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Darren,
My experience is you can't get enought inward focuser movement with the dob but the barlow moves the point of focus (out) so you don't need to rack the focuser in as far and can therefore achieve focus.
I suspect with the ED80 you have to rack the focuser out a lot to achieve focus without the barlow and when you use the barlow you don't have enough outward travel. I'm guessing if you put a spacer between the focuser and the barlow on the ED80 you will be able to achieve focus with the barlow as the spacer effectively gives you more outward focuser travel.
Peter
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