= Coma
{disambiguate=optics}
{wiki}
= Coma
{synonym}
<Coma> makes an off-axis <optical point source> appear asymmetric, often with a compact head and a flaring tail like a comet. Different parts of the <optical pupil> send its <light rays> to different transverse image positions. A sharp central <star> image therefore does not guarantee sharp images across an <optical telescope>'s <field of view>. In an idealized, aligned system with <rotational symmetry>, <coma> vanishes on the <optical axis>; misalignment can introduce <coma> even at the nominal field centre.<Amateur Telescope Optics by Vladimir Sacek/Coma aberration>[Amateur Telescope Optics, §4.2, opening paragraphs and Figure 39: field coma versus misalignment coma]{ref}
<Coma> differs from <spherical aberration>, whose dependence on <optical pupil> radius can blur an on-axis point without selecting a transverse direction. <Astigmatism> gives different <optical foci> in two perpendicular sections of an off-axis bundle; a through-focus sequence can change its elongated image's orientation. <Diffraction> persists when these <optical aberrations> vanish. A real asymmetric <point spread function> may mix several effects, so its appearance alone is not a unique diagnosis of <coma>.
The classical primary description belongs to the nineteenth-century <Seidel aberration> classification of five primary monochromatic image errors.<Amateur Telescope Optics by Vladimir Sacek/Seidel aberrations, wavefront aberration function>[Amateur Telescope Optics, §3.5.1, opening paragraph: historical classification and equation 5]{ref} It remains useful in modern design: OpticStudio's <Seidel Coefficients by ANSYS, Inc.> documentation lists <coma> as $W_{131}$ and tabulates each surface's contribution as well as the system sum (Discussion). Cancellation of contributions can reduce <coma> without making every surface individually free of it. The same documentation limits this calculation to specified systems with axially symmetric surfaces; higher-order terms and broken <rotational symmetry> require more general <ray tracing>.
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