Collimator sight

[3] The basic layout of a collimator sight is a closed tube with a lens at its open end and a luminous reticle mounted near the closed end at the focus of the lens, creating an optical collimator.

This allows the brain to superimpose the aiming reticle onto the target, creating a composite image.

There have had more modern incarnations such as the Armson Occluded Eye Gunsight (OEG) and the Normark Corp.

Collimator sights have had uses as a small arms sight for low light situations (such as twilight or "room clearing" operations) since there is no intervening optical window that can block the light, allowing the use of both eyes with a relatively un-obstructed field of view.

Collimator sights are also used in astronomy as Finderscopes to aim a telescope at a designated celestial object.