which is negative since f' and g are positive. The required form of the primary is thus hyperbolic and the aspheric form of the plate is the opposite of a Schmidt plate correcting a spherical primary. If bs1 has the value for an RC primary, then we have the Gascoigne PF aspheric corrector [4.7] [4.9]. The necessary position of the plate is given at once by transforming (4.9):
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