The telescope is operated by the MMT Observatory, a join venture of the Smithsonian Institution and the University of Arizona.
The MMT operated between 1987 (when its construction was completed) and 1998 with 6 mirrors with a diameter of 1.8 meter each, having the equivalent gathering area of a 4.5 meter telescope.
One of the reasons for its original multiple mirror design was the difficulty on casting large mirrors. This was solved by Roger Angel of Steward Observatory, in the University of Arizona, by casting mirrors with a honeycomb structure in the interior of a rotating oven. This made feasible the replacement of the six mirrors with a six meter one. The original building and part of the structure could be reused. The new mirror was casted and polished on the Mirror Lab, at the University of Arizona.
In May 2002 the new MMT was rededicated.