To render parallel to a certain line or direction; to bring into the same line, as the axes of telescopes, etc.; to render parallel, as rays of light. [1913 Webster]

Collimating eyepiece, an eyepiece with a diagonal reflector for illumination, used to determine the error of collimation in a transit instrument by observing the image of a cross wire reflected from mercury, and comparing its position in the field with that of the same wire seen directly. -- Collimating lens (Optics), a lens used for producing parallel rays of light.

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OXFORD DICTIONARY

collimate, v.tr.
1 adjust the line of sight of (a telescope etc.).
2 make (telescopes or rays) accurately parallel.

Derivative

collimation n.

Etymology

L collimare, erron. for collineare align (as COM-, linea line)

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