Fw: [ATM] what causes coma?

[stuart452 at earthlink.net (Stuart Hutchins)]

Please disregard my previous post on this subject.  Little error in math:
derivative of x^2 is 2x..  It's been a few years since I did this myself.
Corrected below:


 As others have said, coma in a mirror image is caused by the geometry of a
 parabolic reflector.  It can be demonstrated by tracing only three rays in
a
 single plane.  Off axis rays don't converge to a single point.  If you can
 accurately graph the function Y = .125X^2 for X from -1 to +1, you will
have
 drawn a parabola that is approximately f/1.  At X = 1, the value of Y is
 .125.  The slope is  .25 which you can use to construct the normal to the
surface.  You actually only need to plot X = -1, 0, and 1.

 Using a scale of 10cm = 1, it will fit nicely on standard paper. Then you
 can draw the reflected rays using a compass to make angle of reflection =
 angle of incidence.

 That shows the effect, not the cause, which unfortunately is in the math
 (geometry).  The popular book, Telescope Optics, by Rutten and van Venrooij
 from Willman-Bell is a readable source for understanding all kinds of
 aberrations.

 Stuart
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