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The brightness of the image as seen visually is determined by exit pupil alone no matter what f ratio your telescope is.
E.g. take two telescopes, one at 200mm aperture, f5, 1000mm focal length and the other 200mm aperture, f10, 2000mm focal length.
Exit pupil = aperture/magnification
also
exit pupil = focal length of eyepiece/focal ratio of telescope
Take the first equation with no reference to any eyepiece.
Say you want 50x magnification. In the first telescope (200mm aperture) exit pupil = 200/50=4mm. In the second telescope exit pupil =200/50=4mm. Both telescopes give the same exit pupil and therefore the same image brightness at 50x, or any other common magnification irrespective of the f ratio of the telescope.
Now to achieve 50x in the 1000mm focal length telescope which is f5 you need a 20mm eyepiece. Exit pupil also is 20/5=4mm. To achieve 50x in the 200mm telescope which is f10 you need a 40mm eyepiece. Exit pupil is 40/10=4mm. Same exit pupil at different f ratios gives same brightness by using different f.l. Eyepieces.
My planetary telescope of choice is a large Newtonian (10 to 16”) with premium mirrors that acclimatise quickly (and stay that way) with around 20% central obstruction certainly no more that 25% and mounted on a tracking platform. When seeing permits this type of telescope excels on planetary viewing.
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