the second term in the round bracket being the supplement compared with the classical telescope. With the normalized geometry of Tables 3.2 and 3.3, this supplement is very large, giving in the Cassegrain case
a value some three times higher than that of the equivalent DK and nearly thirty times higher than that of the classical telescope (Table 3.3, Cases 3 and 6).
In the afocal case, substitution of the relations (2.55) and (2.72) in (3.141) leads to the non-singular form
4Ra upr 1
which can also be deduced at once from Table 3.6 and (3.137).
We now derive the field astigmatism of the SP form. We assume, as for the DK form, that the stop is at the primary: otherwise there will be a very large stop-shift term from the third term of (3.61) because of the huge value of (E S//)sp. Then
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