List of largest optical reflecting telescopes

Location in the northern or southern hemisphere of the Earth can also limit what part of the sky can be observed, and climate conditions at the observatory site affect how often the telescope can be used each year.

The combination of large mirrors, locations selected for stable atmosphere and favorable climate conditions, and active optics and adaptive optics to correct for much of atmospheric turbulence allow the largest Earth based telescopes to reach higher resolution than the Hubble Space Telescope.

[1] Another advantage of Earth based telescopes is the comparatively low cost of upgrading and replacing instruments.

Multiple mirror telescopes that are on the same mount and can form a single combined image are ranked by their equivalent aperture.

[20] The mirror for Subaru was cast by Corning and polished at Contraves Brashear Systems in Pennsylvania, USA.