Above: A typical Dobsonian telescope
There are two basic categories of telescope mounts: alt-azimuth and equatorial. Alt-azimuth--short for altitude-azimuth--mounts allow a telescope to move up-down and left-right. This movement is parallel and perpendicular to the horizon, making this mount intuitive to point and leaving the eyepiece in a convenient position. Equatorial mounts allow for automatic tracking, essential for astrophotography. However, for visual observing, the Dobsonian alt-azimuth mount is extremely easy to set up and use.
How a Dobsonian Mount Works
The Dobsonian telescope is a relatively recent invention. While alt-azimuth telescopes have been around for centuries, what makes the Dobsonian unique is that the optical tube is held in place only by friction. In previous alt-azimuth scopes, once the telescope was aimed, it had to be locked in place. By employing Teflon and precise balance, the Dobsonian can be moved very easily from place to place in the sky, but when let go it stays perfectly in place. This telescope design was invented and popularized during the 1960s and 1970s by John Dobson in northern California. After a few articles were published on the design, and it began appearing at public showcases like the Riverside Telescope Makers Conference, the popularity began to grow. It is now one of the most common designs due to its ease of use and low cost. A Dobsonian is by far the most bang for the buck.
As long as the telescope is well balanced, movement is simple and achieved by pushing and pulling the optical tube around the sky. Teflon bearings allow easy motion but hold the telescope in place while observing. The Dobsonian really is ingenious in its simplicity.
Advantages of a Dobsonian Mount
Compared to an equatorially-mounted Newtonian telescope, a Dobsonian is much easier to set up, lighter weight, simpler to use, and less expensive.
Setting up a Dobsonian telescope is as simple as putting the optical tube onto the mount. It's a one-step setup for most telescopes. Only the very largest sizes (14"+) may require a little more setup, but even these scopes are very easy. An equatorial mount on the other hand, requires assembling more components, balancing, and polar aligning. Dobsonians, having fewer components, are much lighter than an equatorially-mounted Newtonian. Equatorial mounts require counterweights approximately equal in weight to the optical tube itself, which can have a lot of mass if the telescope is large.
Using a Dobsonian is as easy as it gets. You simply point the telescope wherever you want to look. The movement is very intuitive since it is just up-down, left-right. The fluid motion allows for easy tracking of objects as Earth rotates. Many Dobsonians are now computerized as well. While most do not track automatically (although some can), the computer makes finding objects in the night sky even simpler.
One of the main reasons for the popularity of Dobsonians is their low cost. A Dobsonian mount gets you the most telescope for the money. Newtonian optical designs are relatively inexpensive already, since the mirrors are easy to manufacture, but an equatorial mount (especially for a large telescope) can be pricey. On the other hand, even very large-aperture Dobsonians are fairly inexpensive. This allows beginners to get a lot of telescope for very little money.
Disadvantages of a Dobsonian Mount
The primary disadvantage of a Dobsonian mount is that it does not automatically track the sky as Earth rotates. This means long-exposure photography is not possible and sharing the view with more than a couple people becomes a bit of a hassle as the telescope must be adjusted every minute or two to keep the target in the field of view. It is possible to install a tracking system on a Dobsonian, but this is only practical and cost-effective for very large sizes. But as long as tracking is not necessary, a Dobsonian telescope is hard to beat.
Dobsonian Mount Prices
Dobsonian telescopes include the entire optical tube as well as the mount, so prices are for the complete system. Small Dobsonians, popular with beginners, range in size from 4.5" to 8" in aperture and cost from $200-600 depending on features such as computerization. Mid-sized Dobs for intermediate observers range from 10"-14.5" in size and can cost anywhere from $600-4000 depending on features and optical/mechanical quality. The largest Dobsonians are enormous telescopes, up to 36" in aperture, requiring ladders to observe and trailers to transport. While a low-cost 16" Dob might cost around $1500, the best telescopes in this range will run into the thousands of dollars, with the biggest costing well over $10,000. However, you simply can't get more telescope and a better view for the money.
Is a Dobsonian Mount Best for Me?
For beginners who want a simple-to-use telescope that will provide great views, and for advanced observers who want to see as much as possible, the Dobsonian is by far the best choice. Unless automatic tracking and photographic capability are essential, the Dobsonian is tough to beat.