An imaging catoptric optical unit has at least four mirror, which image an object field in an object plane into an image field in an image plane. A first chief ray plane of the optical unit is prescribed by propagation of a chief ray of a central object field point during the reflection at one of the mirrors. A second chief ray plane of the optical unit is prescribed by propagation of the chief ray of the central object field point during the reflection at one of the other mirrors. The two chief ray planes include an angle that differs from 0. In an alternative or additional aspect, the imaging optical unit, considered via the image field, has a maximum diattenuation of 10% or a diattenuation that prefers a tangential polarization of the imaging light for a respectively considered illumination angle. The result of both aspects is an imaging optical unit in which bothersome polarization influences are reduced during the reflection of imaging light at the mirrors of the imaging optical unit.