1. A device for optically reducing the effects of undesired background radiation on an optical system, comprising: means for receiving an incident beam of light containing light radiated from a desired object and background radiation from a background radiation source and for splitting said incident beam of light into first and second beams; and means for combining said first and second beams to generate an interference fringe pattern on the background radiation and a representation of the image of the desired object at a predetermined focal plane, the representation of the image of the desired object being disposed on a dark fringe of the interference pattern of the background radiation, said combining means including means responsive to the curvature of the wavefront of the light radiated from the desired image and to the curvature of the wavefront of the background radiation for rendering said representation of said image movable along said focal plane upon relative movement of said desired object and said device while maintaining the position of the interference pattern of said background radiation relatively independent of the relative position of said device and said background radiation sources.