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22513RE: Some flexure perhaps?

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  • dan_kordella
    Dec 19, 2013

      Sorry - more confused now. Are you saying you are using a flattener WITH the GTF81? Doesn't that defeat the purpose of the built-in flattener? Or did you remove the flattening elements? 

      Additionally, what gear do you have in the optical train? I've run into an issue with spacing when I've tried using other telescopes without built-in flatteners. For example, I've found that with a Televue TRF-2008 the backfocus is supposed to be 56mm, but I only got a flat-field when I had a total of 46mm backfocus (my camera is 17mm and my filter-wheel is 29mm). Ideally I want to fit an OAG into my optical train, and I have a Starlight Xpress thin OAG (with 10mm backfocus), but with the Televue, for example, this produced some very noticeable field curvature (in two different scopes, both ED-doublets from Stellarvue and Sky Watcher, haven't tested in a triplet system but I wouldn't think the results would be that different). 

      Anyway, this is why I went with the GTF81 - to fit an OAG into the train and to not have to worry about spacing requirements for a stand-alone flattener or reducer. But - if you are adding an additional flattener, why use the GTF81?

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