Fig. 3.41. Spot-diagrams for a Wright-Vaisala telescope of aperture 400 mm and f/4.0
Petzval sum (E SI v = 0). All systems fulfil the conditions ^2 Si = E Si I = Siii = 0, giving therefore a corrected flat field in front of the primary mirror. The different types differ only in the position of the corrector plate. Type A is the shortest - the length from plate to primary can be reduced to only 3 f' or less - but requires appreciably aspheric mirrors and a very aspheric plate. Baker also proposes a front baffle which loses some of the advantage of the very short plate length. (It is important to remember that, as a result of the stop-shift formulae (3.22), the stop position is - to third order accuracy - of no consequence in these systems because ^2 S i ... E S Iv are all zero). Types B and C are dimensionally almost identical: type B has a spherical secondary and lightly aspheric primary, type C a spherical primary and lightly aspheric secondary. Type D corrects the further condition of distortion (E Sv = 0) but requires significant asphericities on the mirrors, as with type A. As a consequence of the fulfilment of the condition ^2 S IV = 0 requiring |f2| = If |, all the Baker systems require an image position in front of the primary to achieve an acceptable obstruction ratio Ra. For a Cassegrain telescope with the following quantities defined as positive from Tables 2.2 and 2.3, we have from (2.57) and (2.58)
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