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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Hi Barry , if the image scale was just one side of the frame , yes it would be about 61% ,but it's the other way round as the image scale is greater with greater focal length .However we are looking at a 2 dimensional surface over which the light is spread to form the image , thus it is a division of the square of the two focal lengths to compare the two image scales. ie (900x900)/ (1457x1457)=0.38 or 38% smaller for the refractor (that's why more fits on the sensor at shorter focal lengths). Then compare the light gathering power of both scopes using the square of the aperture and taking into account the obstruction on the c9.25" (~7cm diameter) . ie (120x120)/ (235.5x235.5)- (70x70) = 0.28 or 28%
Thus the c9.25 captures 72% more light than the refractor . That comes to focus over a greater surface area than the refractor. Thus the advantage of greater light gathering power is to a large extent cancelled out by the same size of the sensor in each case . The extra light is outside the perimeter of the sensor for the c9.25" compared to the refractor.
Hope this makes sense regards philip