Abstract: A projection exposure system (10) for microlithography. The system includes projection optics (12) configured to image mask structures into a substrate plane (16), an input diffraction element (28) which is configured to convert irradiated measurement radiation (21) into at least two test waves (30) directed onto the projection optics (12) with differing propagation directions, a detection diffraction element (34; 28) which is disposed in the optical path of the test waves (30) after the latter have passed through the projection optics (12) and is configured to produce a detection beam (36) from the test waves (30) which has a mixture of radiation portions of both test waves (30), a photo detector (38) disposed in the optical path of the detection beam (36) which is configured to record the radiation intensity of the detection beam (36), time resolved, and an evaluation unit which is configured to determine the lateral imaging stability of the projection optics (12) from the radiation intensity recorded.