The constructional data and surface parameters of a tilted two-mirror optical system with eliminated spherical aberration, coma and astigmatizm at the center of the field may be sloved by analytic method. By ray tracing with those solved data, the image size at the center of the field is less than 0.2 arc second. For reducing the image at the outter part of the field, it is necessary to add higher order terms in the equation of the bicurvature mirror, and at the same time deviate some parameters solved by equations. For an F/11 system, we may expect 1.5″～1.9″ image spot size at ±0.6° field. The meridional and sagittal focal lengthes of such a system are not equal, the difference being less than 1%.