Abstract: A plurality of sample beam wavefronts derived from selected portions of a wide-aperture outgoing optical beam wavefront enter an entrance pupil (11) located adjacent a focal surface (12) on which the sample beams are individually focussed. A relay lens system (13) transmits the sample beams to a beam splitter (14), which divides each of the sample beams into an undeviated component and a diviated component. The undeviated components of the various sample beams image the entrance pupil (11) on a first scanning mirror (15), and the deviated components image the entrance pupil (11) on a second scanning mirror (16). The focal surface (12) is reimaged by the relay lens system (13) onto focal planes (17 and 18), respectively, so that the foci of the individual sample beams appear as spots on the focal planes (17 and 18).