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Hi GR,
I can't really offer any explanation because I'm very much a novice but I've had exactly the same issue when using my Skywatcher ED72 and GSO 2" 2xbarlow.
I found a 1.25" TS optics branded barlow in the cupboard which is 80mm long and with this barlow I can achieve focus.
I'm keen to hear the advice from others though because I'd prefer to use a 2" barlow myself. The 2" lens seems to make the eyepiece assembly a lot more rigid.
Cheers
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