Sky science
Secondary Mirror
Direct answer
Secondary Mirror: A Secondary Mirror intercepts and redirects or further modifies light after the primary mirror in many telescope designs.
It routes the converging beam to a usable focus.
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At a glance
- Found in
- Astronomy & Skywatching
- Related
- Objective Lens, Primary Mirror, Refracting Telescope, Reflecting Telescope
It routes the converging beam to a usable focus.
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The story behind Secondary Mirror
Newtonian, Cassegrain, and other reflectors use distinct secondary geometries.
A one-minute story
The Wandering Stars
How the moving planets were watched, named and found, and what a transit is.
Why Secondary Mirror matters
Secondary Mirror’s size, alignment, and shape affect obstruction, contrast, and aberration correction.
How to read Secondary Mirror
Collimate it according to the telescope design rather than by appearance alone.
Where readings of Secondary Mirror go wrong
Misalignment degrades images, and solar energy remains hazardous throughout an unfiltered optical train.
Sources and editorial method
Research references used for this entry include:
- European Southern Observatory: Technology for Telescopes - Observatory reference for aperture, angular resolution, atmospheric seeing, mirrors, and telescope technology.
- NASA Science: Eclipse Viewing Safety - Official solar-viewing guidance for eyes, cameras, binoculars, and telescopes.
- Source
- The Great Almanac Dictionary
- Category
- Astronomy & Skywatching
- Last updated
- 2026-09-26
Trace the folded telescope beam
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Common questions
What is a Secondary Mirror?
A Secondary Mirror intercepts and redirects or further modifies light after the primary mirror in many telescope designs.
What do people get wrong about a Secondary Mirror?
Misalignment degrades images, and solar energy remains hazardous throughout an unfiltered optical train.
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