Down Time at Work

The Steward Observatory Mirror Lab, where I work, is shut down for the Christmas and New Year's Holiday, as is the entire University of Arizona campus. But while nothing is going on this week and most of last, there was a flurry of activity the last few weeks. The big projects are the Giant Magellan Telescope, the Large Synoptic Survey Telescope and the San Pedro Martir Telescope. I'm involved with the first two, but it has been months since I last updated on the Mexican project.

The last time I posted about the San Pedro Martir mirror was when it was being cast last August. It was an uneventful cooldown, even though it happened during our rainy season (power outages are a particular concern). Interestingly, whether we are casting a large mirror or "small" (6.5 meters or less in diameter), the cooling process is driven by how uniformly you can cool the thickest part of the mirror, in our cast, only a few inches thick. So nearly all our mirrors have a cooldown/anneal cycle that lasts about 3 months.

The oven was opened just before Thanksgiving in November and the brand spanking new 6.5 meter diameter blank looked great! The casting crew had removed the inconel bands that wrapped around the circumference during the heat of casting before the Thanksgiving break. Upon their return they worked to remove the tub walls and clean up the soft refractory from the outside and inside diameters of the casting. Here you can see Jim spraying some water on the refractory to keep the dust down, as it is quite bad for the lungs. I also included a photo that illustrates how the machined soft refractory makes the edge profile that includes an extended faceplate and backplate.


By early December the crew had finished the cleaning process and were preparing the lifting fixture. Here the now-lonely-looking mirror blank sits on the oven turntable. Of course, the mold is still encased in the once-molten glass, so the next step is mold cleanout. The lifting fixture the casting crew is preparing will lift the 10 tons of glass and nearly as much mold and refractory material. While being held on it's side the next few months, the mold material will be cleaned out with a high-pressure water spray, leaving hollow hexagonal voids in the glass to make it lightweight, yet stiff.


Finally, just before the holiday break, the casting crew installed the lifting spider. The 36 annular pads are built up with a .25" (6mm) thick layer of RTV (bathtub caulk) which is allowed to cure, then additional RTV is added to the glass/RTV layer and applied to the blank, then allowed to cure for several weeks. When we come back, they will cut off at least one or two samples to verify they have cured completely through the thickness of the pads. Only then will we lift the mirror off the oven hearth plate and move it to the mold washout stand. After the hard work of centering and gluing, here the entire casting crew takes a breather. From left is Jim, crew leader Randy, Phil, John and Damon. So even during the holiday shutdown, progress is being made, if only curing of the RTV!