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Hmmmm.... maybe I am missing something, but the focal length of a scope is determined by the optics, not the tube. Extension's are used so that a camera can reach focus in a setup designed for eyepieces. My guess is the coma corrector is the actual issue.
My system is really only interested in the image scale, if that is correct, focal length is irrelevant.
Good to know it is working, as that is what counts!!!
Malcolm
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