a retro - reflector which comprises two spherical mirrors , namely a concave main mirror and a convex secondary mirror , which are coaxial to one another and whose respective radii are in the ratio 2 : 1 , the secondary mirror facing and being spaced from the main mirror by a distance at the most equal to its radius . means are provided for forming a circular pupil centered on the center of curvature of the main mirror and a regulatable diaphragm positioned in front of the secondary mirror . an optoelectrical shutter can be positioned in the pupil to modulate the reflected beam and provide the optical responder function .