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As a follow-on question to What's the largest optical lens put in space? I'd like to ask about mirrors this time.

Question: What's the largest optical telescope mirror ever put in space?

Optical could include infrared, visible and ultraviolet, but not microwaves. It also needs to be a single mirror, not an array of separate sub-mirrors like the JWST design. It could be a telescope's objective mirror or a part of an optical assembly of mixed elements, or it could still be in its shipping crate. It could be curved or flat, as long as it's used in an optical telescope.

Answers to What's the largest aperture telescope sent beyond the Earth-Moon system? are in the 0.5 to 1.4 meter ballpark for deep space, but what about closer to home?

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The winner (as of 2019) would be the ESA Herschel Space Observatory with a focal mirror of 3.5M/11.5ft (Image Source)

enter image description here

Optical could include infrared, visible and ultraviolet, but not microwaves.

Herschel covers both far infrared and microwaves (55–672 µm) and so doesn't show up on some lists of optical telescopes, but it counts as far as the current question is defined.

NASA's Hubble Space Telescope would be second at 2.4M/8ft.

If/when the James Webb Observatory makes it up, it will be the largest at 6.5M/21.3ft. While the question excludes JWST, it should be noted that it probably will not be possible with current rocket technology to put a Hubble-esque mirror of that size into space

If the Hubble Space Telescope's 2.4 meter mirror were scaled to be large enough for Webb, it would be too heavy to launch into orbit. The Webb team had to find new ways to build the mirror so that it would be light enough - only one-tenth of the mass of Hubble's mirror per unit area - yet very strong.

JWST vs Hubble

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