Dobsonian telescope
A reflecting telescope on a simple, stable altazimuth mount, designed to provide large apertures at relatively low cost.
Newtonian telescope: A reflecting telescope that uses a concave primary mirror and a diagonal secondary mirror to direct light to a side-mounted eyepiece. Most Dobsonian telescopes use this optical arrangement, while the Dobsonian mount is a separate design choice.
Aperture (optics): The effective diameter of an optical system’s opening, which determines how much light it collects and influences resolution. A Dobsonian’s defining practical advantage is making large apertures affordable.
Deep-sky observing: Visual observation of astronomical objects beyond the Solar System, including nebulae, star clusters, and galaxies. Large apertures make faint deep-sky objects a central use of Dobsonian telescopes.
John Dobson: An American amateur astronomer and educator who developed and promoted an inexpensive large-aperture telescope design. His name became attached to the mount and observing approach he popularized.
Schmidt–Cassegrain telescope: A catadioptric telescope that combines a corrector plate, primary mirror, and secondary mirror in a compact optical tube. Its compact tube is easier to transport than a comparably capable large Dobsonian, but typically costs more.
Parabolic reflector: A reflecting surface shaped as a paraboloid that brings parallel incoming rays to a common focus. Large Newtonian mirrors are commonly parabolic to focus light from distant objects sharply.
Focal ratio: The ratio of an optical system’s focal length to its aperture, indicating how quickly it forms an image. It helps compare Dobsonian designs and anticipate their field of view and focusing demands.
Planetary observing: Visual or instrumental study of planets and other Solar System bodies through a telescope. A well-collimated Dobsonian can show planetary detail, especially under steady skies.
Sidewalk astronomy: Public stargazing conducted in accessible outdoor places, often using portable telescopes and informal explanations. Dobson promoted low-cost telescopes as tools for bringing astronomy to the public.
Refracting telescope: A telescope that forms images primarily by bending light through lenses. Dobsonians use mirrors to make large apertures more affordable than comparable refractors.