What Is A Dobsonian Telescope?

A Dobsonian telescope is any telescope that features an alt-azimuth mount and a Newtonian-telescope tube assembly, along with several innovations by the amateur astronomer John Dobson. The "classic," "hard-tube," or "first-generation" Dobsonian telescope was intended to be affordable, easy to make, and portable. It was optimized for deep-sky observing star clusters, nebulae, and galaxies that require a large objective mirror with light-gathering capability. Since deep-sky observation demands that the observer be in a dark location, away from city lights, the Dobsonian telescope is compact and more rugged than the standard Newtonian telescope.

The Original Design Features:

• Thin mirrors made out of ship-porthole glass from salvage yards. (The mirrors are supported by a simple cell with a backing of indoor/outdoor carpet.)
• A hard telescope tube originally made from Sonotube, which is used to pour concrete columns. (The hard tube is extremely durable, thermally stable, and nonconductive.)
• A square mirror box, sometimes hinged in the back to allow easy mirror removal. (This feature provides a rigid, flat surface to which the carpet support and altitude bearings can be attached.)

The alt-azimuth mount features a ground board on which rests a rotating box with semi-circular depressions cut into the top for the altitude bearings. The azimuth (side-to-side) motion is the result of Teflon blocks turning on a flat, Formica-covered surface, and the altitude (up-and-down) motion is the result of a large-diameter axle turning on Teflon bearing blocks attached to the altitude cutouts. The Teflon bearing surfaces and the large diameter of the bearings reduce the amount of friction. As a consequence, a clamp mechanism is not necessary to prevent the unintentional movement of the telescope.