Abstract: Apparatus for real time measurement of optical beam parameters includes an assembly of known dimensions, carrying multiple apertures, occupying parallel planes. The assembly can be moved so that the apertures sequentially intersect an optical beam which is not propagated in the plane of the apertures. At least one pair of the apertures are non-circular and identical with accurately established positions and which are not disposed in the same angular alignment when they intercept the beam but are disposed in the same plane in order to allow determination of the two-dimensional position of the centroid of the beam in that plane.