Dobsonian telescopes have become popular among amateur astronomers since they're simple but rugged, and can be built for little money. The term refers to any telescope that uses a Newtonian style assembly (one of the first reflective telescope assemblies) with an alt-azimuth mount, and several innovations that John Dobson helped popularize. Dobson did not invent these features, but his use of them in the 1950s meant that they became much more common. He did combine them all into one very large, but cheap and simple to use telescope meant only for astronomical observation.
The original design used very thin mirrors, made from salvaged ship porthole glass. The mirror could be supported in a very simple cell, using a carpet backing to support it evenly. Hard tubes, often made from a construction material used in pouring concrete columns, were also used in these first Dobsonian telescopes. Their advantages included being cheap, light, and durable when transported. Since these tubes were also nonconductive and thermally stable, they didn't convey heat from the users hands into the light path (which can introduce distortions.)
Early Dobsonian telescopes also used a square “mirror box”. This is a plywood box used to house one of the mirror. Sometimes, it was hinged to allow easy replacement or removal of mirrors. The box allowed a rigid surface to be used for attaching the carpet backed mirror, and also made it easy to place altitude bearings. Dobsonian telescopes also use a distinctive type of mount, where a flat platform acts as a base. A rotating box is placed on top, with semi-circular depressions cut into it. This is called a rocker box, and the depressions are used to hold the altitude bearings. Side to side motion is made possible using Teflon blocks on a Formica surface. Teflon allows for a smooth action without need for a clamp.
Since Dobsonian telescopes are a popular design for amateurs and self-builders, not all of these telescopes feature all the same features. However, they tend to have a number of them in common, and often have a very similar shape. The Dobsonian telescope has proven to be easy to use, good for deep sky viewing, and compact for its aperture size. However, because this type of telescope is optimized for portability and expense, it lacks an equatorial drive, meaning the user has to nudge it every few minutes to compensate for the Earth's rotation. It also suffers from balance issues if heavy items are attached. This type of mount is also difficult to focus on objects near the zenith, because it does not handle small movements well.
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