giving with L = 0.350 the value m2 = -1.538. The mirror separation d1 = -0.4225. Now the secondary is defined as spherical in this system, giving from (3.41) with bs2 = 0

4 Vm-2 + 1/ The first two equations of (3.292) then give <5S| = -0.621 Z = -0.924

and substitution of the values in the third equation shows that the conditions are fulfilled. Eq. (3.30) gives bs1 = +0.016 , a slight oblate spheroid but only 1.6% of a parabolic deformation of opposite sign.

In general, such a system can be set up with any geometry by solving Eqs. (3.292).

For the Baker system B above, the aspheric plate requires ¿S| = -0.621, about 2.5 times the value of -0.25 given by Eq. (3.229) with fi = -1 for the Schmidt of the same focal length. This is the price to be paid for reducing the length of the system. Since the chromatic errors increase linearly with the asphericity on the plate, Baker proposed an achromatic plate of the type discussed in § Without this, it is clear from the stop-shift equations (3.22) that the spherical aberration induced by the spherochromatism will lead to coma and astigmatism if the stop is shifted from the plate. Such chromatic effects are considered in detail by Schroeder [3.22(a)]. Because of these, it is no longer considered normal good practice to shift the stop from the plate as proposed by Baker (Fig. 3.42). The same remark applies to any system with a strong singlet aspheric plate producing appreciable spherochromatism.

A Baker-type telescope with plate diameter 84 cm was built in 1949 for the Boyden station (South Africa) of the Harvard College Observatory in association with the Armagh and Dublin Observatories (ADH - telescope) [3.39(c)]. The flat field has a diameter of 4.8° (26 cm) with relative aperture f/3.7. A similar system following a design by Linfoot [3.29] with an aperture of 50 cm was built for St. Andrews University in 1953. Linfoot [3.45] proposed a design with 2 spherical mirrors, but with appreciable astigmatism and chromatic aberration. Slevogt's modification [3.44] [3.46] of Baker's Type B system to make both mirrors spherical and accept a slight amount of astigmatism seems a more practical solution.

Further details of the performance of the Baker flat-field cameras are given by Schroeder [3.22].

Linfoot [3.45] [3.47] proposed a further interesting basic form, the concentric (monocentric) Schmidt-Cassegrain (Fig. 3.43). From its basic geometry, this is the most fundamental, indeed obvious, extension of the basic Schmidt to the Schmidt-Cassegrain. However, the monocentric form puts a special constraint on the radius of the secondary relative to that of the primary and rules out immediately the possibility of the flat field given by the Baker constraint of r2 = r1. In the general case, without restriction on the final image position, we have from the concentricity:

Also, from Eq. (2.54), we have for the equivalent focal length of a Cassegrain system in air

Fig. 3.43. Monocentric (concentric) Schmidt-Cassegrain proposed by Linfoot (1944)

11 1 d1

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