[ATM] what causes coma?

[stuart452 at earthlink.net (Stuart Hutchins)]

As others have said, coma in a mirror image is caused by the geometry of a
parabolic reflector.  It can be demonstrated by tracing only three rays in a
single plane.  Off axis rays don't converge to a single point.  If you can
accurately graph the function Y = .25X^2 for X from -1 to +1, you will have
drawn a parabola that is approximately f/1.  At X = 1, the value of Y is
.25.  The slope is also .25.  You actually only need to plot X = -1, 0, and
1.

Using a scale of 10cm = 1, it will fit nicely on standard paper. Then you
can draw the reflected rays using a compass to make angle of reflection =
angle of incidence.

That shows the effect, not the cause, which unfortunately is in the math
(geometry).  The popular book, Telescope Optics, by Rutten and van Venrooij
from Willman-Bell is a readable source for understanding all kinds of
aberrations.