There is provided an illumination system for lithography with wavelengths of .ltoreq.193 mn. The system comprises a first optical element, which is divided into first raster elements and lies in a first plane. The first plane defines an x-direction and a y-direction, the first raster elements each have an x-direction and a y-direction with an aspect ratio, and at least two of the first raster elements have aspect ratios of different magnitude.