly. As a rough general rule, they are very sensitive
to red and near-infrared light, moderately sensitive to yellow
and green light, and somewhat poorly sensitive to blue light,
though clearly superior to film at most wavelengths. Their
capacity for spatial resolution is potentially less then fine
grain film, though much amateur astrophotography is limited by
local seeing and telescope tracking, and CCD imaging can
approach the sharpness found with most film astrophotography. |
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