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ED vs SD
Hi All, Many thanks for all your replys, I went and looked at the William-Optics wedpage to compare the Zenithstar ZS66D-SD vs the ZS66P-ED the first is claimed to be an APO, an SD doublet, and the Longitudinal Abberation graph shown makes it look fantastic, no graph is given for the ED Scope to compare it with, I gather that the larger the objective the more difficult it is to get all spectra to focus together, as I looked at the ZenithStar Fluorite doublet Long. Abb. Graph and its way out by comparison. So on a small scope of 66mm Obj. at say F-4 to F-6 (wide Field) is there going to be that much difference between an SD scope and a Fluorite doublet/triplet for Astrophotography, I can see the need on a large refractor for Fluorite Glass and even a triplet (big bucks but nice), your thoughts please.cheers.....Jim
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