A catadioptric projection objective has, along an optical axis, a concave mirror; a positive lens group with positive refractive power arranged adjacent to the concave mirror and spaced apart therefrom; a first prism and a second prism each having respective first fiat surfaces and second flat surfaces, wherein the second flat surfaces are arranged adjacent to the positive lens group and on opposite sides of the optical axis, and wherein the first flat surfaces are arranged adjacent to an object plane and an image plane, respectively, of the projection objective. The projection objective has unit magnification and at least one focus at a wavelength in a design wavelength band that includes mercury g-, h- and i-lines. At least the thickest optical element of the positive lens group and the first and second prism are made from optical material having an absorption coefficient k<0.001 cm.sup.-1 in the design wavelength band.