We can use this 2-mirror formulation also for the PF case by taking the limit case of a flat secondary in Cassegrain geometry, giving a positive focal length /'. This gives the same result as Eqs. (3.219).

In these equations, is the contribution of the aspheric plate to Sj and spi is the distance of the plate from the primary in object space, a positive spi corresponding to a virtual plate position to the right of the primary with the light incident from the left. It was shown by Burch [4.10] that the above formulation can be extended to any number of plates with (5S|)i, (5Sj)2 ... and (spi///)1, (spi//')2 ..., the plate effects being additive. Furthermore, the effect of an aspheric plate in the convergent beam forming the image (Fig. 4.2) is the same as a plate at the conjugate point at spi in object space whose

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