Lab Tour - Steward Observatory Mirror Lab
Most readers know thta Tucson is home of one of the largest collections of world-renowned experts on IT storage. But what you may not know, is that Tucson is also the home of experts for optical sciences. This week, I was part of a delegation of IBMers invited on a tour of the Steward Observatory Mirror Lab [SOML].
SOML was built in 1990 underneath the football stadium at the University of Arizona. Why under the stadium? Their motivation was [Chicago Pile-1], the world's first nuclear reactor, built by Enrico Fermi under the football stadium at the University of Chicago.
We got to see all aspects of the process to develop the huge mirrors used in large telescopes. SOML did not always offer lab tours. Back in 1993, two dozen members of the Earth First! terrorist organization [attacked the lab with hammers and monkey wrenches to destroy and dismantle the mirror lab]. Now, security is tight to ensure no-one damages these mirrors, some of which fetch as much as $30 million dollars.
At other mirror labs, mirrors start as a large, heavy, flat piece of glass and then ground and polished to the correct parabolic curve. SOML created a new process that works a lot better, similar to making a [Pineapple Upside Down Cake]. For those who are not familiar with this cake, you arrange sliced pineapple rings on the bottom of the baking dish, then pour the liquid cake batter that fills in and around the pineapple slices, then bake.
The first step is creating a base of 1,690 hexoganal tubes made of Aluminum Silicate. These are like the pineapple rings in the cake. The tubes are bolted to the baking dish that is 8.4 meters wide. These tubes form the base of the [parabolic shape] that focuses starlight to a small focal point. The tubes are spaced with about an inch of space in between. The Aluminum silicate feels like clay.
Once the base is built, chunks of glass are placed on the surface. Rather then pouring on the cake mix of molten glass, these chunks will be melted in place. This isn't normal glass, but a special Boron Silicate glass that does not expand or contract much during changes in temperature, made by the [Ohara Corporation] in Japan.
The oven is then lowered onto the baking dish. Once the temperature reaches 700 degrees, the entire system is then rotated at 7 RPM. This allows the glass to melt and take its parabolic shape through [centrifugal force]. The people who run the oven are called "oven pilots", and they monitor the entire process to make sure nothing goes wrong.
This particular mirror is one of the two that will go into the [Large Binocular Telescope]. The mirror will be 36 inches thick at the edges, and 18 inches in the middle. If the glass cools down to quickly, it may crack or form crystals, so instead the oven is kept in place and the temperature lowered slowly over the course of a few months. This is called annealing.
Once a mirror has annealed, 24 suction cups are glued to the top surface to pull the mirror out of the baking dish. It is then tipped on its side so that all the bolts can be removed and the hexagonal tubes washed out, leaving behind a honey-combed effect on the bottom of the mirror. This means the mirror is 80 percent air, making it strong and lightweight.
The next step is grinding the surface with diamonds. In most cases, the process of spinning creates the correct shape so little grinding is required. However, for this mirror here for the Large Synoptic Survey Telescope [LSST], about five tons of glass will be ground out of the center. This will actually have two parabolic curves, the outer curve is shallow, and the inner curve is deep. This will allow for the LSST to survey a wide area of space at a time.
Once the glass is ground to the right shape, it will be polished with Cerium Oxide, what is commonly known as Jeweler's Rouge. How smooth does it have to be? If this mirror were the size of the United States, there would be no bump higher than 2 inches tall!
Most mirrors are symmetrical, so the polishing can be done on a spinning platform, but this mirror is not. The Large Magellan Telescope will consist of seven mirrors, one in the middle that is symmetrical, and surrounded by six other mirrors that will all continue the parabolic shape in each direction. This is one of the outer mirrors, which means that each part of the polishing process will be controlled by computers to get exactly the curve required.
Here is a small scaled-down model of the Magellan Telescope. Each of the seven mirrors will be 8.4 meters wide. At this point, one person asked why all the mirrors were 8.4 meters wide. I joked that this was the size of the oven! It reminded me of [the story where newly-wed had to ask her grandmother why she cut the ends off the pot roast]. The actual reason was that the posts of the football stadium are 8.5 meters wide, so any mirror made inside the lab larger than that could not be removed easily for transportation.
The LMT will be installed on [Cerro Tololo] in Chile, where my father worked earlier in his career. Why Chile? Observatories need high altitude, dry climate and clear skies. That is why Arizona is home to many observatories, including Kitt Peak National Observatory and the Vatican Observatory on Mount Graham. Cerro Tololo in Chile is close to the equator and meets these requirements.
Once operational in 2020, it will gather 6 TB of images every evening. That got all of the IBMers on the tour very excited!
To verify the polishing is complete, it is put on three red stands and measured with a laser. Once the measurements are complete. The surface will be coated with aluminum to provide the reflective surface. You can't just paint the surface with a roller! Instead, the aluminum is vaporized and allowed to land on the surface of the mirror evenly, in a layer that is only three molecules thick. There is more aluminum in standard size beer can than on the surface of one of these 8.4 meter size mirrors!
So that was the tour. It took almost 2 hours. If you are ever in Tucson, consider contacting the SOML and arranging a tour for yourself. There is no other mirror lab like it!