[ATM] what causes coma?

[dgood at aha.org (Good, Donald)]

Bob May replied:
Coma has its error in the angle of the beam so refocusing won't fix it at
all.  When you look at a comatic image, you will see that the light from the
center area is all pointed at the tip of the coma where the comet would be
and, as the light comes from further out on the mirror, the light goes
further up the tails.  This makes it difficult to find any particular point
at which the focus is best although the focusing to the center of the mirror
produces the sharpest of the comatic shape.

Don> I agree.  But the question was where is the best focus for coma.  My
point was that coma is fuzzy through its focus range (just like spherical
aberration), so go for the best focus at the center.</Don

Mike Peck replied:
At 07:43 11-02-04 -0600, Good, Donald wrote:
>precise than the best focus of the on-axis image with no coma.  So you
might
>as well go for the best central focus and the off-axis field will probably
>be in or near the best focus range that is characteristic of spherical
>aberration.

You can't compensate coma with defocus.

Don>I never said you could.</Don

Here's the best one sentence description of coma I've seen, from Smith's 
"Modern Optical Engineering," (3rd edition):

     "Coma can be defined as the variation of magnification with aperture." 
(p. 67).

Don>I don't agree.  Choose another parabolic mirror with double the
aperture.  Does the coma at the center change?  No, it is still zero.  A
statement about coma had better say something about angle.</Don

Combine with the next sentence to get the best two sentence description of 
coma I've seen:

     "Thus, when a bundle of oblique rays is incident on a lens with coma, 
the rays passing through the edge portions of the lens may be imaged at a 
different height than those passing through the center portion."

Notice there is no mention of spherical aberration here. There's a reason 
for that.

Don>That sounds a lot like spherical aberration.  The rays from the edge
focus at E and the rays from the center focus at C and C is greater that E.
The reason it was not mentioned might be that the author didn't think of it.

Clear skies
Don