Cheshire eyepiece

A Cheshire eyepiece or Cheshire collimator is a simple tool that helps aligning the optical axes of the mirrors or lenses of a telescope, a process called collimation.

Through a lateral opening, ambient light falls on the brightly painted oblique back of the peephole.

Images of this bright surface are reflected by the mirrors or lenses of the telescope and can thus be seen by a person peering through the hole.

[2][3] Some modern models of Cheshire eyepieces in common use include extended sight tubes and are equipped with crosshairs.

When inserted into a Newtonian telescope whose primary mirror is marked in its center, such aids allow the user to adjust the position and tilt of both the secondary and the primary mirror.