1. (noun) a small telescope attached to a large telescope to use in setting the line of the larger one

• scope
• telescope

2. (noun) optical device consisting of a tube containing a convex achromatic lens at one end and a slit at the other with the slit at the focus of the lens; light rays leave the slit as a parallel beam

• optical device

1. (noun) a small telescope attached to a large telescope to use in setting the line of the larger one

• collimate