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exit pupil

American  

noun

Optics.

  1. the ridge of the entrance pupil of an optical system, indicating the place where the pupil of the eye should be placed to view the object.

exit pupil British  

noun

  1. the smallest cross section of the beam of light from the eyepiece of a telescope through which all the light from the eyepiece passes. Its diameter is equal to the ratio of the focal length of the eyepiece to the focal ratio of the telescope

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Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.