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Chapter 2. Geometrical optics
in (2.216, 2.217) represent isotropic coma and anisotropic coma, respectively. The aberration figure for coma is plotted in Figure 2.13.

Each circle represents the aberrated transverse position in the

Figure 2.13: Coma.
Gaussian image plane for a given radial coordinate in the aperture plane. The smallest circle corresponds to the zone nearest
to the center of the aperture, while the largest circle corresponds
to the zone nearest to the rim of the aperture. The figure with
all of the circles is plotted for a single point in the object plane.
Physically, coma arises because the transverse magnification varies
with radial coordinate in the aperture plane. Isotropic coma has
a figure which is aligned with the radial direction in the Gaussian
image plane, while anisotropic coma has a figure which is oriented
in the azimuthal direction. The intensity represnts a blur which
resembles a comet, hence the name coma. One can show that the
angle between the two lines which form the envelope for all of the
circles is always 60 degrees.