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Take this with a grain of salt as I'm an absolute newbie, but I was under the impression that a guide scope of nominally 30/120 is only effective up to approximately 1000mm focal length, and up to 50/150mm or 60/240 and longer up to approximately 1500mm focal length. But these would also be dependent upon the pixel pitch of the guide camera, and also at their "best" performance on a completely rigid mount.
For longer focal lengths Off Axis Guiders (OAG) are sourcing their targets through the same image chain as the primary optical image pathway, This means that any movement within the scope (such as SCT mirror slip) can be compensated for, whereas an external rigid guide scope can't "see" any changes to the primary optical path and can't compensate.
But also, an OAG is also using the focal length (give or take a bit) of the primary scope itself, and so a 1500mm SCT would act somewhat like a 1500mm guide scope by diverting a "very small" part of the primary optical path to the guide camera, whereas a guidescope is only around the 150-700mm focal length.
But again, I am only too happy to be corrected by those who know a hell of a lot more about it than I do!!!
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