- Optics.
- a fixed telescope for use in collimating other instruments.
- an optical system that transmits parallel rays of light, as the receiving lens or telescope of a spectroscope.
- a device for producing a beam of particles in which the paths of all the particles are parallel.
- a small telescope attached to a larger optical instrument as an aid in fixing its line of sight
- an optical system of lenses and slits producing a nondivergent beam of light, usually for use in spectroscopes
- any device for limiting the size and angle of spread of a beam of radiation or particles
