[ATM] what causes coma?

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

It is a geometric characteristic of a parabolic mirror that incoming rays
that are parallel to the axis will focus at a point (neglecting the airy
disk).  When you look through the eyepiece (assuming properly collimated
optics), that is the object that you see in the center.  Parallel to the
mirror axis is the important point.  It is a common misconception that all
the incoming rays are parallel.  Certainly, one would think that coming from
hundreds of light-years, this must be the case.  But it is not.  What you
see through the eyepiece is a field of view.  The actual field of view (as
opposed to the apparent field) can be calculated from the telescope optics
and configuration.  This value is not zero.  That means that some rays not
entering parallel to the axis focus someplace else.  That place is some
distance horizontally from the center of view as viewed through the
eyepiece.  If this point were in the focal plane, then there would be no
coma.  In actuality, it is also some distance toward or away from the mirror
(further away for a parabolic mirror), making it a little out of focus.
This set of focus points traces out a curve when plotted near the focus
looking perpendicular to the mirror axis.  This is the caustic that you may
have heard about.  The out of focus from the focal plane is the coma.

-----Original Message-----
From: lance clarke [mailto:alaskawolfjoe@hotmail.com]
Sent: Monday, February 09, 2004 9:00 AM
To: atm@atmlist.net
Subject: [ATM] what causes coma?


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

Thanks
Lance Clarke
Toronto

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