The fifth mirror for the GMT's 7 segment primary mirror is being cast at the Richard F. Caris Mirror Laboratory at the University of Arizona. In this image, a worker at the lab places the last piece of glass for mirror 5. Image: Giant Magellan Telescope Organization Astronomy, News, Observatories They Just Began Casting the Giant Magellan Telescope’s 5th Mirror. What a Monster Job. Article written: 7 Nov , 2017 by Evan Gough The fifth mirror for the Giant Magellan Telescope (GMT) is now being cast, according to an announcement from the Giant Magellan Telescope Organization (GMTO), the body...