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Thanks again for the above Don. It's all starting to make sense.
My focuser focal plane requires 20mm out-travel. My eyepiece requiring the most in-focus (Morpheus 12.5mm) in the 1.25" adapter requires the focuser to be 9mm out. So there is a difference of 11mm.
The calculation then is 75 - 46 - 11 = 18mm additional spacing required.
I've just ordered some spacers but in the mean time have found an adapter that replaces the GSO 46mm eyepiece holder with a 70mm holder which gives me 75 - 70 - 11 = -6. This is still within the 65-85mm operational range for the EP above but I might as well aim for spot on.
I now only need to parfocalise my other eyepieces to match the Morpheuses and should be good to go.
PS: also, just a note for anyone who has never had to find the focal plane of their scope before, if your translucent tape is low quality and striates as you unroll it, you may struggle to find focus. Holding up a white sheet of paper on the focuser was clearer considering the low quality tape I had.
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