
6.2. Two-Mirror Telescopes 115
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,K^,
and w^ 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 verif ...