"Mike Ruskai" wrote

Brian Tung wrote:

To see why it favors a tail, I think you would have to do the math, or see
a ray-trace diagram, or something like that. I'm not sure there's a good,
simple, first-order explanation in words alone.

A rotatable 3D diagram would probably be required for an intuitive grasp.

I think I have it now. I was confusing the shape of the mirror or lens with
the angle a ray of light hits the slope of the mirror or lens. A round mirror
would always be round, but the shape of a star depends on where on the
mirror the rays of light hit WRT how the face of the mirror is shaped.

So plotting the coma would have two functions: how far from the center of
the field you are, and how far off-focus you are. A star in the center would
have no coma; one on the edge would have maximum coma; in focus least
coma; out of focus the coma would increase until the image disappeared.

From that it follows that coma favors one side of the image because that
corresponds to the points on the mirror or lens that are farthest from the
object.

Would that also mean that an egg shaped mirror would produce egg shaped
stars even in the center of the field?

--
Craig Franck

Cortland, NY