Monday, July 20, 2015

Old Business at the Mirror Lab

It has been a while since posting about work at the UA's Mirror Lab. For one thing, it has gone through a name change! It will now be known at the Richard F. Caris Mirror Lab - named for an Arizona businessman who recently made a 20 million dollar donation in support of the UA towards the Giant Magellan Telescope (GMT). Richard Caris founded the company Interface Advanced Force Measurements, which makes load cells for industrial applications, including the supports for the polishing and telescope cells - used in the lab for decades. We're still getting used to the name change, but eventually we'll get past calling it the Steward Observatory Mirror Lab any more, which we've known and used for 25 years!

The other big news is that we finished another mirror! The Large Synoptic Survey Telescope (LSST) will revolutionize astronomy in so many ways. It utilizes an 8.4 meter mirror, but there any resemblance to previous project ends! While the outer part of the mirror looks normal, the inner part is hollowed out to a much steeper curve - yes, a second curve on the same mirror substrate! I've seen it done on small substrates, but nothing on this scale. Doing it this way assures that the M1 and M3 curves will always be in alignment in the 3-mirror system. At left is the optical layout of the telescope, showing the 2 curves on the primary mirror. The unusual primary is just the start - it has a huge mosaic of solid-state imaging chips at the focus, allowing a 3.5 degree field-of-view (7 full moons across!), unheard of for such a large telescope. The wide field will be used to cover the entire sky every couple days to search for anything that changes with time - comets, asteroids, variable stars/galaxies... It is predicted to be a real game-changer, making data publicly available immediately on-line. Go to the link above for more images and info on the LSST project.

We actually finished the primary mirror last Fall, but then, as normal for a large mirror, we did metrology, measuring it 6 ways from Sunday through the end of the year, then finally boxing it up and shipping it to a storage facility when it was finally accepted in May. Way back in February, we were given approval to give it a protective coating and start preparations to box it up. I took a series of images that made it into a time-lapse sequence. Asking permission to post it, I was asked to wait till acceptance negotiations were complete, but since there have been newspapers articles about the mirror completion, as well as the move out of the lab to airport storage, I figure I can now post my few images! At left is a shot of the finished mirror, showing the two curvatures in the uncoated mirror, still on the Large Polishing Machine (LPM). The crew is assembling in the background to apply the "Opticote" protective film. At right they are applying the liquid while another crewman used a roller to spread it out to a thin, even coat. Because there are solvents in the liquid, everyone must wear forced-air respirators. Since I have a beard, I've had bad luck with respirators, so couldn't be in the room. I set up a camera to run automatically, then took a few shots from the second floor before I left the building. The shot at right shows them starting work on the inner curve. Note that we often walk on the finished optical surface in stocking feet as shown here.

Once dry, the Opticote surface protects the precision surface with a thin plastic layer. We use an array of suction cups to lift the mirror out of the polishing cell to place it in the transport box, and that layer keeps any dust from scratching the polished surface. When we want the glass to be exposed again, it peels off easily, any remnants can be quickly cleaned off with a little acetone. At left, the coated mirror sits on our test tower, with the edge of the mirror pressure washed to remove remnants of polishing compound visible in the above image. Some of the steelwork structures over the mirror are used for various aspects of measuring the optical surfaces in the lab.

So anyway, on that Saturday back in February, I set up my camera to take an image of the crew every 8 seconds. Assembled into a time-lapse sequence, it makes an interesting clip showing the 70 minute process that marks the conclusion of work on the project. Note however its move to airport storage is not permanent - when the telescope cell is finished, it will return to the lab for continued testing before boxing up again for transport to Chile. So we were glad to see it go, but it will return once more before leaving permanently... Here is the Opticote clip - go full screen and HD if you can!

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