"Good defined as tangential coma that measures 1 arc-sec.

With coma as the aberration that limits the size of the field, the placement of the aperture stop at any other position does not improve the image quality. Coma is independent of stop position when spherical aberration is zero, as noted in Section 5.5, and reducing astigmatism has little effect on the overall blur size.

The remaining items in Table 6.1 are related to field curvature. Here Kp,Km, and um are, respectively, the Petzval curvature, median curvature, and sag of the median surface midway between the tangential and sagittal focal surfaces. Given the limiting field radii in Table 6.2, the reader can verify that for any practical focal length / the median image surface is essentially flat.

In summary, then, the paraboloid telescope is limited to small fields with coma setting the field limit. All other aberrations are negligible over this field.

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