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