## F di f m2di

u prl upri

from (2.75). Since L ^ 1 in the normalized system of Table 3.3, the astigmatism of the classical telescope is markedly higher for the same final focal length than that of the 1-mirror telescope given by Eq. (3.88). If we set m2 = —1 in Eq. (3.96), we have the limit case of a folding flat without optical power for the Cassegrain secondary and the astigmatism is the same as for the 1-mirror telescope with the stop at the primary.

In the afocal case, we have the limit case of the Mersenne telescope (Fig. 1.3 (a) and (b)) consisting of two confocal paraboloidal mirrors. Since f!, L and m2 are ro, the focal forms (3.94) and (3.96) would be singular in the normalized forms of Table 3.3 without their initial factors (v1/f!)3 and (v1/f )2 respectively, but reduce to zero with these factors. This is confirmed from the general afocal formulation of Table 3.6. From this, it is clear that, if bsi = bs2 = —1

then

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