## Cd

Fig. 3.47. Spot-diagrams for a Slevogt aplanatic Schmidt-Cassegrain with 400 mm, f/2.0 - f/3.25, and an achromatic corrector plate. The field is flat

It can easily be shown that the change of dispersion required for the second glass is very modest, so that this proposal by Bouwers can be a practical proposition if the glass costs are not a dominant matter. Eq. (3.284) gave the concentricity departure of the second surface of a Maksutov meniscus as

This is, then, the change dr2 required to correct the longitudinal chromatic aberration CI. Consider this as a change in power of the right-hand half of the concentric meniscus shown in Fig. 3.48, initially having the same glass as the left-hand half. With the approximation of thin lenses, the power Km+ of the right-hand positive half is given by

giving

It can be shown [3.3] [3.6] that such a thin lens gives the longitudinal chromatic aberration y2

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