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Allan,
From my experience I wouldn't recommend anything more than 1000mm for a guide scope. I used to have 8" SCT reduced to 1260mm as a guide scope and it worked very bad with PHD. It did strange things during calibration, sometimes it was loosing a guide star or didn't want to accept it at all displaying "low SNR" message.
Now I'm back to a short FL guide scope(3.5" f/5.3 telephoto lens) for my VC200L with reducer as an imaging scope and it works much better.
cheers,
Mike
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