At least three incoming light beams (10a-c) are combined partly combined into a null beam and partly into fringe tracking beams. The relative phase difference between the incoming beams (10a-c) is controlled so that a phase relationship between in the null beam corresponds to destructive interference of the main beam components in the null beam. In the fringe tracker beams this leads at least partly to constructive interference between the main beam components, with an intensity that depends on the phase differences. Measurements of the intensities of the fringe tracker beams are used to control the relative phase differences of the incoming beams (10a-c) to realize destructive interference of the main components in the null beam. To form the null beam successively more of the light beams are combined into the null beam in a series of beam combiners (14a, 14 b). Part of the light is split from the incoming light beams into the fringe tracking beams, said splitting occurring at least partly between a start and end of said series (14a, 14b) or at said start.