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This site describes one of the most popular amateur astronomy telescope designs around, the Dobsonian. You'll see that it's a clever adaptation of
the Newtonian. Invented by John Dobson, who called the design a sidewalk
telescope, it has become a favorite for a couple of important reasons.
The Dobsonian mount is known for its simplicity and stability. While
the largest model I've owned was an 8 inch, it is not uncommon for people
to build 18 inch telescopes and larger.
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Before you spring money on a commercial DOB, you might check out DOB plans
at Building a
Dobsonian Telescope.
The Secret Is Simplicity
A very clever adaptation of the Newtonian, invented by
John Dobson, is a Newtonian telescope on a Dobsonian mount. As shown, the
Dobsonian mount is a simple altazimuth mount made primarily of wood.
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Using a model telescope, the above set of images shows the simple
azimuth/elevation movements of a Dobsonian telescope. The solid but simple
mount rests on Teflon pads that are spread out to nearly the diameter of the
base, making for a super solid mount. Movements are simple, and the
altazimuth design keeps the eyepiece always in a comfortable configuration,
with its only motion being up and down.
The Dobsonian mount has provided two very useful benefits to the amateur
astronomer. It makes available telescopes that are amazingly inexpensive do to
the largely wood constructed mounts, and it allows fairly large telescopes to
be mounted at reasonable cost. The era of the Dobsonian design has
ushered in the popularity of relatively short focus Newtonian telescopes,
which offer large apertures and work well on the simple altazimuth mount.
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The instrument shown is a design from StargazeSteve (Steve Dodson). It uses counterweights on the
main mirror mount to move the center of gravity very low, allowing the mount to
have a very low profile. It also makes the telescope steadier on the mount.
The design is so solid that it is one of only two telescope I have that I can
focus the instrument without causing the image to wobble.
The Dobsonian typically uses a metal surface (often aluminum) on Teflon
pads for a simple low-friction bearing. The instrument shown has a brake
on the elevation axis to allow different weight eyepieces and attachments
to be used without causing a balance problem. Simple pressure on the side
of the main tube allows easy positioning of the instrument.
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In all other aspects, the Dobsonian is a Newtonian telescope with a concave
primary mirror, a flat secondary suspended by 1 to 4 vanes of thin metal (the
spider), and a focuser. Since it's a Newtonian in optical design, it has all
the advantages and disadvantages of any Newtonian. That is, mirrors are
exposed to the elements during observing sessions, and thus must be
occasionally carefully cleaned. This of course necessitates realigning
the optics. Periodic alignment tweaking is also recommended to obtain peak
performance.
The altazimuth mount's main advantage is sturdiness and simplicity. It's
disadvantage is that it is only good for visual work, and both axes must be
nudged to keep an object in view. There are suppliers of equatorial driven
platforms that can adapt a Dobsonian to do photographic work. Generally
the cost of the equatorial platform is comparable to the cost of the
telescope.
I've used Dobsonians effectively for years, and for many of those years
it was all I had.
If you think a Dobsonian might be right for you, first give a moment to
examining the following chart, which shows which types of telescopes are
generally used for what kind of observing.
Telescope/Observing Preference Table
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Use this astro-customized search to browse some excellent telescope
vendors.
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Personal Notes about Dobsonians
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I've owned 3 Dobs myself, a 4.25 inch, an 8 inch, and my current 6 inch.
The 4.25 inch was built with a mirror from Edmund Scientific, and was an
excellent performer. I was a fool to part with it. The 8 inch was constructed
using a mirror ground by a friend of mine. I used the instrument for 20 years.
I learned on that model, and experimented with the mount repeatedly.
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The 6 inch I have now is heads above my own design, and I can easily see
why Steve Dodson is a master telescope designer. His models, and most
others that can be purchased commercially, have clever designs with
brakes to eliminate elevation slipping, eyepiece shelves, etc.
Some commercial models have computers on them holding star almanacs. The
units aren't motorized, but have setting circles or encoders so one can easily
point the telescope to an object from computer instruction.
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