The invention relates to a lithography system for processing a target, wherein the lithography system comprises a final projection system arranged for projecting a pattern on the target surface. The lithography system comprises a mark position detection system arranged for detecting a position of a position mark on the target surface. The mark position detection system comprises an optical element arranged for projecting a light beam on the target surface and a light detector arranged for detecting a reflected light beam. The optical element may be positioned adjacent to the final projection system and the light detector may be positioned inside a frame.