Primary mirror

A reflecting telescope’s main light-collecting mirror. It gathers incoming light and directs it toward the telescope’s focus or secondary optics.

Focal length: The distance from an optical element to the point where parallel incoming rays converge. The primary mirror’s curvature sets the telescope’s focal length.
Aperture: An opening or effective diameter that limits the light entering an optical system. The primary mirror’s aperture governs how much light a telescope can collect.
Newtonian telescope: A reflecting telescope that uses a concave primary mirror and a small flat diagonal secondary mirror. Its primary mirror sends light to a side-mounted eyepiece through a flat secondary.
Refracting telescope: A telescope that forms images primarily by bending light through lenses. It gathers and focuses light with an objective lens rather than a primary mirror.
F-number: The ratio of an optical system’s focal length to its aperture diameter. It relates the primary mirror’s diameter to the telescope’s focal length.
Specular reflection: Reflection from a smooth surface in which incoming rays leave in a well-defined direction. A polished primary mirror redirects light through specular reflection.
Cassegrain telescope: A reflecting telescope that sends light from a concave primary to a convex secondary and back through a hole in the primary. Its perforated primary mirror supports a compact, long-focal-length optical path.
Achromatic lens: A compound lens designed to bring two wavelengths of light to nearly the same focus. Refractors use lens corrections to reduce chromatic blur, which mirrors avoid by reflection.
Parabolic reflector: A reflector with a parabolic surface that brings parallel rays to a common focus. Many telescope primary mirrors use a paraboloid to focus on-axis starlight without spherical aberration.
Optical aberration: A departure from ideal image formation that prevents an optical system from producing a perfect point image. Primary-mirror shape errors appear as optical aberrations in telescope images.

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