Tuesday, April 24, 2012

Glassy Unveil

A few months ago, I posted about a weekend work day at the Mirror Lab which coincided with the casting of a second mirror for the Giant Magellan Telescope.  The first segment for the telescope is nearing completion in our polishing lab and our casting crew can certainly cast them a lot faster than we can polish them!  The casting schedule lasts about 3 months from start to finish, so the casting oven was opened about 10 days ago, and the crew has been working hard disassembling hardware from around our new pride and joy.

The first job in exposing the new mirror is the removal of the top and sides of the oven and stowing them out of the way.  That done, this last Thursday I caught them removing the inconel bands which wrap around the mold during the casting progress.  Yesterday they removed the hard refractory tub walls and scraped away the soft machined refractories to expose the outer edges of the glass blank.  So you can see if you blink, you can easily miss a step as the crew hustles to expose their handiwork. 

At left, staffers Bruce and Tom get a close look at the surface of the mirror.  You can see the hexagonal structure of the casting - here the glass entombs the mold hexagons, but soon the mold material will be removed to only leave lightweighting hollows in the mirror blank.  In the right photo you can see the mold cores go through the full thickness of the mirror leaving an inch thick layer of glass at the bottom and a couple inches of glass on top for the mirror surface.  You can also see the 36 meter radius of curvature on the top caused by the 4 rpm rotation of the oven while the glass was molten.  In the coming months a lifting fixture will be fastened to the face of the blank and it will be moved to the cleanout stand where the mold will be removed leaving a hollow structure leaving a "light weight" mirror that weighs a little under 20 tons.  Then we'll get it, likely about the start of next year to start the task of fabricating it into a fine jewel of a telescope mirror.  Stay tuned for updates!