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Way back when...

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#1 oldtimer



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Posted 05 December 2012 - 06:19 PM

Back in the 60's many newt advertisers and newt mirror makers claimed 1/20 or 1/10th wave optics. Then sometime later like in the 80's the wording got changed to 'diffraction limited' which was supposed to be interputed as 1/4 wave. Now I notice the big Orion dobs say nothing about mirror specs. I have 'heard' that most of the 12 through 16" are no better than 1/3 wave. I also see that many who buy these big scope immediately send the mirrors off to be refigured. Once againn its the old adage "You get what you pay for". (or do you?)

#2 David Knisely

David Knisely


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Posted 05 December 2012 - 09:18 PM

Well, often, those 1/20th wave figures were for surface accuracy and not wavefront accuracy. 1/20th wave surface means about a 1/10th wave wavefront for mirrors, which is a fairly decent figure, although maybe not quite as good as some custom mirror makers can do now. The other way of "hiding" problems came with quoting an RMS figure instead of p-v. A good smooth 1/14th wave RMS figure might roughly correspond to a 1/4 wave p-v wavefront figure if the surface wasn't too irregular. Also, limiting testing zone numbers and using wavelengths considerably longer than 5500 angstroms were also ways of fudging things a little. I do have two custom mirrors: one with about a 1/19.4 wave p-v over seven zones, and another with a somewhat better figure, but if I can get a really smooth mirror that is 1/10th wave p-v or better, I would be pretty happy with it. Clear skies to you.

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