A New Catadioptric System

A catadioptric objective system is described which, it is claimed, has considerably improved performance in terms of aperture-ratio and diameter of field compared with other designs of a similar kind. This is combined with greater simplicity and economy of construction. In addition to the full-aperture spherical primary mirror there are two separate refracting elements (which are achromatised individually) which have about one-third of the diameter of the full aperture, the whole forming a positive-negative-positive combination as in the Cooke Triplet while using the principle of the Cassegrain reflector. Although the main advantage of the system over refracting telephoto lenses for small-format cameras lies in its compactness, in large sizes up to many metres focal length, its freedom from secondary spectrum, the shortness of the full-diameter tube—about one-quarter of the focal length—and the comparative ease of construction should allow it to compete, in some degree, with the Schmidt camera; particularly as the focal plane is flat and in an accessible position. A typical specification is included for an objective of 50 cm focal length.