Dobsonian Telescope Basics
Cosmology 101 Items',
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
Orion 8945 SkyQuest XT8 Classic Dobsonian Telescope
, and even in a very portable, acclaimed (and cute)
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.
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).
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.
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.
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
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
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.
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
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 Field||General Purpose||Narrow Field
|15" f/4.5 DOB||10" f/10 DOB||12" SCT
|12" f/4.5 DOB||10" f/6 DOB||6" Refractor
|6" f/5 Newt||8" f/10 SCT||6" Maksutov
|6" f/5 DOB||6" f/8 Newt||6" f/10 Newt
|3.5" f/8 Refractor||4" f/11 Refractor||4" f/15 Refractor
|4.5" f/4.5 DOB||5" f/10 SCT||5" f/15 Maksutov
|2.4" Refractor||4.5" f/10 Newt||3.5" f/15 Refractor
|Binoculars||3" f/10 Refractor||3.5" 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