An exact analytical solution for the design of an optical system without spherical aberration that satisfies the sine condition is presented. The system consists of two parts. The first two mirrors form a strictly afocal system with a spherical primary and the second convex aspherical mirror. The next two mirrors form a focusing system that compensates the coma aberration of the afocal system. A complete description of the optical design, with system data, is given. The spherical primary mirror has a diameter of 10 m. The focal length of the entire system is 152 m, length is 13.5 m, angular field is 7 arcmin, and its theoretical resolution is 0.03 arcsec.