[ATM] what causes coma?

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

My previous post omitted equations on purpose to describe the cause of coma
rather than to evaluate its prominence.  It was also requested - no math.
My conclusion for coma is that that it is a slightly distorted form of
spherical aberation that occurs in parabolic mirrors (assuming no correctors
as in a Newtonian) at the off-axis focal field point.  It is non-existant on
the axis and increases proportionally to its maximum at the edge of the
field of view (FOV).  Like spherical aberation, it best focus is less
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.

A parabolic mirror does have a slight curvature of field.  You generally
have to accept the curvature that you get for visual purposes along with the
curvature of the eyepiece and the manner that the two interact.  You may be
able to pick an eyepiece to compensate, but that is beyond this post.  For
photographic use, a curved film plane might help.  But coma is not a field
curvature effect (as my first post incorrectly implied).

A knife edge is usually placed to evaluate the on-axis focal point.  I am
not sure how you could use it to evaluate an off-axis field point.  Also, I
thought that it would be used in a Foucault test setup rather than in a star
test which examines the on-axis star image with an eyepiece.  If you could,
I think it would be similar to examining on-axis spherical aberation on a
spherical mirror, except that the focus line of the off-axis star image is
both tilted and curved:

                           \C F C/
(focal plane)               ) | (


~  ~
(lots of space)
~  ~

_____________________________________________________________
(parabolic mirror)

| at F = the best focus range of on-axis spherical aberation of a spherical
mirror.
) at C = the coma focus curve of a parabolic mirror.  Imagine that they have
some tilt like the slash characters above them.  Note that the off-axis
spherical aberation of a spherical mirror would be straight like the
slashes.

Don


-----Original Message-----
From: Mutalib Abdallah [mailto:omegatroid@hotmail.com]
Sent: Tuesday, February 10, 2004 8:10 PM
To: Good, Donald; mdholm@telerama.com; atm@atmlist.net
Subject: RE: [ATM] what causes coma?


Where is the best focus when you have coma?  Are off-axis stars focused best

on the same flat focal plane as on-axis stars, or is a curved focal plane 
required?   Perhaps some (but not all)of the coma could be conquered with a 
curved focal plane.

Also, I am still wondering what off axis coma looks like in a star test when

you replace your eyepiece with a knife edge in a film can.