with E = spi //' for the virtual plate in object space at a distance spi from the primary, as shown in Fig. 4.5. A paraxial ray must be traced back from the real plate at distance g inside the secondary image to the conjugate point in the object space. If the paraxial parameters ri, r2 and di are first eliminated to give an expression in /', L, m2 and g (g being defined as positive and the other quantities having the signs given in Chap. 2 for the Cassegrain and Gregory cases), one can derive f J f, f, sp = f I g - L

The second term of this expression can be simplified by eliminating m2 in terms of d1, a negative quantity in both cases, giving f, L

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