Collimators, Autocollimators and Collimation Demystified
Updated 14 July 2020
An Autocollimator is a device that projects an image, usually focused at infinity, and allows the reflection of that image (typically by a mirror) to be observed through an eyepiece and small angular deflections of the image to be measured accurately against a graduated reticle.
In industry, autocollimators are most frequently used for the non-contact measurement of small angles. Angles as small as 1 arc second (1/3600 degree) can be measured. This can be done effectively and accurately at long distances.
During the U.S. Apollo manned space program, the arms connecting the Saturn V rocket to its tower were aligned using autocollimators.
In optical testing of photographic and cinematographic lenses, autocollimators are used with special reticles to observe focus. In conjunction with a Focal Plane Micrometer very accurate measurements of focus, down to 0.001 mm [0.00004 in], can be made with properly calibrated equipment and proper procedures.
Back focus of photographic and cinematographic lenses is always set at infinity focus (∞ mark on the focus scale), using an autocollimator, or a non reflex collimator and focal plane microscope.
Other distances on the focus scale of a cine lens are normally calibrated at the appropriate finite distance using a high resolution focus target and a focal plane micrometer. Cooke Optics in UK for example has a 10m [33 ft] focus system mounted on one end of the lens assembly floor.
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