L Ra

giving Ra = L/f' = L with our normalization f' = 1. Baker's systems have L = 0.350 with the geometry he proposed. This is a rather high value, bearing in mind the field supplement required which brings the real physical obstruction up to about 0.5. But it can still be acceptable in view of the other advantages. If the image position were in the more conventional position behind the primary, the obstruction ratio would normally be unacceptable.

Baker considered Type B the most interesting because it reduced the length to some two thirds of that of the classical Schmidt while allowing a spherical secondary and only slightly aspheric primary. The spherical secondary is a great advantage in manufacture: Baker pointed out that the primary can be used as an interference proof plate, since its central area hardly departs from the sphere. We will now take the Type B system as an example for third order analysis. The data of the system [3.5] [3.23(c)] [3.43]

Table 3.13. Optical data of Baker Schmidt-Cassegrain Type B with f/3.0 and f' = 1

r

d

D'

bs

(Plate)

+ 1.394

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