
Casting a $20M Mirror for the World’s Largest Telescope [video] - sohkamyung
https://spectrum.ieee.org/video/aerospace/astrophysics/casting-a-20-million-mirror-for-the-worlds-largest-telescope
======
JshWright
Fun fact, this facility is underneath the football stadium at Arizona
University.

Tom Scott recently posted a guest video from the Active Galactic Videos
YouTube channel about this process.

[https://www.youtube.com/watch?v=BP9HNVuGb-g](https://www.youtube.com/watch?v=BP9HNVuGb-g)

~~~
erentz
And if you're ever in Tuscon you can visit the facility, they do tours:
[https://mirrorlab.arizona.edu/tours](https://mirrorlab.arizona.edu/tours)
(Also recommend visiting Kitt Peak and doing one of their night observing
programs.)

~~~
snowwrestler
The Kitt Peak program I did was a great experience. It's in the middle of a
desert wilderness, so get there early for incredible views and a sunset. We
got a tour of many of the telescopes. Bring lots of warm clothes because it is
quite cold and windy after the sun goes down. And then at the end, for one
more bit of excitement, you drive down the (steep, winding) mountain road
without headlights--just following the brake lights of the person in front of
you.

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leon_sbt
I have always place diamond turning and aerospace composites part/tooling very
high on my list of high stakes manufacturing.

The process that this mirror goes through, just redefined high stakes for me.

Think about it, if somebody or something makes an error with any significance.
Bolt wasn't secured, sensor calibration was off, etc. The entire project is
derailed by years. Just getting new material, will take at least a year based
on the video.

In aerospace, usually it's alot of money to scrap a part. But then you call up
your supplier, and they can make another one "quickly".

With this, you have no wiggle. At all.

I'm curious how they manage risk/failure modes on their team/facility/machines
with a project so critical.

Especially since the entire project has a QTY output of less than 10 over a
few years.

~~~
batbomb
You think the manufacturing is high stakes?

They ship all the equipment for telescopes together on the same ship, because
if you lose the mirror you're basically fucked anyway. You're at the mercy of
the interstate, the shiphyards, the ship, the seas, the canals, the other
shipyards, the other highways, the roads, the drivers, and a bunch of other
things.

Manufacturing is important, but it's a lab environment. Getting the stuff
where it needs to go is the messy bit.

This even goes into the design. The reason all the telescopes above La Serena
are curiously near 8.4M wide is because there's a tunnel which is about 9M
wide.

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analog31
My workplace includes a small optics shop where they polish optics that are
roughly 2 or 3 inches in diameter. Even in such a modest little shop, the
process is just fascinating. The whole shop runs in slow motion, and there is
a high level of skill and judgment necessary to make and test the optics while
at the same time maintaining the condition of the abrasives and tooling.

Optical manufacturing processes are a wonderful mixture of old and new. Many
of the basic techniques are centuries old, but have been updated with
sophisticated laser measurements and computer controls.

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bjpirt
The timescales involved in the manufacture of this mirror are amazing. I love
the way this guy just casually states things like:

"we then cool the glass ... for a few months"

"the mirror will sit for the next year while we machine the back surface flat"

"that final polishing process ... takes about 18 months"

It's kind of mind blowing for it to take so long to make a single object.

~~~
woodandsteel
>"that final polishing process ... takes about 18 months"

That's a lot faster than it used to be. The 200 inch mirror at Mt Palomar took
13 years to grind and polish. I guess they have greatly improved the
technology.

