On 2004-06-17, Craig Franck wrote:

In 4.2.2 they discuss coma and attribute it to "the intersection of
rays not being symmetrical." Shouldn't "off axis light" come into the
telescope in a symmetrical fashion when confronting an evenly distributed
light source? If one were to rotate the lens or mirror, would the coma
rotate as well?

Look at a cross section of a parabolic mirror. With light rays parallel
to the axis of the parabola the rays striking the left hand side of the
mirror make the same angle with the surface of the glass as the rays
striking the right hand side of the mirror and the rays all meet at the
center. With rays coming in off axis the rays striking the left hand
side of the mirror no longer make the same angles as rays striking the
right hand side of the mirror and the rays don't all meet at the center.
This is what causes coma. With parabolic mirrors the image of an off
axis star spreads away from the center. The diagrams in the book
show exactly what happens. Since lenses and mirrors are figures of
rotation, rotating the lens or mirror on it's axis doesn't change
anything.