Eliminating s2 and f1 from (2.71), (2.72) and (2.76):
Eliminating L from (2.74) and (2.79) gives f2 = dl(b - dl) (2.80) 2 f - b + 2dl ()
We now introduce a further important constructional parameter P in Figs. 2.11 and 2.12, representing the distance between the primary and secondary images. This is a positive quantity in both the Gregory and Cassegrain forms and is defined by
Transposing (2.63) gives
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