Ramsden eyepiece

American

noun

Optics.

an eyepiece consisting of two plano-convex crown-glass lenses of equal focal length, placed with the convex sides facing each other and with a separation between the lenses of about two-thirds of the focal length of each.

Etymology

Origin of Ramsden eyepiece

1840–50; named after Jesse Ramsden (1735–1800), English maker of astronomical instruments

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Ramsden eyepiece

The equivalent focal length of a positive or Ramsden eyepiece is equal to three fourths of the focal length of either of the lenses.

From Pleasures of the telescope An Illustrated Guide for Amateur Astronomers and a Popular Description of the Chief Wonders of the Heavens for General Readers by Garrett Putman Serviss

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Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023