[ATM] what causes coma?

[Hugues.Laroche at ses-astra.com (Hugues.Laroche@ses-astra.com)]

Hello Lance,

>I know coma is occurs when parabolic mirrors are used,
>and I know it gets worse the faster the mirror,
>but what I can't figure out is the reason for this.
>Can anybody give an easy explanation for this effect?

I can try, at least.

Contrarily to spherical aberration, coma is an off-axis
aberration. That means that a perfect paraboloid mirror
will put no coma in the image of a star that is perfectly
along the optical axis. OTOH, for a star that is not on the
optical axis, the light hits the mirror in a tilted
angle, and on one hand the different parts of the mirror
do not focus anyore exactly toward the same point, which
causes coma, and on the other hand the mirror as a whole
does not show the same curvature along the plane that
contains the optical axis + star as the curvaure of
the other plane (perpendicular), which
causes astigmtism. Well the 2 effects are a bit difficult
to separate conceptually, but in more or less fast
newtonians (dob or not) the coma is the most troublesome.

In short, a perfect paraboloid gives a "perfect"
(or at least, diffraction-limited that is, such that
you would not see the difference) image of a
vicinity of the sky around the optical axis.
This vicinity becomes vanishingly small as
the f/ratio goes under 3-4.

>Also, is this a problem in catadioptric systems
>where the primary is parabolic, ie a classical
>cass or gregorian?

The Schmidt camera has in principle no coma and
somehow the SCTs are catadioptrics build on top
on this design. Others probably know more than me
to say if they have or not coma (but they may have
residual color, spherical aberration at some mirror
offsets etc...)

The classical cass and gregorian both have
coma and astigmatism, and also curvature of field.
Now the famous Ritchey-Chretien and aplanatic
gregorian are nothing else than a cass or a greg
which shapes have been tweaked to cancel coma
without giving up on the spherical aberration.
(for the RC, both primary and secondary are hyperbolic,
while in the aplanatic greg, both are elliptic).
And they are free of chromatic aberration. As everything
is a matter of compromise, they are not free of other
inconvenients.

You are still with me?
Well I hope I did it right :-)

Have a good day, and a still better night!

Hugues
Luxembourg, Europe




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