For the same camera/chip, image scale is determined solely by focal length of the imaging train. Your C9.25 @ f/6.3 would be 1457 mm, whereas your 120ED native would be 900mm. So the C92.5 will give an image scale 61% smaller than the refractor. It will also gather photons faste than the 120ED, because the former is f/6.3 and the latter is f/7.5.
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