For years, this has been published as a theoretical limit , and thought to be pie in the sky . So 8.5 is a reasonable naked eye limit after all. Let s talk about these one by one. First, telescope aperture is pretty obvious. The light gathering power is proportional to the aperture squared.
Telescopes are big visual funnels, pouring more and more photons into the observer s eye.
Bigger funnels gather more photons. Magnification helps because as you magnify a field, the background sky is darkened. Technically, the surface brightness of the whole field is reduced. But since stars are point sources, they are reduced much, much less. So magnification has the effect of increasing contrast.