Glass Sorting!

One of the more interesting operations at the Steward Observatory Mirror Lab is the glass sorting before casting one of the mirror projects. Coming up next month is the spin casting of the 6.5 meter diameter substrate for the San Pedro Martir Observatory. To make as perfect a mirror as possible, over 11 tons of glass must be inspected and sorted before loading it into the mold to form the lightweight casting. My last couple posts about the Mirror Lab have talked about the mold making, so refer back for that info.


Besides handling large quantities of the magical looking blocks of glass, the sorting crew is a wonder to behold - everyone has their job and move as if part of a smoothly oiled machine. The boxes of glass individually wrapped in foam are unpacked, cleaned of particulates, inspected under crossed polaroid filters (more about that in a minute), sorted by seed content, reboxed and weighed. Meanwhile the scraps of foam are corralled and boxes recycled. Sorting the 10 tons of glass took a day and a half, so they were quite efficient indeed. Note that everyone wears gloves - the cleaved blocks are extremely sharp and gloves help keep that nasty blood off the glass!


Randy, head of the casting crew, individually inspects every piece of glass. Using a light box with a polarizing filter, the crossed polarizer in his glasses reveal stress in the glass. Usually associated with localized impurities or stria (index of refraction variations), remelting the glass doesn't always relieve the stress, so there is a reject box. If the stress is in a small area or near an edge, it can be cleaved out of the block. What does it look like under the polaroids? A "clean" chunk is featureless as shown at left. In the presence of stress at right, the plane of polarization is rotated, so appears as a bright spot or zone with the analyzing filter (click to enlarge images). This example was quite bad so was rejected.





The glass is also sorted for seeds - tiny bubbles or impurities within the blocks. The glass with fewest defects is set aside for placement on the top of the casting. These blocks will help form the faceplate of the telescope mirror and using the clearest glass correspondingly helps form a mirror surface with fewer defects.


And yes, as can be told from the name (Ohara Glass Corporation), it is a Japanese product. They go to a lot of work for the lab making a consistent batch-to-batch product, then cleave the batches into these blocks so that they melt together cleanly without trapping a lot of air bubbles. No other manufacturer has offered to go to that effort to assure the consistency of results on which we depend. We've had excellent results in the past, and expect to continue to use them in the future...