Hi all, Say I have an optical system consisting of a HeNe laser, rotating mirror and achromatic lens. The scenario is: 1. The laser beam propagates towards the rotating mirror. 2. Rotating mirror is mouted on motor axis with slightly oblique angle and the motor axis itself is creating an angle of 45deg with a plane defined by the laser beam. This way a holow circular cone is traced out upwards. 3. Above the rotating mirror an achromatic lens is set. If - 3.1 the distanse from the laser beam hit point on the mirror to the lens is as lens's focal lenght and, 3.2 optical axis of the lens is coincident with the hit point on the mirror then (I guess) holow cone trace will become to hollow circular cylinder trace. My question is how do I callibrate the system for the first time when is built, to assure precision, because the diameter of a cylindrical trace profile after the lens should be about 5mm. What will I get if optical axis will be slightly aside from the hit point? Will I get a trace of elliptical cylinder? Will I be able to determine the digression from a perfect circular profile? Thank you in advance!