Sagittal astigmatism (field ± 1°)
— 7 arcsec
Tangential astigmatism (field ± 1°)
In its geometry, the Schwarzschild telescope suffers from the major drawbacks of a relatively high obstruction ratio (> 0.5 with the field effect), a system length of 1.25f and, above all, the exposed position of the final image which is difficult for baffling and awkward of access. For the telescope sizes of his time, however, these latter two drawbacks were less serious than they are today. The given geometry was determined by the effective "flat-field" condition as follows.
The astigmatism of an aplanatic telescope is given from Eq. (3.119) by
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