Friday, August 14, 2009

Last Step Before Casting!

Wednesday was the glass-loading into the mold for the San Pedro Martir 6.5 meter telescope. I've been making regular posts about this telescope's recent mold construction and glass inspection, and the loading of the mold is the last big step before the casting starts week after next.

In "the old days" of casting large mirrors (I'm thinking Palomar 200" vintage), the glass was mixed and melted nearby (mixed in large batches for uniformity) and the molten glass was ladled into the mold. This has many inherent problems, and would be nearly impossible to implement into our spinning oven. Yes, you heard right - at the Mirror Lab, we spin the mold and glass whenever the temperature is over 700C (the softening point of the E-6 Pyrex we use) primarily to put the steep curve into the mirrors we make. The curve in this 6.5 meter diameter mirror will be about 30cm (12") deep in the center. Spinning the curve into the molten glass saves not having to add another 10 tons of glass into a non-spinning mold, but also saves the work and effort of removing it again with diamond tools to put the curve back in! We can simplify the casting by using the glass blocks shown - Ohara Glass goes to a lot of work to make a very uniform product for us, saving us from having to deal with mixing or dealing with large quantities of molten glass. The spinning process also insures that the thickest part of the glass mirror is only a couple inches thick, which saves annealing time - the slow cooling of the filled mold to insure no internal stress is built up in the glass. That Palomar mirror, cast in the '30s was about a foot thick and took over 13 months to cool. Our process, including the heat up from room temperature is finished in 12 weeks.

So Randy's well-oiled casting crew was again in action carefully placing the 11 tons of glass into the mold. The mold core material - a machinable aluminum silicate ceramic, is quite fragile. You can easily pick out little chunks with your fingernail, so the sharp glass edges can easily damage the cores. Indentations in the cores can usually easily be seen in the finished mirror castings as bumps in the back of the faceplate. So care is exercised, yet at the same time, the task was finished in record time. The crew started loading at 6am and by lunch was lowering the cone (top and sides of the oven) down over the now-filled mold.

After electrical hookup (heat comes from resistance heaters - visible in the inside-oven shot at left) and checkout of all control systems, the casting starts in about 10 days. Stay tuned!

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