Thanks for the link, explains what i am trying to chase down.
It tells me if the focuser drawtube is not centred in the secondary mirror reflection, while everything else appears centred, a longitudinal adjustment is required (longitudinal error)
I have the secondary mirror up as high as i can possible get it, so it would then assume that i will need to undo the primary mirror screws and lower it down as far as it can go and go from there, that is about the only thing left to move.
At least i know know what was causing this effect.
Thanks.
Peter.
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