To: atm{at}shore.net
From: "James W. Burrows" 
Reply-To: "James W. Burrows" 


 >From: "Bill Mitchell" 
 >
 >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
         --              mailto:burrjaw{at}earthlink.net
         --              http://home.earthlink.net/~burrjaw
         -- Seattle      N47.47233, W122.36620 (WGS84)

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