OPTICAL SYSTEM OF A MICROLITHOGRAPHIC PROJECTION EXPOSURE APPARATUS

In a method for improving imaging properties of an illumination system or a projection objective of a microlithographic projection exposure apparatus, which comprises an optical element having a surface, the shape of the surface is measured directly at various points. To this end, a measuring beam is directed on the points, and the reflected or refracted beam is measured, e.g. using an interferometer. Based on deviations of the measured shape from a target shape, corrective measures are derived so that the imaging errors of the optical system are improved. The corrective measures may comprise a change in the position or the shape of the optical element being analyzed, or another optical element of the optical system. The target shape of the surface may, for example, be determined so that the optical element at least partially corrects imaging errors caused by other optical elements.