For large-aperture ground-based telescopes operating in the visible spectrum, the seeing limit is the dominant effect. Atmospheric turbulence is a bigger factor

For large-aperture ground-based telescopes operating in the visible spectrum, the seeing limit is the dominant effect. Atmospheric turbulence is a bigger factor than the precision of your optics, in other words. See https://en.m.wikipedia.org/wiki/Astronomical_seeing https://en.m.wikipedia.org/wiki/Astronomical_seeing (Space telescopes are diffraction-limited but have other constraints, like mass budget and cost, that keep them from being as large as ground based facilities.)