Like other mirrors produced by the RFC Mirror Lab, the GMT mirrors are made so that their front surface forms a paraboloid—the shape taken on by water in a bucket when the bucket is spun around its axis; the water rises up the walls of the bucket while a depression forms in the center.
However, the GMT’s unique off-axis design requires the shape of its outer mirror segments to be asymmetric in profile, so that all mirror segments together assemble into one giant paraboloid mirror.