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A Ronchi eye piece can tell you alot, and you should be able to track down details on glass used (if it is a refractor). There are charts available online that show test details of different glass element combinations typically used in refractors. Some Newt builders, like GSO, publish details on their mirrors, many other do not, but again a Ronchi eye piece can tell you alot in that case. Have a look at the Gerd Neuman test photos taken through Ronchi EPs.
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