To achieve a quick and accurate protection of a lens by providing a slide cap movable between the position of opening the end face of a lens frame of a collimation telescope and the position of closing it to slide the slide cap.

A straight traveling guide groove 22b is formed on a support plate 22 and halved slide caps 24 are made movable to the position of opening the end face of a lens frame of an objective lens 16 or to the position of closing it along the straight traveling guide groove 22b. A cam groove 23c is formed on a pair of operation blades 23b of an operating ring 23 and when turning the operating ring 23 normally or oppositely, the pair of half split slide caps 24 is advanced or retracted symmetrically pertaining to the center of the objective lens 16. When the operating ring 23 is operated to turn normally or oppositely centered on the optical axis of the objective lens 16, the pair of half-split slide caps 24 is moved symmetrically pertaining to the center of the objective lens 16 by an engaging relationship of the cam groove 23c and a cam follower 24a to be closed keeping the objective lens 16 open.