(a) Scheme of the Montel-type optics. Rays are reflected from both surfaces simultaneously unlike KB systems. (b) Side view of the mirror system showing the Invar corner piece and a chamfer along one edge of each mirror surface. (c) Representation of a laterally graded bilayer (A and B) showing the thickness variation from one end to the other. (d) Reflection geometry of a parabolic-shape mirror.