The effective field curvature is given from Eq. (3.203) and corresponding to Table 3.3 by from (3.289) and (3.290). Thus the optimum field is flat in the Wright-Väisälä telescope, as in the original Schwarzschild telescope of Fig. 3.9 but with a far more favourable constructional length.
From (3.289) and Table 3.4, the astigmatism of the Wright-Vaisala telescope is only half that of a spherical or parabolic primary with the pupil at its pole and only about 1/8 of that of an RC telescope of the same focal length (see Table 3.3), as well as giving the further advantage of a flat optimum field. Its field can therefore be more than twice as large as that of an RC telescope, comfortably covering 1° or more. But, because of its astigmatism, it cannot compete with the field of the classical Schmidt.
The classical Schmidt gives with Eqs. (3.219), setting spi = +2/1 and f'
bs1 = 0, the plate deformation SSj = + f41. Eqs. (3.287) show therefore that the Wright-Vaisala telescope requires twice the plate asphericity of the classical Schmidt and that the mirror has an asphericity of the same amount as a parabola but of opposite sign. The required form is therefore an oblate spheroid - see Eq. (3.13). Dimitroff and Baker [3.23(a)] list three cameras of the Wright type built in the United States with apertures up to 20 cm at f/4. Vaisalä built one in Turku, Finland [3.42]. In general, the steep aspherics have been a disadvantage compared with Maksutov solutions, in spite of the favourable length.
Figure 3.41 shows spot-diagrams for a Wright-Vaisala telescope of aperture 400 mm and f/4.0. The image quality is solely limited by the uncorrected astigmatism, growing as the square of the field.
Schmidt-Cassegrain systems: In the general case, since Schmidt-Cassegrain systems provide three separated aspheric surfaces, the three aberrations £ Si, Sii and £ Sni can be corrected according to Eqs. (3.220) for any geometry of the optical system. Of course, if the separations or power parameters of the mirrors are unfavourable, very high asphericities can result. Conversely, favourable choices for the geometrical parameters can produce excellent solutions with zero or low asphericity on the mirrors. Such possibilities were first systematically analysed in a classic paper by Baker [3.43]. Shorter, but excellent accounts are also given by Bahner [3.5], Dimitroff and Baker [3.23(c)], Känig-Kähler [3.30(a)], Slevogt [3.44], Schroeder [3.22] and Riekher [3.39(b)].
Baker proposed four different systems, types A, B, C, D (Fig. 3.42), which largely covered the range of interest of the 3-element Schmidt-Cassegrain. The systems in Fig. 3.42 are shown on the same scale of the equivalent focal length, the fifth case being the classical Schmidt for comparison. In all four Baker systems, the mirrors have equal radius of opposite signs, giving zero
File : C:\ZEMAX-EE\WRIGHT.RAY Title: WRIGHT-VAISALA Date : Fri Mar 03 1995
GENERAL LENS DATA:
System Aperture Flay aining Gaussian Factor Eff. Focal Len. Total Track Image Space F/# Working F/# Obj . Space N.A. Stop Radius Paras.Ima. Hgt. Paras. Mag
Entrance Pupil Dian Off
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