From: "James E. Gort" 
To: Bianco Giuseppe 
CC: atm{at}shore.net
Reply-To: "James E. Gort" 


Giuseppe,

You are correct - with an explanation.  To parabolize, we must get the
paraxial rays to focus closer to the mirror (relative to the marginal rays)
OR get the marginal rays to focus further away.  In the process of doing
either, we get an ellipsoid.  If we overshot it and the marginal rays
focussed further from the mirror than the paraxial rays, we'd have an
hyperboloid.

So which of the two options are easier to do?  The classical method is to
reduce the radius of curvature of the central region (relative to the outer
region) because there's less glass to remove and perhaps easier to control.
 But one could also lengthen the radius of curvature of the outer regions
if one chose.

Hope that helps.

James Gort

--- BBBS/NT v4.00 MP