Quote:
Originally Posted by Amaranthus
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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Not sure whether my understanding of image scale is the same as yours ? In my mind the bigger the scale the bigger the image that is projected on a surface . The longer the focal length the larger the image projected , the bigger the scale. The c9.25" would have a bigger image than the sw120 at prime focus . The sensor is a fixed size , so more of the image produced by the sw120 would fall on it than the c9.25". Hence more of the light captured by the sw120 would fall on the sensor than the light captured by the c9.25"( a lot of which would be lost outside the sensor ).
Philip