Re: ATM : what is Coma

["Michael P. Lindner" <mikel@att.net>]

Ashish, Balaya (IE10) wrote:
> 
> ...what is coma and why in
> heaven's name can't a concave mirror be figured to remove coma?

>From "Telescope Optics Evaluation and Design" by Rutten and van
Venrooij, Willman-Bell - a good resource for the ATM, by the way...

"Coma occurs in an oblique bundle of light when the intersection of the
rays is not symmetrical, but is shifted with respect to the axis of the
bundle. Off axis light rays passing through the lens near its edge
intersect the image surface at different heights than those that pass
through the center of the aperture."

Basically, for off-axis light, the light hitting the center of the
mirror comes to focus at a different place than thelight hitting the
edge of the mirror.

"In order to be free of coma, a system which is free of spherical
aberration already must comply with the Abbe sine condition. Discovered
by Ernst Abbe, the sine condition requires that in a system free of
coma, every exit ray of an oncoming beam of rays parallel to the axis
will satisfy the condition:

        h/sin(U') = C

where h is the ray height before it enters the system, U' is the angle
between the ray and the optical axis as it travels toward focus and C is
a constant which we will define below. This implies that somewhere
between the entrance pupil and the focal plane there exists and
imaginary spherical surace connecting the parallel entering rays and the
exit rays with its center at F, the focal point."

It's a lot clearer with the diagrams, but basically, a single surface
cannot be made free of spherical aberration and coma at the same time.

--
Mike Lindner