Achromatized plate-mirror systems

A variant of the Schmidt-Cassegram optical system is described in which the single aspheric plate is replaced by two plates, of different glasses and opposite asphericities, whose colour-errors compensate each other. The use of two plates makes it possible to obtain not only apochromatism but also flat-fielded anastigmatism with both mirrors spherical. Design data are given for such two-sphere two-plate anastigmats and their distortion is shown to be small. Next, the effect of relaxing the strict anastigmatism condition is considered and it is shown that the admission of a small amount of Seidel astigmatism leads to a large extension of the range of available systems. Finally two aplanat types are selected which seem well suited to astronomical application.