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And to add to what Rob has posted there, the difference between the 5" refractor and the 10" dob is a factor of 4 in the light gathering capability, so that the 10" will collect 4 times the light of the 5". Not surprisingly, the difference between a mag 13.1 and a mag 14.7 star is approximately 4 times the amount of light (4.3 to be more exact....2.512^1.6).
However, there are other factors that come into play in the comparison, such as focal length (f), any aberrations that maybe present in the optics (chromatic/spherical aberration, coma etc), quality of the optics, obstructions in the light path etc etc etc.
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