[ATM] what causes coma?

[mpeck1 at ix.netcom.com (Michael Peck)]

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
>aberation.

You can't compensate coma with defocus.

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).

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.

Mike Peck

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