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Telescope Mounts
A sturdy mount is obviously important for visual observing--the lack of a
quality mount is a primary complaint about cheap department-store telescopes--but the
mounting under a telescope is every bit as important for CCD imaging as the
telescope itself. The mount's ability to hold the telescope steady during
a long exposure (and in breezy conditions), as well as the tracking accuracy are
important factors.
A significant advantage of CCD imaging over film astrophotography concerns
"guiding". Long exposure astrophotos must be accurately guided to remove
any possible tracking errors. While some mounts are better than others,
all suffer some tracking errors due to slight imperfections in the gears driving
the mount. Guiding cancels out these errors so that the images are sharp. Since CCD chips are so much more sensitive than film,
it is possible to take short, unguided CCD exposures. If the exposures are
short enough, no tracking errors will be detected. Using telescopes with
long focal lengths (about 1000mm or greater) only the very best mounts will
provide unguided exposure capabilities. But with short focal lengths and
fast focal ratios, unguided exposures are easy with typical telescope mounts.
A focal ratio of around f/2 to f/4, combined with a focal length
of 400-1000mm can produce excellent, unguided 60-second CCD images. This is long
enough at such a fast focal ratio to capture the spiral arms of the Whirlpool
Galaxy, or the dark outline of the Horsehead Nebula. By stacking multiple
exposures, a long-exposure can be created from many short shots.
Above: 10-minute exposure of NGC 2903 in Leo taken by
stacking multiple 60-second unguided exposures.
Many of the popular Schmidt-Cassegrain telescopes are sold with computerized
mounts. There are generally two classes of mounts: those intended
for visual observation, and those for imaging. Since not all amateur
astronomers are engaged in CCD imaging or astrophotography, manufacturers make
telescopes with less accurate drive systems (which are less expensive than
telescopes with more precise drives). For visual observations, even at
high magnifications, the errors in the drive cannot be seen. However, for
imaging purposes, these errors are too great and more accurate systems are sold
as well. For example, Celestron's NexStar 8i and Meade's LX90 are identical
in optical design to these companies' higher-end models. They are intended
primarily for visual observations. For CCD imaging, both companies
recommend their next model up (Celestron's NexStar 8 GPS and Meade's LX200GPS),
both of which have more sophisticated tracking systems capable of long-exposure
CCD imaging.
Above: Computerized fork-mounted telescope. All goto, alt-az
telescopes require an equatorial wedge for long-exposure CCD imaging.
In the case of other optical designs, such as apochromatic refractors or
Ritchey-Chrétien telescopes, or in the case of SCTs sold as an optical tube
only, a separate mount is necessary. Almost exclusively, a German
equatorial mount is employed. German equatorial mounts provide exceptional
stability and can be used interchangeably with various telescopes.
Above: German Equatorial Mount with a 14" SCT and CCD camera.
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CCD Imaging Page
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