This is identical with the result of Eq. (3.87) for a 1-mirror telescope if we replace f1 by f , the focal length of the 2-mirror combination. For the cases of Table 3.3 with y1 and the final focal lengths normalized to unity, the field coma is identical for the 1-mirror and classical Cassegrain and Gregory cases, an important result. However, it must be remembered that the classical 2-mirror telescopes will give a lower value in practical cases if their relative aperture is smaller.
The field astigmatism is given by Eq. (3.61). With E SI = 0, the last term is again zero. The other stop-shift term in the bracket can be written f (2di£ + f!) , which contains in the bracket twice the stop-independent coma of Eq. (3.60). In the general case, then, the classical telescope has the astigmatism
since £ = 0. If the stop is at the primary, this reduces to
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