The casting crew opened the oven last Tuesday just before Thanksgiving, and have been unpacking it since then. Today (3 December), they are planning a big unveil for partners and VIPs, and since I've been out of town since the holiday, don't have current pictures, and have had to hold these until after the event. Above is a 5-frame panorama assembled by the Microsoft ICE program. Jim and Phil are unloading the white insulation that sealed against the vertical walls of the oven. Visible are the inconel bands that hold the tub walls together against the force of the molten glass as the oven spins. The bands are terminated with the air cylinders that allow for the band's expansion and contraction as the casting cools from 1200C to room temperature. Note there are more bands on the bottom since there is more force when you add the weight of the liquid glass at the bottom of the mold... At right, John helps Jim load up the insulation for disposal. Note that there are lots of dangers in handling not only the insulation, but most of the other refractory materials, so all the casting crew routinely wear respirators and gloves in handling them.
The casting appeared to my eye to be about as perfect as they come. A casual inspection didn't reveal any surface flaws one normally sees - bubbles, seeds, bubble veils that normally occur in any casting. This one looked great! Another 5-frame panorama is shown at left - the one thing you can see is that there is a fine scattering of black dust that happens during the casting after the glass solidifies at about 800C. I've been told it is microscopic particles of the heater elements that flake off during the process. You can see near the center where someone wiped it off at one spot. It literally is a surface feature and will wipe off easily. You will note also that there is no center hole on this casting, identifying it as one of the off-axis mirrors of GMT. The next mirror to be cast, GMT4 will be the one mirror with a center hole, nearly 1.5 meters in diameter. At some point in the fabrication process, we will be coring a 5cm (2") diameter hole in the center for drainage of generator and grinding/polishing fluids. At right, our self-appointed Mirror Lab documentarian, Ray Bertram is documenting the mirror unpacking process too.
And, of course, I felt the urge to take a few stereo shots. The band termination posts are a natural focal point for the depth of the stereo pair, both perhaps better show how the bands wrap around the tub walls. These are both cross-eyed stereo - cross your eyes slightly so that you view the left picture with your right eye, and vice-versa. There will be a center image that your brain interprets having depth... It is usually easier to practice on the small images here before clicking for the full-size views. Serious stereographers usually mount a pair of cameras that take images simultaneously, but I do it by lean left, snap, lean right, snap again, any motion between frames results in something weird in the stereo view. Since Ray is included in the right pair, he moved slightly in the half second or so between frames, so there is a blur in the stereo pair there...I'm sure that the efficient casting crew has removed the bands and hard refractories by now and only the naked mirror remains for the unveiling today. I believe the schedule calls for attaching the lifting spider to it, allowing the RTV attachment to cure the couple weeks over the holiday shutdown. Then in January or February, it will be lifted off the oven hearth and mold cleanout will start. The finished mirror will likely be available late Spring or early Summer for us to start fabrication, though we'll likely not be ready for it yet - if only we could polish them as fast as they cast them!
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