Fig. 3.46. Spot-diagrams for the aplanatic Schmidt-Cassegrain system of Fig. 3.45 with 400 mm, f/2 - f/10, and an achromatic corrector plate Further developments of meniscus-type systems. The first obvious extension of the Bouwers-Maksutov concepts discussed in §§ 3.6.1 and 3.6.3 is the Bouwers-Cassegrain or the Maksutov-Cassegrain.

The basic principles of the layout of such a system are essentially the same as those of the Schmidt-Cassegrain, the role of the corrector plate being replaced by some meniscus-type system. However, the theory of aspheric plates is particularly elegant and simple, as was shown in § 3.4. Eqs. (3.270) and (3.282) showed that the effect of a meniscus is more complex, both monochromatically and chromatically. Nevertheless, the formulae given in § 3.6.3 enable a Maksutov-Cassegrain to be laid out in a way similar to the Schmidt-Cassegrain. The asphericity term of the plate is replaced by the meniscus contribution to si, respecting of course, the chromatic condition (3.283) for the Maksutov-type meniscus.

Quasi-concentric (monocentric) Bouwers-Cassegrain: Bouwers [3.32] proposed a two-glass concentric form of meniscus to get round the problem of longitudinal chromatic aberration. This can be used either in a prime focus Bouwers telescope or in a Bouwers-Cassegrain, as shown in Fig. 3.48. Instead of departing from concentricity according to the Maksutov formula (3.283) to achieve longitudinal achromatism, Bouwers proposed an achromatic concentric meniscus with two glasses separated by a plane surface. Since the effect of the meniscus is that of a negative lens, the positive half of the meniscus will have the higher dispersion glass. Monochromatically, the system remains strictly monocentric (more so than the monocentric Schmidt-Cassegrain because of asymmetries of oblique pencils at the aspheric plate) if the refractive index for the central wavelength is the same for both glasses, i.e. on a horizontal line of the glass diagram of Fig. 3.32. The convex, right-hand half of the meniscus must have the same dispersive power with its (weaker) concentric surface as the non-concentric right-hand surface of the Maksutov meniscus provides. The requirement of a central wavelength index which is the same for both glasses (and preferably good blue transmission for the high dispersion glass) pushes the designer towards expensive optical glasses. This is no doubt the reason why this form has been little used by both amateurs and professionals. However, its correction possibilities are extremely interesting, since the complete symmetry together with the basic correction of si by the Bouwers concentric meniscus give for a theoretical field of 180° apart from vignetting by the stop:

Sj = Sjj = Sjjj = 0 siv = 1/f', for the normalization H2 = 1. This is the same as for the monocentric Schmidt-Cassegrain, but is valid for a much larger field. Of course, the concentric meniscus will still limit the f/no because of fifth order spherical aberration.




System Aperture Ray aining Gaussian Factor Eff. Focal Len. Total Track Image Space F/# Working F/# Obj . Space N.A. Stop Radius Parax . Ima.. Hgt. Parax. Mag. Entr. Pup. Dia. Entr. Pup. Pos. Exit Pupil Dia. Exit Pupil Pos. Maximum Field Primary Wave Lens Units Angular Mag.

Fields :

Field Type: Angle # X-Value

1 0.000000 2 0.000000 3 0.000000

Entrance Pupil Diameter Off

0.000000 1300 1807.51 3.24999 3.25001 2e-008 200 34.0416

Value 0.365000 0.405000 0.486000

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