A method for a measuring instrument is disclosed, for separating the angular deviation of a rotational axis of an instrument body from a corresponding true rotational axis due to imperfections in at least one rolling-element bearing effectuating the rotational mounting of the instrument body into different parts corresponding to type of imperfection. The method comprises detecting rotary position of the at least one rolling-element bearing, and determining angular deviation of the rotational axis from the corresponding true rotational axis in a plurality of rotational positions of the instrument body, wherein the instrument body is rotated a plurality of successive full or partial revolutions about the rotational axis. There is also disclosed a measuring system and a measuring instrument to be used in such a measuring system.