Ordnance Dept, page 82, THE COLLIMATOR
"Wikipedia"
Optical collimators can be used to calibrate other optical devices, to check if all elements are aligned on the optical axis, to set elements at proper focus, or to align two or more devices such as binoculars or gun barrels and gunsights.
"Wikipedia"
Collimator sights are a relatively old idea, being used in many forms for almost 100 years.
"Wikipedia"