Abstract: A method for swapping an optical system, such as a DUV mirror, of a projection exposure apparatus, comprises: a) raising the optical system along a centre axis of the optical system so that mount struts of the optical system pass out of contact with frame struts of a frame carrying the optical system; b) rotating the optical system about the centre axis so that the mount struts are arranged between the frame struts; c) lowering the optical system along the centre axis; and d) shifting the optical system perpendicularly to the centre axis so that the optical system is moved out of a housing.