Re: [ATM] parabolic versus spherical volume of glass

["vladimir sacek" <v959@msn.com>]

Still not done. Guy did me good with his question,
because it heleped me clear some misconceptions. A
70% zone doesn't focus at the circle of least confusion -
as I unjustly accused the literature of claiming - but 
at the best focus of the spherical mirror. It is so obvious, 
wonder how I haven't noticed it. Guess my brain was in
one of its dumb modes.

After putting things together, I crunched some numbers,
which showed that least amount of glass to remove is really
for the parabola focusing at the best focus, where similar 
(greatest) depth of glass is taken from both center and 
edge, and 70% zone is worked the least the least (~0.0025 ci 
for 16.5" f/5 mirror). Working center somewhat more than the edge,
and ~87% zone the least, requires somewhat more glass
removed (~.0038 ci ), but has the advantage of less activity 
around the edge. Deepening from the center out requires 
~0.009 ci removed, and from the edge in (least desirable)
~0.011 ci. (the former parabola focuses at the marginal focus
of the initial sphere, the latter at the paraxial).

Why do I have to spend time trying to figure out things that are 
already known?

Vlad








----- Original Message ----- 
  From: vladimir sacek<mailto:v959@msn.com> 
  To: atm<mailto:atm@atmlist.net> 
  Sent: Monday, January 03, 2005 10:21 PM
  Subject: Re: [ATM] parabolic versus spherical volume of glass



   >... What
    I regularly see in the literature is that the fastest 
    parabolizing method (center&edge, with the 70%
    zone least affected) produces best-fit parabola
    that focuses at the circle of least confusion of the
    initial sphere...<
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