Patent ID: 7573655

Claim:
A projection objective having an object plane, an image plane and an optical axis, the projection objective comprising: a concave mirror along the optical axis; a positive lens group along the optical axis, the positive lens group having positive refractive power, the positive lens group being arranged adjacent to the concave mirror and spaced apart therefrom, the positive lens group comprising optical elements; a first prism along the optical axis; and a second prism along the optical axis, wherein: the first and second prisms each have first and second flat surfaces; the second flat surfaces are arranged adjacent to the positive lens group and on opposite sides of the optical axis; the first flat surface of the first prism is arranged adjacent to the object plane; the first flat surface of the second prism is arranged adjacent to the image plane; 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; the optical elements of the positive lens group include a thickest optical element having a maximum thickness of optical elements within the positive lens group; at least the thickest optical element of the positive lens group and the first and second prisms are made from optical material having an absorption coefficient k<0.001 cm −1 in the design wavelength band; the projection objective is a catadioptric projection; and the projection objective has three or more common foci at respective three or more wavelengths in the wavelength band.