An optical system for shaping an incoming beam having a divergence with an angular distribution at least in a first direction comprises at least one angle selective optical element for clipping the angular distribution in the at least first direction. The approach according to the present invention bases on using an angle-selective device operated by the principle of total internal reflection to reduce divergence of the incoming beam, in contrast to a spatially-selective device as for example a field-stop or slit. The method according to the present invention has the advantage that no physical sharp edges have to be exposed at high energy densities. Thus, thermal impact and demands on the optical elements to withstand a high power laser beam are significantly reduced.