Abstract: A mechano-optical device comprises a support adapted to receive at least one substantially parallel incident beam, of known axis of incidence, and a reflecting dihedron, which is substantially straight and rotatable relative to the support about an axis of rotation which is substantially comprised within its plane of bisection and substantially perpendicular to its edge. The axis of incidence is a generatrix of a first surface of revolution about this axis of rotation, and the reflected beam is a generatrix of a second surface of revolution. When the device is being used as a rotary joint, the dihedron rotates at an angular speed which is half that of the incident ray, which permits the reflected beam to remain stationary on its surface of revolution.