The Dobsonian is a simple, low-cost telescope design popularized by San Francisco amateur astronomer John Dobson in the 1970s. It consists of a Newtonian tube assembly riding on a simple, wooden altazimuth base. A large primary mirror at the bottom of the tube collects light and concentrates it onto a small secondary mirror, which diverts the light cone out the side of the tube and into the eyepiece.
Their affordability and simplicity of operation make Dobsonians a great first telescope for entry into the hobby of amateur astronomy, especially those in the 6" to 8" aperture range. Larger Dobs, though bulkier to transport, are highly popular among experienced observers, thanks to their tremendous light-gathering prowess and, again, their affordability compared to refractors or catadioptric telescopes. Even relative to similar-aperture Newtonians on equatorial mounts, Dobsonians are delightfully economical.
A Hands-On Telescope
A Dobsonian is a telescope you can push around-literally. You point it by simply nudging the tube up or down, and left or right, with your hand. Side trunnions on the tube rotate on low-friction plastic bearings, allowing it to move up and down (altitude). The boxy base rotates horizontally (azimuth) around a center pivot. The motion on both axes is "buttery smooth," letting you guide the scope with just the lightest touch from one part of the sky to another. You don’t need to loosen and retighten clamps when targeting objects, as you must with other mounts.
Working with Gravity
The Dobsonian mount has one big advantage over other mount designs-it doesn’t try to fight gravity. The center of gravity lies directly over the center of rotation in both directions, so no matter where the telescope is pointed, weight is evenly distributed through the mount to the ground. This unique characteristic results in a dramatic reduction in vibration and oscillation, the inevitable downfall of many telescope mounts. Whereas in many telescopes the image seems to bounce around forever after the scope has been touched, this is not the case with well made Dobsonians.