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Fig. 3.61. Double-meniscus system due to Wynne [3.59] (schematic)

Double or multiple meniscus systems without corrector 'plate: The paper of Maksutov in 1944 [3.38] was remarkably complete with regard to the possibilities with a single meniscus. Shortly after, in 1947, Wynne [3.59] [3.60] extended the analysis in an important paper to two menisci more or less concentrically disposed round the stop. These systems are also discussed by Maxwell [3.52], who refers also to such a stop-symmetrical two-meniscus system designed earlier by Bennett [3.61]. Bennett stated that his system was self-achromatic. Wynne optimized the balance of the centres of curvature of the surfaces of the system with regard to the stop (Fig. 3.61). He showed that not only the longitudinal chromatic aberration C1 could be much improved in this way, but also that the zonal (fifth order) spherical aberration could be considerably reduced for a given total meniscus thickness. Furthermore, the symmetry balance of the centres of curvature of the surfaces shown in Fig. 3.61 improves the field correction. The system can therefore be used at higher relative apertures than a single meniscus system, or the field may be extended since transverse chromatic aberration C2 and astigmatism can also be influenced.

Wynne also investigated in detail the effect of an asymmetry of the thicknesses of the menisci, giving the system shown in Fig. 3.62. This asymmetry permits both the longitudinal and transverse chromatic aberrations Ci and C2 to be fully corrected. A further advantage is that fifth order spherical aberration can be largely eliminated, giving excellent axial performance at high apertures (f/1); but the off-axis correction is made worse. For wide fields, therefore, the more symmetrical design of Fig. 3.61 is to be preferred.

Maxwell [3.52] also mentions the principal advantages of an aspheric extension of the Wynne symmetrical double-meniscus system of Fig. 3.61, in which the two inner meniscus surfaces are aspherics. This gives an extremely powerful control of both fifth order spherical aberration and spherochroma-

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