[https://en.wikipedia.org/wiki/Hale_Telescope](https://en.wikipedia.org/wiki/Hale_Telescope)

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anon1253
And here I'm sitting with my 8" Schmidt Camera and 12" Newton … the amount of
engineering that goes into this is incredible. It's not just the mirror, which
is of course important, but also having a mount that can precisely track at
the sidereal rate with all the mirrors on it. And then, judging by the image,
they'll also do Laser Adaptive Optics to correct for atmospheric seeing
artifacts, which means adjusting the mirrors and/or camera at a very high rate
and /very/ precisely. It basically works by painting and artificial star in
the sky with a high powered laser, and by measuring the difference between
where it should be and where it actually is, you can then adjust the optical
train to compensate. And, then there's the issue with temperature
compensation, collimation (looks like a Gregorian-type design), and of course
the cameras themselves. For the cameras I'm assuming the illuminated field is
quite large, so either this means a HUGE CCD, or stapling several together
with all sorts of noise artifacts involved. Very exciting. Very hard.

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SamPutnam
For those wondering _why do they have to go to Chile?_ , I presume it's for
the same reason as given here for the ELT (extremely large telescope, to be
completed in 2024): The location was chosen because of its high percentage of
cloudless nights and low light pollution [1] and (notably but with a lower
weight of importance) the temperature is cold enough that there will not be
differences in temperature between the interior and exterior of the telescope
which could cause the rays of light to defract (ever so) slightly [2].

[1] [https://newatlas.com/extremely-large-telescope-
construction-...](https://newatlas.com/extremely-large-telescope-construction-
begins/49747/) [2]
[https://astronomy.stackexchange.com/a/21413](https://astronomy.stackexchange.com/a/21413)

~~~
mjsweet
My Mrs is an astrophysicist and has been to Chile about 10 or so times over
the last 8 years. She says the Atacama desert is ideal because of Chiles
political stability as well as stability of the atmosphere. A high and dry
western coastline is good for great “seeing” conditions because highly stable
sea air is less prone to fluctuations in temperature which is the bane of any
astronomer (this is what creates twinkling of stars). As it happens she was
working on the data reduction pipeline for an instrument with adaptive optics
(AO) on the Gemini South telescope. AO helps reduce atmospheric disturbances
even further so they are now getting better results than Hubble from a ground
based telescope.

~~~
gattr
The same goes for South California mountains, especially if you're into solar
imaging (the Big Bear Solar Observatory is there for a reason).

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sizzzzlerz
So if a crack did develop in the glass as it was cooling or some other flaw
appeared that would make it unusable, could the glass be broken up and be
remelted or has its chemistry changed such that it won't meet its specs any
longer?

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pkaye
Found a longer video on Youtube.
[https://www.youtube.com/watch?v=2GufF3pfIyA](https://www.youtube.com/watch?v=2GufF3pfIyA)

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feniv
Here's another video about the mirror's construction:
[https://www.youtube.com/watch?v=BP9HNVuGb-g](https://www.youtube.com/watch?v=BP9HNVuGb-g)

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jagger11
Given that we're talking about telescopes under construction, the largest one
will be EELT, no?

~~~
sohkamyung
You are right. Based on an image in this article [1], the EELT would be the
largest optical telescope, once completed in 2022, two years after the GMT, if
the dates are correct.

And in the battle for superlatives, the cancelled OLT would have been the
largest of them all.

[1] [https://medium.com/starts-with-a-bang/worlds-largest-
telesco...](https://medium.com/starts-with-a-bang/worlds-largest-telescope-
will-revolutionize-the-future-of-astronomy-d31386b92605)

~~~
ceejayoz
I love the way telescopes are named.

Can't wait for the "JFCTABT".

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dnate
It would be good to know what off-axis or on-axis means in the context of
telescope mirrors.

~~~
eesmith
[https://en.wikipedia.org/wiki/Reflecting_telescope#Off-
axis_...](https://en.wikipedia.org/wiki/Reflecting_telescope#Off-axis_designs)

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rdtsc
1nm precision of the curve and 20nm for surface irregularities. That's
incredible. Wouldn't just thermal expansion cause it to deviate from the 1nm
precision? What about dust, precipitation just bacteria creating a film on the
surface.

