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Fig. 3.49. Spot-diagrams for an achromatic, monocentric Bouwers meniscus-Cassegrain telescope as in Fig. 3.48, but with a singlet field fiattener added. The geometry is lightly modified to 400 mm, f/3.11 - f/6.0, and the stop is shifted to the meniscus to

Fig. 3.50. Classical Bouwers telescope with additional weak lens at the stop

of the f/3 Schmidt with a singlet plate as Fig. 3.29. For the prime focus, we take the large angular field of ±3°. Fifth order spherical aberration causes an axial image core of about 1 arcsec. If the relative aperture is reduced to f/3.5, this effect declines rapidly to give about 0.6 arcsec.

Since the results of Fig. 3.51 are achieved with only spherical surfaces, this modified Bouwers system must rate as a very interesting solution. If further sophistication is desired, the fifth order spherical aberration can be corrected by aspherising one face of the weak lens, permitting higher relative apertures. This then becomes a mixed plate-meniscus solution, a type discussed in § 3.6.4.3.

Figure 3.51 can also be directly compared with the spot-diagrams of prime-focus Maksutov designs of the "short" type given in Figs. 3.35 and 3.37. Because of the weak lens at the pupil in the centre of curvature, the modified Bouwers form is appreciably longer than the "short" Maksutov.

As a concentric Cassegrain, the extended Bouwers telescope of Fig. 3.50 is also an attractive form. As with the Linfoot concentric Schmidt-Cassegrain, m2 = -2. Figure 3.52 shows spot-diagrams for a system with aperture 400 mm and f/3.0 - f/6.0. The field correction is so good that the angular field of ±1° is limited solely by the obstruction.

The Maksutov-Cassegrain: Maksutov solutions, in general, accept a departure from concentric symmetry. The most complete accounts of the possibilities are given in two books mainly intended for amateurs, since the amateur market with modest apertures has made most extensive use of such systems. The two references concerned are Rutten and van Venrooij [3.12(d)], frequently referred to above, and Mackintosh [3.48]. The latter is a collection of articles from the "Maksutov Circulars" for advanced amateurs.

Here we shall demonstrate the power of this solution by giving the results in detail for a "short" Maksutov-Cassegrain of the non-flat field type with an aperture of 400 mm and a primary working at f/3.5. This is slightly "slower" than the examples given in [3.12(d)] and relaxes the "zonal error" (fifth order spherical aberration).

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