RE: [ATM] Calculate Astigmatism

[Jim Burrows <burrjaw@earthlink.net>]

At 2004-03-24 06:41 -0500, Robert Turner wrote:

>Ok maybe I am still a little confused on this calculation. Can you explain
>here what the variables are a little more? Is your calculation for the 8"f6
>say that the Radius of curvature at 0 and 90 degrees need to be within
>.014mm to be passing? Maybe a little elaboration might help understand.

The calculation says if the surface cross-section on every diameter is 
parabolic, and the R changes smoothly by .014 mm from 0 to 90 degrees (this 
is primary astigmatism), the surface RMS will be 5 nm for an 8"f/6.  The 
surface RMS of a "quality optic" is less than 20 nm, so you can tolerate up 
to an R difference of .056 mm for that size mirror.

At 2004-03-24 16:45 -0500, Douglas S Angle wrote:

>On one diameter, I input R=2500, then the program re-calculates to 2498.6 
>mm.  On
>the other diameter, I input the same measured R=2500, and the program
>re-calculates this to 2499.0 mm.  Does that difference of 0.4mm really
>represent physical difference in focal length on the two diameters?  And if
>so, is that the number that I need to compare to the 0.0037mm calculated
>through the tolerance formula?  In short, can I use the sixtests calculated
>R to evaluate astigmatism?

I think it would ONLY if each whole KE reading (not just the usual 
increments) from the vertex of the mirror were accurate to the level being 
discussed along each diameter.


         -- Jim Burrows
         -- mailto://burrjaw@earthlink.net
         -- http://home.earthlink.net/~burrjaw
         -- Seattle N47.4723 W122.3662 (WGS84)  

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