~~~
kurthr
Yes, the "nanometer level" is local flatness not the position. The mirror
itself is only polished ~20nm flat. It is temperature controlled and made of a
low expansion ceramic (zerodur?), but still bent by active optics to make the
focus of "guide star" perfect (and to deal with any high altitude atmospheric
"seeing" artifacts).

~~~
danbruc
_It is temperature controlled and made of a low expansion ceramic (zerodur?)
[...]_

Borosilicate glass. I also don't think they control the temperatures of the
mirrors but I am not sure about that.

 _[...] but still bent by active optics to make the focus of "guide star"
perfect (and to deal with any high altitude atmospheric "seeing" artifacts)._

For the GMT atmospheric compensation happens at the secondary mirrors. The
primary mirrors are still active to maintain the optical alignment but slowly
- less than 1 Hz - as compared to atmospheric compensation in the kHz range
and I guess this also compensates for thermal expansion.

~~~
tesseract
> I also don't think they control the temperatures of the mirrors but I am not
> sure about that.

I know they typically air-condition telescopes during the day to keep the
mirror at the anticipated nighttime air temperature, to avoid convection
currents caused by a warm mirror. That requirement would seem to preclude
controlling the temperature to a specific value, I think.

------
lamacase
If anyone is interested in reading about a large telescope project from start
to finish, I highly recommend this book: "The Perfect Machine: Building the
Palomar Telescope". The project started in the 1920s, but the Palomar
telescope (now called the Hale telescope) is still in active use.

If anyone knows of a comparably comprehensive account of a more modern
telescope, I would love to hear about it.

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sporkologist
> more than 17 kilograms of special glass

That's not very much glass for such a large area, that would make it just a
very thin layer... is this correct?

~~~
barryp
The Youtube video says 17,481.45 kg of glass. Maybe the IEEE article meant to
say 17 metric tons.

~~~
Double_a_92
I never understood why , and . where used as separators... They look so
similar especially in hand writing.

I would propose 17'481.45 kg.

~~~
ino
That would be the weirdest reason to move to Switzerland or Liechtenstein.

[https://en.wikipedia.org/wiki/Decimal_mark](https://en.wikipedia.org/wiki/Decimal_mark)

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viraj_shah
Holy shit, this is awesome.

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delinka
Once these mirrors are mounted and put to work in the telescope, what's the
process for cleaning dust off the things?

~~~
rtkwe
Each mirror is actually coated on site and periodically when the performance
gets bad enough the mirror is removed and taken back to the coating facility
where it's cleaned, which actually removes all the aluminium coating the
mirror then it's recoated and put back into service.

[https://www.space.com/37730-how-to-clean-very-large-
telescop...](https://www.space.com/37730-how-to-clean-very-large-telescope-
mirror-video.html)

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pier25
Imagine having to invent new technology just to be able to create a module for
your next programming project.

~~~
nathancahill
So just a normal day for a JS developer then?

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mhb
Why does it matter if the raw glass has imperfections if it's going to be
melted?

~~~
rtkwe
An imperfection in that would mean there was some problem with the creation of
the glass probably some impurity which would cause issues with the thermal
expansion properties. If the mirror doesn't expand evenly or as expected it'd
ruin the focus which would make the mirror pretty much useless.

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toomanybeersies
Apparently this telescope will have a focal length of 18,200 mm (18 meters).

That's pretty big.

~~~
Gibbon1
I really like this group photo

[http://www.gmto.org/gallery/mirror-
lab/#/lightbox&slide=48](http://www.gmto.org/gallery/mirror-
lab/#/lightbox&slide=48)

From here

[http://www.gmto.org/gallery/mirror-lab/](http://www.gmto.org/gallery/mirror-
lab/)

~~~
mturmon
A great photo. Here's the not-quite-so-big (6.5m) multi-element JWST mirror:
[https://www.flickr.com/photos/nasawebbtelescope/26602405532/...](https://www.flickr.com/photos/nasawebbtelescope/26602405532/in/album-72157629134274763/)

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wglb
My brother made some of the instrumentation for one of the earlier telescopes
in Chile at the NOAO in Tucson.

