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When built, the GMT will be the largest optical telescope in existence. At the heart of the giant observatory are seven off-axis mirrors, each spanning 28 feet, produced in the UA's Richard F. Caris Mirror Lab. (Artists' concept: GMTO) When built, the GMT will be the largest optical telescope in existence. At the heart of the giant observatory are seven off-axis mirrors, each spanning 28 feet, produced in the UA's Richard F. Caris Mirror Lab.