How to make seven really large mirrors

November 25, 2017 2:36 PM

Tucked under the east wing of the UA football stadium is the University of Arizona's Richard F. Caris Mirror Laboratory. Since 2005, they've been casting and polishing the giant mirrors for the Giant Magellan Telescope in Chile: each one's 8.4 m / 27′ in diameter. So, how do you build a mirror for one of the world's largest telescopes? Seemingly very slowly, while spinning.
posted by scruss (17 comments total)

So, it's made of borosilicate glass, like old school Pyrex. I love that they can rough form the mirror just by spinning the mold. I wonder how they will be shipped to Chile?
posted by Bee'sWing at 3:18 PM on November 25, 2017

They travel by tractor-trailer on land, and by freight ship from California to Chile.
posted by Dr. Twist at 4:09 PM on November 25, 2017 [2 favorites]

Spinning, spinning towards the Future!
posted by mikelieman at 4:10 PM on November 25, 2017 [4 favorites]

"That orbiting telescope [The Hubble] was a generous gift to the next generation from the people who worked on the project for decades before it launched. HST’s deep space images amazed, motivated and inspired many of us on Earth. The GMT project team dreams of passing on a similar gift for future generations."
posted by lungtaworld at 5:54 PM on November 25, 2017 [4 favorites]

From the third link...

Our tool, containing Silly Putty enclosed by a thin rubber diaphragm, slowly moves over the surface of the mirror while simultaneously rapidly orbiting around itself. The Silly Putty is stiff over the quick period of the orbit, which smooths out small-scale irregularities in the mirror surface. Over the longer time it takes to move across the mirror, the Silly Putty flows easily, so the tool always matches the surface’s shape.
posted by justsomebodythatyouusedtoknow at 6:27 PM on November 25, 2017 [3 favorites]

Aw, yay! I work in communications at the UA, and I used to work with Dae Wook Kim from the third link. He's the sweetest, most enthusiastic guy. Loves his work, and he's great at explaining it.
posted by kwaller at 6:36 PM on November 25, 2017 [3 favorites]

I was just afraid they'd drop it.
posted by scruss at 6:38 PM on November 25, 2017

I know that my father had the idea to float the mirrors for some of the early xray telescopes in a mercury bath so that their roundness could be assessed without gravitational distortion. As for transporting them, they used custom fabricated containers carried by tractor-trailers designed for very high end art, e.g., with special shock absorbers, etc.

One of my prized possessions is the waste edge from one of the mirrors made for the Einstein Observatory. It is beautiful.
posted by carmicha at 7:10 PM on November 25, 2017 [11 favorites]

Oh and regarding dropping the mirrors, for Einstein (and, I believe, the subsequent xray telecope mirrors made at Perkin Elmer) they tested the robot arm used to lift it out of the polishing, um, vise (not the right term but descriptive) with a dummy... and it fell during the first attempt.
posted by carmicha at 7:13 PM on November 25, 2017 [1 favorite]

This is the best use of a football stadium I've ever heard of. Kudos to them.
posted by el io at 9:21 PM on November 25, 2017 [5 favorites]

I can't resist the temptation offered by the title: Drop one really really large mirror.
posted by Chitownfats at 11:54 AM on November 26, 2017 [2 favorites]

I visited this place early in my undergraduate astronomy degree thanks to a scholarship that had attached international travel funding. The brief was basically "go and see cool astronomy stuff" and there really wasn't much more to it than that. Name a place, make contact with some astronomers/technicians there, go and visit them, see their telescopes, ask them about their work. I picked the area around Tucson and visited UoA, the observatories on Kitt Peak, Mt Hopkins, Mt Graham, Mt Lemmon. Got to join in an observing run on Kitt Peak. Accidentally got locked in a telescope after dusk. Then went up to Flagstaff to visit Lowell and the Navy observatory. Got to drive the Discovery Channel Telescope - and panicked when I thought I'd broken it...

For a kid from a small town/country (New Zealand) this mirror lab was amazing! They told me that they had to shut it down during football games because if they kept operating then the vibration from the crowds would ruin the optics. It was the first time I got a good sense of both the scale of scientific endeavour and the scale of college football in the US.
posted by dashdotdot dash at 1:01 AM on November 27, 2017 [9 favorites]

I wish there was more information about the last step of making an actual mirror, i.e. the reflective coating. All I found was this: "After polishing, the surface is coated with a thin layer of aluminum to achieve maximum reflectivity." So I'm gonna assume something more fancy than applying the World's Biggest Sheet of Aluminum Foil gets slapped on?
posted by Bron at 7:21 AM on November 28, 2017

Bron, I suspect it's some form of vapour deposition. Basically coating the glass backing uniformly with an atoms-scale thickness of aluminum.

fwiw, that's an article published in 1934.
posted by porpoise at 5:58 PM on November 28, 2017

I wrote about this and some other cool astronomy stuff in the Tucson area a while ago.
posted by gottabefunky at 11:07 AM on November 29, 2017 [2 favorites]

I suspect it's some form of vapour deposition. Basically coating the glass backing uniformly with an atoms-scale thickness of aluminum.

Yep.

After the glass surface is polished to a very precise figure, it is turned into a front-surface mirror by applying a very thin metallic coating. The function of the glass substrate is to hold the shape of this thin metal layer which reflects the light. Aluminum is most frequently used, although silver or gold is used in some applications. Aluminum is the preferred coating since it has a high reflectivity over the whole visible spectrum whereas the reflectivity of silver decreases significantly shortward of a wavelength of ~ 400 nm. The thickness is typically ~ 100 nm ( 4.0 x 10-6 inch) thick and weighs only a few grams. Aluminum is applied in a vacuum chamber by evaporating a small amount of metal and allowing it to bond to the clean glass surface. For astronomical telescopes, the mirror coating (aka aluminization) is normally done at the mountaintop observatory site.

For an idea of what the coatingg process looks like, here's a video of the Anglo-Australian Telescope mirror being cleaned and aluminised.
posted by zamboni at 9:15 AM on December 1, 2017 [1 favorite]

Thanks, that was a fascinating video!
posted by Bron at 1:49 PM on December 20, 2017