If focused near the smallest image blur circle, the aberration function of misalignment is similar to the complicated form that appears in Fig. 6-2. That figure scarcely does justice to the deformation. The phase surface is like a drumhead that has just taken an enormous off-center blow. One half-circle is deformed downward, and the other half-circle has a balancing bulge upward. Added to this form is a small amount of the saddle or potato-chip deformation characteristic of astigmatism. The aggregate result is a wonderfully interesting surface to investigate by calculations, but it is bad news if it describes the wavefront of a telescope.
The functional form of the coma aberration is j coma
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