What is a Dobsonian Telescope and How does it Work?

The Dobsonian telescope has gone through a wide variety of iterations and innovations since it was first invented at the dawn of the Space Age, as well as a great deal of criticism. 

However, today Dobsonians dominate observing fields, are common even at astronomical observatories, and are widely considered the best beginner telescopes, if not the best telescopes made for visual observation altogether. 

Origin and design

The Dobsonian telescope was invented in the 1960s in San Francisco by a Hindu Vedanta monk named John Dobson. Dobson had worked as a chemist during World War II, and over time became interested in cosmology and spiritual studies. 

Much of Dobson’s studies and work as a monk focused on the nature of the Universe and reconciling it with Hindu teaching, and Dobson had the bright idea of making his own telescopes. 

But at the time, even a homemade 6” Newtonian of the conventional kind was out of reach for Dobson. Telescopes were constructed with expensive precise parts and were often not even portable.

Origin and design of the Dobsonian Telescope

The thick Pyrex mirror blanks, heavy duty equatorial mounts, and high-quality eyepieces were far too expensive for a monk like Dobson, with the typical cost of building or purchasing such a telescope being near the cost of a few weeks’ wages for the average person. 

While a monk with little spare change, Dobson was certainly not the average person, and set out to build low-cost telescopes that he could show people the Universe with. 

The result of this experimentation was the telescope that now bears his name, though he refused to call it anything but a “sidewalk telescope”. 

Dobson’s initial telescopes used porthole glass for the primary mirrors, ground with sand and polished with tools made of roofing tar and jeweler’s rouge. He silvered them with nitrate compounds made from fertilizer, and supported the mirrors in the completed telescopes with slings made out of seat belts – or clips made out of pencil erasers in the smaller ones. 

What is a Dobsonian Telescope and How does it Work? 1

The tubes were cardboard, and the whole telescope pivoted up and down much like a cannon. His eyepieces were scavenged from war surplus binoculars. 

Today’s Dobsonians are a little different. The portholes have largely been replaced by optical glass, because Dobson bought them all. Tubes are now thin-walled metal, or replaced entirely by aluminum skeletons. The largest mirrors still use slings, but they are generally Kevlar or steel cables. They’re still remarkably easy and cost-effective to make, though imported scopes from overseas are of course a simpler option for many people. 

How Dobsonian Telescopes Work

Dobsonian telescopes are really just an adaptation of the basic Newtonian reflector, which uses a parabolic primary mirror to focus light with the final image deflecting off a small flat secondary mirror off to the side of the tube. You look into an eyepiece attached to a focuser on the side of the tube, and the image appears upside down – but there’s no up or down in space, so it really doesn’t matter much.

Dobsonian Telescopes Set

The “Dobsonian” part of a Dobsonian is really the mount and tube design. The telescope tube is attached to a pair of altitude bearings or trunnions with some kind of plastic surface on them, which glides against Teflon plastic for smooth up and down motion. 

To move in azimuth (side to side) the Dobsonian uses more plastic on three Teflon pads. There’s just enough friction that the telescope stays in place when you let go of it, but it’s easy to push on when you need it to move. 

Some mass-manufactured Dobsonians have too-small bearings which need additional tension with springs or locks to not tip over when swapping accessories on the front end of the telescope, some have bearings which can be slid across the tube, and the best Dobsonians have very large altitude bearings that simply don’t have the center of gravity move much relative to them. 

Dobsonian Telescope  Origin and design

Most Dobsonians are all-manual; you find targets by using your smartphone or star charts and you slowly nudge the scope to track the sky. This makes it a lot faster to set up, lightweight, and of course cheaper than any other style of mount. The eyepiece also stays on the same side of the telescope, and generally doesn’t end up in a very awkward position.

Benefits of Dobsonian Telescopes

The biggest benefit of Dobsonians is that they give you a lot of aperture for the lowest price. An equatorial-mounted 8” reflector costs easily double that of an 8” Dobsonian, an 8” Schmidt-Cassegrain is triple or quadruple, and an 8” refractor tends to cost about as much as a decent automobile. As such, for many people, the only option for a decent sized telescope is going to be a Dobsonian.

Dobsonians are, additionally, much more portable and easy to set up than any other telescope type. An 8” Schmidt-Cassegrain can be slightly more compact than a Dobsonian, but assembly tends to be more complex. 

8” Schmidt-Cassegrain

An 8” Newtonian on an equatorial mount is awkward, heavy, bulky, and often unstable. And an 8” refractor is basically a permanent observatory instrument. 

Scale things up and the situation is even worse – a 12” equatorially mounted instrument of any kind needs an observatory and might weigh half a ton. A 12” Schmidt-Cassegrain is ‘portable”, but setting the heavy telescope tube on an equally-heavy tripod proves to be a struggle. 

Meanwhile, 20” and even 30” aperture Dobsonians are not uncommon and can be pretty easily transported and assembled by one person. 

Dobsonian Brands We Recommend

Our top 5 guide highlights our favorite Dobsonian telescopes for a variety of budgets and apertures, but in general, here are some of the brands and models of Dobsonians we can vouch for:

  • Orion – SkyQuest XT, XX lines, SkyLine, StarBlast 4.5”, SkyScanner 100mm – avoid the other SkyScanner and StarBlast products, they are low in quality
  • Zhumell – Z series Deluxe and tabletop Dobsonians
  • High Point/Apertura – AD and DT series
  • GSO – Almost anything, they are an OEM for the aforementioned three brands
  • Sky-Watcher Dobsonians
  • Bintel
  • Omegon
  • Saxon
  • New Moon Telescopes
  • StarStructure
  • AstroSystems
  • Taurus

Conclusion

Dobsonian Telescope Top Design

The Dobsonian telescope is a versatile design that fulfills the needs of nearly anyone looking for a telescope for visual observation, and they are capable of some photography too. Dobsonians are, quite simply, unmatched in their ability to deliver on large aperture, portability, and convenience – there’s a reason most scopes at a star party tend to be Dobs, and why they place highly so often on our top telescope recommendations. 

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