Abstract: An improved aperture beam splitter comprises an intercepting mirror positioned at an aperture image plane that is remote from any focus so as to deflect one half of the energy in a beam of incident radiant energy. the aperture image plane corresponds to an aperture stop and one half of an incident beam of radiant energy is lost at the aperture image plane. The remaining energy supplies the input to an optical system. The radiant energy from the output of the optical system reaches an image plane corresponding to the aperture image plane. Preferably, the optical paths of the incident and returning radiant energy are sufficiently coincident in space that the aperture image plane returns to the intercepting mirror. If the optical system forms the radiant energy into an odd number of foci, the image formed at the aperture image plane experiences a mirror symmetry reversal.