Thursday, January 22, 2009

LSST Mirror Move

Yesterday we moved the primary mirror for the Large Synoptic Survey Telescope (LSST)from the casting lab to polishing in preparation of starting optical work. While the total distance moved was less than 200 feet, it was the successful completion of a major milestone in the project, and was a carefully choreographed process than took 10 people the better part of a day!

When last blogging about LSST a week ago, we were just starting plugging the backplate holes to seal out generating coolant and subsequent grinding and polishing slurry. With that process complete, it was time to head over and prepare for diamond generating of the backplate. The photo shows the blue-tinted silicon mold material that seals well enough to keep liquids out of the mirror substrate, but will come out cleanly when it is time to remove the plugs. We need to remove something like .25" of the backplate to get the thickness we need, so there is plenty of bevel on the backplate holes to leave some for grinding and polishing and also in the final surface.

The red handling ring, which is perhaps better illustrated in a previous post, has been holding the support structure (called the spider) whose 54 supports, in turn are fastened to the mirror substrate's faceplate with a silicone bathtub caulk. It held the mirror vertical for the mold washout, and will now hold the mirror for backplate fabrication. When the back of the mirror is complete, the telescope interface loadspreaders will be glued to the backplate and the 54 supports will be removed once the mirror is flipped over and it is installed in the polishing cell.

Today's task was to transfer the mirror and spider to the air cart (used to move mirrors and cells around the lab), and remove them from the handling ring. It seems incongruous to use 45 ton cranes, steel-toed shoes, hardhats and air wrenches around an optics shop, but when handling loads like this (mirror plus spider plus handling ring was just over 42 tons!) you get used to ALL the tools you need to get the job done. The task was mostly uneventful, but there were some hardware changes that required a quick run to the machine shop for some mods (this spider is new and had some slight differences from that used by our other 8.4 meter spider).

So after the mods were done, the spider was centered and set down on the air cart, then the handling ring was detached, lifted and moved away from the now naked-looking LSST substrate. The last image demonstrates that a single person can move 30 ton loads around the lab using the air cart. It's 4 pads can lift incredible loads using air pressure, and the little yellow tractor provides the power and steering for maneuvering around the lab. It is like using a little red wagon to move loads around the back yard!

The next tasks are to install the plumbing troughs, move onto the Large Optical Generator, center and level the substrate to the turntable, and do the thousand little things that need doing before putting diamond wheel to glass. Stay tuned!