ABSTRACT
An optical system which uses only mirrors as imaging elements is termed a catoptric system. A concave mirror, whether spherical or conicoid in form, has several advantages in optical system design. A concave spherical mirror has a focal length of half the radius of curvature. If an aperture stop is positioned at the centre of curvature, coma, astigmatism and distortion are zero. The mirror radius is twice that of the spherical image surface. This property is useful in imaging a flat surface on to a curved surface photodetector, or the reverse for video projection. The possibilities of folding a long optical path and correcting the aberrations of the primary mirror by the properties of the secondary mirror have been explored in great detail by astronomers, and various classical designs using conicoid surfaces. A spherical mirror has only one-quarter the curvature of a lens having the same power, so large apertures are possible without excessive residual aberrations.
