and the change of wavefront aberration is virtually independent of m2 if m2 ^ 1. This is physically obvious if we consider that the change in beam diameter at the secondary depends only on (y //')dd and that the aspheric form of the secondary converges on the parabola as m2 ^ ro.

We have seen above that the DK form is free from despace spherical aberration in the afocal case. A final case of interest is the focal case of a 2-mirror telescope, corrected for spherical aberration and with zero despace sensitivity (DE,0). Setting the bracket of (3.409) to zero gives the condition

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