Collimating lens

Collimate \Col"li*mate\, v. t. [imp. & p. p. {Collimated}; p. p.
   & vb. n. {Collimating}.] [See {Collimation}.] (Physics &
   Astron.)
   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.
      [1913 Webster]