whereas a spherical primary mirror alone would give f '/4. The spherical aberration is therefore of the order of 1/3 of the aberration of a spherical primary. This is over one and half times the spherical aberration of Eq. (4.6) for the single plate correcting coma alone and with the same g as the larger plate of the 2-plate corrector. This illustrates an important consequence of multiplate systems: because of compensations, the individual plates are stronger with corresponding effects on any uncorrected aberrations. The contribution S 1 of the larger plate is
which is almost exactly twice that of a single plate, from Eq. (4.6), correcting coma alone at the same distance g1.
Paul [4.4] drew the logical conclusion that a 2-plate corrector should be used with a hyperbolic primary to correct Si. The general condition of (4.5) can then be written:
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