Patent ID: 6879383

Claim:
An optical system comprising along an optical axis: a concave spherical mirror; a lens group with positive refracting power arranged adjacent the mirror with an airspace therebetween, the lens group comprising in order farthest from, to closest to, the mirror: a plano-convex lens element having a convex surface facing the mirror and a flat surface facing away from the mirror; a first negative meniscus lens element adjacent the plano-convex lens and having a convex surface facing the mirror; and a second negative meniscus lens element adjacent the first negative meniscus lens and having a convex surface facing the mirror; and first and second prisms each having respective first and second flat surfaces, wherein the second flat surfaces are arranged adjacent the fiat surface of the plano-convex lens element on opposite sides of the optical axis, and wherein the first flat surfaces are arranged adjacent object and image planes, respectively; wherein each lens element in the lens group is formed from a different glass type such that the lens group and first and second prisms provide chromatic aberration correction in a spectral region that contains at least g, h and i-line wavelengths.