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Old 23-11-2013, 08:25 AM
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Location: Melbourne
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Originally Posted by Satchmo View Post
I've seen wavefront maps of these large RC surfaces before and after ion milling . The ion milling is more about honing down assymetries and non - rotationally symetrical defects in large surfaces to get them better than 1/4 wave and rotationally symmetric . Such localised polishing by hand would be very time consuming .A 32" F3 Hyperpola for an RC has a wavefront departure of over 120 waves on the wavfront from the base sphere. It is very hard to polish out this amount of glass without non rotationally symetric defects coming in .
A test of the combined wavefront with the highly aspheric secondary ( which also will have non- rotationally symmetric errors gives the operator of an ion milling machine a chance to smooth the wavfront better than 1/4 wave on these large surfaces , by milling down assymetries.

Robert Gendlers photos look good , not because the optics are 1/20 wave but becasue the combine wavefront error of the optical train could definately be better than 1/4 wave in terms of random - non symmetric errors.

I thought that ion milling obtained 1/100th wave performance?

"Post figuring is a common procedure in modern glass optics.
The Keck telescope mirror segments, for example,
were polished to the l/20 (optical) level and then post figured to the l/100 level by ion milling"
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