Abstract: A compound optical system apparatus, having a main optical path with a reflector, a shutter, an objective lens, and an image-receiving surface; and an alternately activated folded auxiliary optical path, with quasi-collimative reflective elements, one closely peripherally disposed to the main optical path, for passing a relatively small auxiliary image through the objective lens and displaying it upon the image-receiving surface on the side of the main image that is opposite to the side where the one reflective element is positioned with respect to the objective lens.