Abstract: Improved catoptric lens arrangements to form beams from the energy emitted by extended sources of wave propagated energy are disclosed. All embodiments utilize at least a pair of mirrors disposed along a common axis, one of such mirrors being convex and the other concave, the generatrices of the reflecting surfaces of such mirrors being curves with noncoincident focal points, such generatrices being always divergent. At least one source of wave propagated energy is disposed between such reflecting surfaces so that all of the wave propagated energy from such a source is reflected back and forth between the mirrors in passing to an exit aperture defined by the mirrors.