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The Auto-collimator is a single instrument combining the functions of a collimator and a telescope to detect small angular displacements of a mirror by means of its own collimated light.

The two reticles are positioned in the focal plane of the corrected objective lens, so that the emerging beam is parallel. This usual configuration is known as infinity setting, i.e the auto-collimators are focused at infinity.

When moving the reticles out of the focal plane of the objective lens, the auto-collimator can be focused at finite distances, and the beam becomes divergent (producing a virtual image) or convergent (real image). This results in a focusing auto-collimator. The shape of the beam -convergent or divergent- depend on the direction in which the reticles are moved

The main components of a standard auto-collimator i.e. focused at infinity are:

• Tube mounted objective lens

• Beam splitter mount which contains two reticles

• Eyepiece

• Illumination device

The illuminated reticle projected over the beam splitter towards the lens is known as collimator reticle. The second reticle placed iin the focus of the eyepiece is the eyepiece reticle.

The beamsplitter mount together with the eyepiece and the illumination device form a main unit called: Auto-collimator head.

A focusing auto-collimator (finite distance setting) is similary built. The auto-collimator head containing the two reticles is now mounted on a draw out tube for focusing adjustment.

Auto-collimation is an optical technique of projecting an illuminated reticle to infinity and receiving the reticle image after reflection on a flat mirror. The reflected image is brought to the focus of the objective lens in which the eyepiece reticle is located. Thus the reflected image of the collimator (illuminated) reticle and the eyepiece reticle can be simultaneously observed.

When the collimated beam falls on a mirror which is perpendicular to beam axis, the light is reflected along the same path. Between the reflected image and the eyepiece reticle -which are seen superimposed- no displacement occures.