Sky science
Exit Pupil
Direct answer
Exit Pupil is the diameter of the light beam leaving an eyepiece, equal to aperture divided by magnification.
It is the beam delivered to the observer’s eye.
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At a glance
- Found in
- Astronomy & Skywatching
- Related
- Focal Ratio, Magnification, True Field of View, Apparent Field of View
It is the beam delivered to the observer’s eye.
A scientific orientation to scale, light, or motion; the article supplies the precise definition.
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The story behind Exit Pupil
Visual optical design used exit pupils to connect instruments with human vision.
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Why Exit Pupil matters
Exit Pupil influences image brightness, eye placement, and effective use of aperture.
Exit Pupil in practice: an example
A 200-millimeter telescope at 100 power produces a two-millimeter exit pupil.
Where readings of Exit Pupil go wrong
An exit pupil larger than the eye pupil wastes some collected light.
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: Skywatching FAQ - NASA guidance for observing stars, planets, constellations, and changing sky events.
- Source
- The Great Almanac Dictionary
- Category
- Astronomy & Skywatching
- Last updated
- 2026-09-26
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Common questions
What is an exit pupil?
Exit Pupil is the diameter of the light beam leaving an eyepiece, equal to aperture divided by magnification.
What do people get wrong about an exit pupil?
An exit pupil larger than the eye pupil wastes some collected light.
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