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The Dream Machine CCD contains a SITe IMG 1024S chip6.
The chip is electronically cooled to prevent excessive dark
current. A detailed discussion of CCD image processing with
respect to bias, dark, and flat field images is beyond the scope
of this report. For the purposes of this project, all raw images
obtained through the Johnson-Cousins filters were processed in
standard fashion with bias and dark subtraction and flat field
correction. Standard bias, dark, and flat field frames were
obtained by median combination of 5 individual bias, dark, and
flat field frames, respectively obtained at the same camera CCD
temperature as the raw images they were used to process.
Approximately one-half of the objects discussed in this project
were imaged with the CCD chip cooled to -35o C. The
other half were imaged with the CCD chip electronically cooled
to -25o C, because halfway through the project the
cooling system for the CCD camera partially malfunctioned, and
it was not possible to cool the camera beyond 25o C.
Johnson-Cousins filters are used for color imaging at the
Grasslands Observatory, because they provide well-defined
spectral responses for the primary colors of red, green
(visual), blue, and near-infrared (figures 1-2). Standardized
scientific filters provide a means for obtaining consistent data
that can be compared across multiple objects and multiple
telescopes. Filters that don’t have standard pass bands or that
don’t have consistent through-put can produce interesting
pictures, but they can not be trusted to produce consistent data
over multiple data sets. A discussion of photometry and the use
of standard photometric filters are beyond the scope of this
project5.
The telescope was guided using an
SBIG
ST-237A CCD camera mounted on a Celestron 8-inch f/10 telescope
that is attached to the side of the 24-inch telescope. The
ST-237A guider is used for long exposures to correct periodic
error drift and other drive errors of the 24-inch telescope.
Color astrophotography with a CCD camera is possible if three
black and white images of an object are taken through three or
more 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. CCD
cameras only produce black and white images. Their spectral
response and sensitivity vary greatly (figure 1). They are very
sensitive to red and near-infrared light, moderately sensitive
to yellow and green light, and poorly sensitive to blue light,
though clearly superior to film at most wavelengths. Their
capacity for spatial resolution is less then fine grain film. |
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