A radiation-collecting optical assembly designed to form an optical entrance of a seeking device for guiding a self-propelled machine, said collecting optical assembly comprising: - a convex mirror , which has a reflecting face and an optical axis (A-A); - a concave mirror , which is placed in front of the reflecting face of the convex mirror and facing said convex mirror, the concave mirror being provided with an aperture (O) through a central portion of said concave mirror, the concave and convex mirrors having same optical axis (A-A), and said optical axis passing through the aperture of the concave mirror at a central point of said aperture; and - at least one window that is transparent to the radiation, and that extends behind the convex mirror , on a side opposite the reflecting face of said convex mirror, the assembly being designed so that part of an electromagnetic radiation that passes through the aperture (O) of the concave mirror in direction of the convex mirror is reflected by said convex mirror then by the concave mirror, then passes through the window , and then propagates behind the convex mirror, and being further designed so that a fluid that passes through the aperture (O) of the concave mirror in direction of the convex mirror is deviated in front of said convex mirror, thereby acquiring a radial velocity component that is divergent with respect to the optical axis (A-A), and then flows between the window and the concave mirror, the convex and concave mirrors being designed and placed so that electromagnetic radiation (F 1 ) that passes through the aperture (O) of the concave mirror parallel to the optical axis (A-A) is focused by the collecting optical assembly on an image point (I 1 ) that is located on said optical axis behind the convex mirror. - a convex mirror , which has a reflecting face and an optical axis (A-A); - a concave mirror , which is placed in front of the reflecting face of the convex mirror and facing said convex mirror, the concave mirror being provided with an aperture (O) through a central portion of said concave mirror, the concave and convex mirrors having same optical axis (A-A), and said optical axis passing through the aperture of the concave mirror at a central point of said aperture; and - at least one window that is transparent to the radiation, and that extends behind the convex mirror , on a side opposite the reflecting face of said convex mirror, the assembly being designed so that part of an electromagnetic radiation that passes through the aperture (O) of the concave mirror in direction of the convex mirror is reflected by said convex mirror then by the concave mirror, then passes through the window , and then propagates behind the convex mirror, and being further designed so that a fluid that passes through the aperture (O) of the concave mirror in direction of the convex mirror is deviated in front of said convex mirror, thereby acquiring a radial velocity component that is divergent with respect to the optical axis (A-A), and then flows between the window and the concave mirror, the convex and concave mirrors being designed and placed so that electromagnetic radiation (F 1 ) that passes through the aperture (O) of the concave mirror parallel to the optical axis (A-A) is focused by the collecting optical assembly on an image point (I 1 ) that is located on said optical axis behind the convex mirror. <EOS>