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SBIG CFW-9
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oxygen line is blue-green in color, and so must pass through both
blue and green filters. The hydrogen-beta line at 486 nm is very
blue, and must be rejected by the green filter. The green filter will
pass less than 10% of this line.
Hints for the User
Be aware that color imaging is substantially more difficult than
using the CCD unfiltered. Whereas before you only needed to take
one image you now need to take three, and each one needs to be
about 4 times as long since only a portion of the total spectral range
is passed by each filter. This aggravates tracking problems, which
are also more pronounced due to the requirement of registering
three images. Hot pixels and cosmic ray hits are more objectionable
since they become strongly colored in the final image (the hot pixel
remove utility in CCDOPS should be used). In spite of all this, you
will find that even subtle colors present in galactic scenes enhance
the image noticeably, and emission nebulas take on beautiful hues of
blue, blue-green and red. Star colors are dramatically enhanced, and
impart a three dimensional aspect to the image.
We have endeavored to create a product that enables true color
pictures of deep sky objects to be produced. We have made certain
philosophical decisions with which some may disagree. For
example, if we traveled in space to where we were quite close to the
Dumbbell Nebula it would appear to be a large gray mass devoid of
color, buried in mostly white stars. If we traveled to other galaxies
they would be no brighter, in general, than our Milky Way. Many
extended objects, such as the Horsehead Nebula, have very low
contrast relative to the sky background, often less than 10%, and
would never stand out sharply. The images you see on the
computer screen are not true, in the general sense, being
considerably
