A time lapse of the glass for the Giant Magellan Telescope’s fourth mirror melting, captured from inside the kiln.  The Giant Magellan Telescope (GMT) will be one member of the next class of super giant earth-based telescopes that promises to revolutionize our view and understanding of the universe. It will be constructed in the Las Campanas Observatory in Chile. Commissioning of the telescope is scheduled to begin in 2021.The mirror is made using a unique “spin cast” process; the furnace is rotated at 5 rpm as the glass melts. This gives the mirror surface a rounded, or parabolic shape. The mirror will still require additional shaping by grinding to achieve optical tolerances, however, this spinning process saves several tons of glass and significantly shortens the annealing and grinding time because the glass is already in a parabolic shape.
Once the furnace is started, the temperature is maintained for four hours to allow the glass to melt and fill the mold. As the glass is heated it gradually becomes more fluid. Eventually it becomes fluid enough that as the oven is spinning the glass rises up the side of the furnace (the same way as if you were spinning a bucket of water). After more heating the glass becomes fluid enough that it can sink down into the mold.The glass is then cooled rapidly to 900°C (1652°F), and then cooled more slowly for three months to avoid strains in the final mirror.