  For a given asphere the grating equation is used to derive the design for a
Computer Generated Hologram (CGH), sometimes referred to as a diffractive null
corrector. The CGH converts a spherical wavefront to the shape appropriate for
the given asphere as required for interferometric metrology. The derivation is
effectively analytic but may require numerical implementation in certain cases
depending on the complexity of the shape of the asphere and the accuracy
required.
