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.
See under Collimate.

See also: Collimate, eyepiece

References in periodicals archive ?
Install the objective and use a sighting tube or a Cheshire collimating eyepiece to align it as you would a standard, non-folded refractor.
After the telescope is assembled, I loosen the top clamp on each monopod and adjust the position of the secondary cage until the cross is centered in a collimating eyepiece. Next, I remove the dust cover, unclamp the mirror, and give the alignment a final touchup using the mirror mount screws.
Even using collimating eyepieces (S&T: March 1988, page 259), the task can be laborious.