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Dobsonian Basics

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Building a Dobsonian Telescope

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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.

Many commercial DOBs are available. Check them out with this astro-customized search engine:

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Before you spring money on a commercial DOB, you might check out DOB plans at the San Francisco Sidewalk Astronomer's website: Building a Dobsonian Telescope.



Shown: A Stargazer Steve 6 inch Dobsonian

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.

The Dobsonian mount has provided two very useful benefits to the amateur astronomer. It makes available telescopes that are amazingly inexpensive, and it allows fairly large telescopes to be mounted at reasonable cost.

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 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.

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 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

Telescope Usage Chart

Use this astro-customized search to browse some excellent telescope vendors.

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Personal Notes about Dobsonians

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.

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.