The present invention relates to an aperture (10) for electromagnetic radiation (100), preferably electromagnetic radiation (100) comprising a wavelength between 1 nm and 20 μm, comprising an aperture body (20) made of a body material (22) transparent for the electromagnetic radiation (100). Further, the present invention relates to a method for aligning an aperture (10), and additionally to a system (70) for applying electromagnetic radiation (100) onto a source material and a dispersing element (90) for such a system (70).