They just cast the first mirror for this and it came out very nicely!
Story at Physorg.com:
First Giant Magellan Telescope Mirror Casting is 'Perfect'
"We're very happy to see this one come out looking so gorgeous," Mirror Lab Technical Director J. Roger Angel said. "We'll see more once the mold is removed, but so far, looking through the front surface, it looks great."
The mirror is the first of seven 8.4-meter (27-foot) mirrors that the Mirror Lab is making for the Giant Magellan Telescope. The GMT is the world's first extremely large ground-based telescope to start construction.
This part of the article gives you a sense of the scale of this project:
The colossal telescope will feature six giant off-axis mirrors around a seventh on-axis mirror. This arrangement will give it a 22-meter (72-foot) aperture, or 4.5 times the collecting area of any current optical telescope. It will have the resolving power of a 24.5-meter (80-foot) diameter telescope, or 10 times the resolution of the Hubble Space Telescope. The GMT is slated for completion in 2016 at a site in northern Chile.
And one more excerpt:
For the casting last July, Mirror Lab workers used 40,000 pounds of Ohara E-6 borosilicate glass. The furnace hit peak temperature, 2,150 degrees Fahrenheit (1,178 Celsius) on July 23. As the furnace rotated at 5 revolutions per minute, glass melted around the 1,681 hexagonal cores in the mold. This created a 'honeycomb' mirror blank with a faceplate of the desired curvature. The honeycomb mirror weighs only a fifth as much as would a solid mirror of the same size.
The rotation produces centrifugal force which makes the molten glass slump out toward the rim. It just happens to form a nice parabola -- this trick cuts a lot of time from the final grinding and polishing. Here is a photo:
Remember, this is one of the seven mirrors that will comprise this telescope. I had the great pleasure to visit the Keck Observatory on Mauna Kea, Hawai'i and that thing is huge.
This puppy will be a bit more than twice the diameter (4.5 times mirror area).
The website for the project is here: Giant Magellan Telescope
Here is an artists' rendition of the finished scope:
