(1) Small telescope attached to a larger optical instrument to fix its line of sight;
(2) optical device for producing a nondivergent beam of light;
(3) any device for limiting the size and angle of spread of a beam of radiation or particles.
1. A small telescope attached to a large telescope to use in setting the line of the larger one.
2. 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. + prikaži više