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Old 23-11-2013, 07:33 AM
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Satchmo (Mark)
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Join Date: Dec 2005
Location: Sydney
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Originally Posted by gregbradley View Post
If your argument were true then there would be no point in getting those Ion Milled RCOS's that essentially had perfect figure. Yet have a look at those Rob Gendler images taken on a 14.5 inch RCOS with Ion Milled optics in WA on his astropic site..
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.
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