Newtonian telescope

A reflecting telescope that uses a concave primary mirror to collect and focus light, then a small flat diagonal mirror to direct it to a side-mounted eyepiece.

Primary mirror: The main optical surface in a reflecting telescope, which collects incoming light and brings it to a focus. Its concave shape gathers light and forms the focus intercepted by the diagonal mirror.
Isaac Newton: An English mathematician and physicist whose work shaped classical mechanics, optics, and mathematics. He built the first known reflecting telescope using the arrangement now bearing his name.
Geometric optics: The model of light as rays traveling along paths and changing direction at optical surfaces. It describes how the primary and diagonal mirrors route light to the eyepiece.
Amateur astronomy: The nonprofessional observation and study of celestial objects. Newtonians offer amateur observers substantial aperture at comparatively low cost.
Cassegrain telescope: A reflecting telescope that uses a concave primary and convex secondary mirror to send light through a hole in the primary. It folds the light path back through the primary instead of sending it to a side eyepiece.
Secondary mirror: A smaller mirror in a reflecting telescope that redirects light from the primary mirror toward an eyepiece or detector. The Newtonian’s flat secondary sends the converging beam sideways without changing its convergence.
Reflecting telescope: A telescope that forms images primarily with mirrors rather than lenses. The Newtonian is a widely used member of this broader telescope family.
Aperture: The diameter of an optical system’s light-collecting opening. The primary mirror’s diameter sets how much light the telescope can gather.
Dobsonian telescope: A Newtonian telescope mounted on a simple, freely moving altazimuth base. This practical mounting made large-aperture Newtonians accessible to many observers.
Gregorian telescope: A reflecting telescope that uses a concave secondary mirror beyond the primary’s focus to direct light back through the primary. Its secondary mirror and folded focal path differ from the Newtonian’s flat diagonal.

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