Re: ATM Parabolizing measurement

["James W. Burrows" <burrjaw@earthlink.net>]

 >From: "Bill Mitchell" <astronutski@yahoo.com>
 >
 >Any ideas how much glass actually gets removed during parabolizing?
 >Are we talking microns, nanometers, angstroms?  I don't mean volume,
 >just the depth at the center, say for a 20" f/5 just to keep the math
 >simple (if possible).

The sphere rises

         r^4/(48R^3)

(r = mirror semi-diameter, R = ROC) above the best-fit parabola at the 
center and edge; half that below the 70% zone.  For 20"f/5 (r=254, R=5080 
mm), if my finger didn't slip on the calculator, 6.61E-4 mm = 661 nm, a 
little bit more than one standard wavelength of light (550 nm).  Running 
the numbers into Sixtests and shortening the "target" parabola's ROC Rp 
until the edge deviation was zero, the Rp was 5075.8 mm and the central 
deviation was 1303 nm.  Probably the true answer is 2*661 = 1322 nm (should 
be able to prove that).

         -- Jim Burrows
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