It seems that almost every article online regarding planetary viewing recommends a telescope with a high focal ratio (f/8 or more), for revealing the finer details of the planets. But I find this a bit confusing because the resolving power of a telescope is solely a function of the aperture, or at least that is my understanding.

Given the above, shouldn't it be a matter of a large enough aperture and then the eyepiece to use to zoom on the detail of choice in formed image?

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  • $\begingroup$ The f/number controls the total light collection per solid angle - if you don't get enough photons, you won't be able to see details due to insufficient contrast. $\endgroup$ – Carl Witthoft Jun 29 at 15:00
  • $\begingroup$ But isn't aperture what decides how many photons you "collect"? $\endgroup$ – cad86 Jun 29 at 18:00
  • $\begingroup$ but also how many photons get to your eyeball depends on f/# because of the effective magnification implicit in the value $\endgroup$ – Carl Witthoft Jun 29 at 18:51
  • $\begingroup$ Here is where it gets confusing for me. Isn't higher focal ratios associated with "slower" image taking? Why is that the case if higher focal ratios equate with more photons? $\endgroup$ – cad86 Jun 29 at 21:00

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