Re: [ATM] Calculate Astigmatism

[Jim Burrows <burrjaw@earthlink.net>]

At 2004-03-23 17:11 -0500, Douglas S Angle wrote:

>I thought my question in 2000 was essentially the same as Robert's last
>week.  But "0.1mm" is a far cry from "almost to the nanometer".  I still
>haven't quite figured out if the sixtest output of focal length is the
>actual focal length, but I was satisfied with the answer at the time, and
>proceeded on that basis.  Now I'm back to being confused.  What am I
>missing?

Geeze, 2000 was very early in my term of office, it's all the fault of the 
last guy <g>.  I ignored Mike Peck saying primary astigmatism was the real 
threat <another g>.  Anyway, the required accuracy dR in the radii of 
curvature on two diameters to achieve the accuracy dRMS from primary 
astigmatism is 2*sqrt(6)*(R/r)²*dRMS, with r the mirror semi-diameter.  To 
get dRMS to 5 nm (20 nm is a passing mirror) for Texereau's standard 8"f/6, 
dR = 2*sqrt(6)*24²*5E-6 = .014 mm.  Oh well, R to the micron instead of 
nanometer - you still can't turn the mirror to that level of accuracy.

The fundamental assumption of the Foucault test is that the mirror is 
non-astigmatic and you're finding the spherical aberration of the 
mirror.  Going through the same calculation as above, it turns out that R 
only has to be known to ~10 mm for that test.

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

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