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dothead
super member
Reged: 08/13/08
Posts: 163
Loc: Heidelberg, Germany
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Re: FET-300/EM-3700
08/15/08 04:35 PM
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And what's furthermore very critical when you compare doublets to triplets is not just the increased mass & area of the (triplet) glass, but also the fact that spacing (which is affected by delta T.s) in a triplet lens system is much more critical than in a doublet. Add to that the fact that, compared to the crown and flint of ordinary achromats, the exotic glass types of an APO have vastly different thermal coefficients (the extreme case is CaF2), and you end up at a situation where I'd guess that the error in sperical correction (of a big APO in rapidly falling/rising temperatures) is something like an order of a magnitude bigger=worse than that of an Achro (of the same diameter) in the same (delta T) conditions.
It's precisely because of this that big Apos should be built as Petzval systems (or at least as doublets - like e.g. Pentax does with its flagship, the 250SD).
No wonder that the Astreya quadruplets ("Super-APOs") haven't become more popular, despite their competitive price: even a 6 inch 4-lens-systems exhibits way too severe problems with undercorrection due to its inability to follow typical temperature changes during the course of an observing night in our real-world climates.
Ralph
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