Therefore the calibration of one of the elements of the measurement system--the multiangular prism--is of extreme importance.
The multiangular prism is a precise polygon that has precise flat mirror faces; the angle between the mirrors is being known and very precise.
The angle between the surfaces of the multiangular prism is considered to be the reference angle and in most cases its error value does not exceed a tenth of an arc second.
Main systematic errors (biases) of the measurements can be caused by both the autocollimator and the multiangular prism.
The systematic errors of multiangular prism are usually caused by:
On the other hand the deviations of angles between the mirror faces of the multiangular prism can be clearly determined, evaluated and quite easily corrected in the course of measurement data processing.