In the optical metrology the collimators and telescopes are mostly used together to measure different optical and geometrical parameters:
Angle measurement
A telescope mounted in the front of a collimator enables the simultaneous observation of both collimator and telescope reticle. When a collimator is perfectly aligned to a telescope the reticles are superimposed and no displacement occures.
The presence of an angle "a" between the the collimator and telescope axes is shown by a linear displacement "d" between the two reticles. The displacement "d" gives the size of the angular disalignment of the two instruments:
a = d / ƒ
where: d = linear displacement measured in the reticle plane (focal plane), ƒ = effective focal length (EFL) of the observing instrument (i.e.telescope).
To ease the angular alignment of equipment with flat surfaces a special type of instruments can be used: the square body collimators and telescopes. The reticles of these instruments are accurately aligned to the outer square body surfaces.
Measurement of Optical Parameter
Measurement of Focal Length
A typical application is the measurement of the effective focal length EFL. A highly corrected collimator set to infinity (emerging beam is parallel) has a reticle with a pair of spaced lines located in its focal plane. The image of the reticle is projected over the lens under test and focused in its focal plane. By means of a microscope (or a telescope with a supplementary achromat) the size of spaced lines is determined and the EFL calculated:
ƒ = ƒcol S' / S
Measurement of Centration Errors
A collimator set to infinity contains a reticle with a dark or bright cross. The lens under test is placed in a precision rotary holder. The image of a reticle projected over the lens under test is observed by means of a telescope with additional achromat having a graduated reticle. While rotating the holder with the lens, the circle described by the reticle image is measured.
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