cancel all third-order aberrations. In general these involve solving a relatively complicated set of equations. A few configurations are simple enough, however, that they could be designed starting from a few intuitive concepts.

Paul telescope

The first were proposed in 1935 by Maurice Paul. The basic idea behind Paul's solution is that spherical mirrors, with an aperture stop at the centre of curvature, have only spherical aberration - no coma or astigmatism (but they do produce an image on a curved surface of half the radius

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