1. A device by which a single lens collimates diverging light from a slit, and then focuses the light on an exit slit after it has passed through a prism to a mirror and been reflected back through the prism.

2. A telescope which has a graduated reticle, enabling an observer to read off the angles subtended by distant objects.

3. A convex mirror at the focus of the principal mirror of a reflecting telescope, which causes light to leave the telescope in a parallel beam.

4. A telescope equipped with an eyepiece designed for autocollimation.