
4.5. Aberration Compensation 61
At this stage five third-order aberrations have been identified: spherical
aberration, coma, astigmatism, distortion, and curvature of field. The first of
these is independent of field angle, but all others depend on some power of
6.
The
first three aberrations in this list affect image quality, while the last two affect only
image position.
From Eqs. (4.2.7b) and (4.4.1) we see that the transverse aberrations depend
on aperture radius y (or r) and field angle 9 according to the relation
transverse aberration a
y^O"",
(4.4.2)
where n-\-m = 3. Hence each of these aberrations is called a third-order
aberration