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Dobsonian Basics

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Dobsonian Telescope Basics



This site describes one of the most popular amateur astronomy telescope designs around, the Dobsonian, like the Orion 8945 SkyQuest XT8 Classic Dobsonian Telescope . You'll see that it's a clever adaptation of the Newtonian, with a simple altazimuth base that though simple, can handle very large telescopes. 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 both 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 on such mounts. DOBS come in many sizes, from the great starter Orion XT 6" Classic that can easily be all you need, to the larger and very popular Orion 8945 SkyQuest XT8 Classic Dobsonian Telescope , and even in a very portable, acclaimed (and cute) Orion StarBlast 4.5 EQ Reflector - Telescope - f/4.0 - reflector.

But before you spring money on a commercial DOB, you might check out DOB plans at Building a Dobsonian Telescope. If you like to tinker, making one might be just the ticket. If you want to start viewing very soon, the previous links can help you get started right away. And no, in today's market you don't need to grind your own mirror unless you want to. Finished mirrors ready to be mounted immediately are available from many sources.



The Secret Is Simplicity

The DOB, as it is often called, is a very clever adaptation of the Newtonian, invented by John Dobson. He put a Newtonian telescope on what's now described as a Dobsonian mount. As shown, the Dobsonian mount is a simple altazimuth mount made primarily of wood.

Dobsonian Parts

Above you see the simplicity of the Dobsonian mount, illustrated by a model of a Dobsonian telescope. It is simply a wooden rocker box that sits on a base, and has a cradle on the top to hold the telescope. The telescope, sitting on the elevation bearings, can tilt up and down (elevation), and the rocker box can rotate around on the base (azimuth).

Dobsonian Mount and Telescope

The above image shows the telescope removed from the rocker box. Most Dobsonian systems allow easy removal of the telescope from the rocker box. This makes the system easy to move around, as one can lift off the telescope and transport it to the observing location, then transport the base to the observing location for quick reassembly. To reassemble, one just places the telescope back onto the rocker box.

Dobsonian Base

The bearings on a Dobsonian are very simple. The above illustration shows the rocker box removed from the base and laying on its side. On many homemade Dobsonian systems, the bottom of the rocker box has a thin metal covering on it. The top of the base has three Teflon pads. When assembled, a bolt (perhaps with sleeve) goes through the base up through the bottom plate of the rocker box. In addition to holding the rocker box on the base, the bolt provides the vertical axis. The metal covered bottom of the rocker box turns smoothly on the Teflon pads of the base.

Elevation bearings can be similarly made, with a metal band around an axle mounted to either side of the telescope, or to a cradle that wraps around the telescope. Then Teflon pads can sit in the cutouts of the top of the rocker box. On an 8 inch Dobsonian I once used just 6 inch wooden disks sitting in a V shaped cut out slot on the rocker box. I sanded the surfaces smooth, and the elevation movement was smooth.

Often, some kind of brake is used to put pressure on the elevation bearing so that switching from lighter to heavier eyepieces won't allow the telescope to tilt on its own.

Dobsonian mount
Dobsonian mount

Using a model telescope, the above set of images shows the simple azimuth/elevation movements of a Dobsonian telescope. 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.

A very popular DOB design and size is shown at left. It illustrates the straightforward design, low profile, and amazingly low cost for the size of the instrument. This particular model is designed to be a very compact, potable, wide field instrument, yet with optics the user can align when necessary.

The Dobsonian typically uses a metal surface (often aluminum) on Teflon pads for a simple low-friction bearing. Simple pressure on the side of the main tube allows easy positioning of the instrument in either azimuth or elevation.

In all optical 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. Kit forms of equatorial platforms are available at Equatorial Platform Kits.

Planetary DOB
My 6 inch Planetary DOB

I've used Dobsonian telescopes effectively for years, and for many of those years it was all I had. My main instrument for about 20 years was a classic 8 inch Dobsonian. Now I use the DOB pictured above, a tailor made planetary DOB designed by Steve Dodson (Stargazer Steve). You can more about this special DOB at my Planetary DOB Review page.

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

(Small Instruments At Table Bottom)

Wide FieldGeneral PurposeNarrow Field
15" f/4.5 DOB10" f/10 DOB12" SCT
12" f/4.5 DOB10" f/6 DOB6" Refractor
6" f/5 Newt8" f/10 SCT6" Maksutov
6" f/5 DOB6" f/8 Newt6" f/10 Newt
3.5" f/8 Refractor4" f/11 Refractor4" f/15 Refractor
4.5" f/4.5 DOB5" f/10 SCT5" f/15 Maksutov
2.4" Refractor4.5" f/10 Newt3.5" f/15 Refractor
Binoculars3" f/10 Refractor3.5" Maksutov


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.

I've come up with another solution, based on the utility image shown above. It's not too hard to put setting circles on a Dobsonian, especially if you can mount a dial or digital style level on the tube, which becomes the elevation indicator. See the Planetary DOB Review page for some details on how to do this. Then -- just use the Star Pointer web page in your smart phone, tablet, Chromebook, or Laptop browser and simply move your telescope to the coordinates given by Star Pointer. The web page shows how to polar align your telescope, and then it displays telescope coordinates (altazimuth or equatorial) for any selected object in any of 3 popular star catalogs.