Thank you and @74hc595 for useful info that I did not previously know!
Most of science and engineering begins with the question “Why not?” 
Much fun was had making these mirrors and scopes in the early days:
http://www-history.mcs.st-and.ac.uk/Extras/Herschel_Museum.html
The engineering knowledge in these comments warms my little Humanities-educated heart.
Oh, yeah, about the back surface needing to be perfect… I remember that the main reason is that the mirror is supported in the telescope by 160 actuators that can apply adjustable pressure to each mounting point, which allows the mirror to be held in the same exact curvature at which it is tested, even as it’s tilted in the telescope to be pointed up at various angles.
Each actuator presses against a triangular cradle that presses against three points on the mirror itself. These mounting points are glued onto the rear of the mirror. They need to be just ever so, to ensure that the mirror is supported within a few tens of nanometers of its required flatness.
Beryllium only makes sense for spacecraft mirrors - it’s way too costly and toxic to use where glass is suitable, which is any place on Earth.
I would’ve thought polishing the back is also necessary to remove stress risers.
Or so the robots (who take skilled American jobs) would have us believe.
Sub-nanometer precision kinda freaks me out.
My background is in civil engineering, and no matter what you put in your plans, the smallest unit of measurement on site is traditionally the foreman’s thumb.
I have a vague recollection of a backyard astronomer here in British Columbia who had a large diameter spinning mercury telescope. He and the wife later divorced and she got the house … along with the unmovable telescope.
Indeed! You have to prove it’s feasible before they let you have a lab to prove that it’s feasible. Roger Angel had his first mirror-making oven in the hallway of the first floor of the Optical Sciences building, where I worked as an undergrad in Tucson. Same with the first silicon crystal growing furnace at Texas Instruments - it was in a hallway because no one in the company would finance such a crazy idea.
While the things I do at work are far less ambitious than the ones you mentioned, the thing about hallway couldn’t be more true 
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I, and a colleague once needed a stress testing machine with 1-2 micrometer positioning accuracy and capable of measuring forces as low as 100 micro Newtons for our PhD theses (we were testing mechanical properties of tissues for pharmacologic experiments). In the lab we only have large machines in 100 kN - 6000 kN range. When I said that we could assemble one for about 1k$, no one really believed, but we got small grant for it.
The machine had a large subwoofer speaker as an actuator, controlled in closed loop (with linear encoder) using modified audio amplifier and CNC controller. To isolate it from external vibration we embedded a bicycle tube in machine table. We assembled the machine in my flat, and when we proved that it works correctly, suddenly there was a lot interest in it
Here’s the machine:
And the the subwoofer inside:
How does one link this cool post to the “Whatchamaking?” thread?
But the scale is also a bit different. I sometimes do optical measurements for colleagues who are researching new types of concrete reinforcement. While for things like composites we typically use samples that are at most few centimetres wide, they use 4 meter beams and for some other samples loads as high as 600 tonnes are not unheard of. When something goes wrong with such experiment, things get really interesting:
On the photo there’s a 4 meter wide basalt reinforced beam that slipped out of the testing machine and is flying forward, soon to cause a lot of destruction 
Also, I’ve once did measurements during stress tests of a new bridge. They loaded it with about 18 trucks loaded with sand. The scale is really impressive.
Thanks! I’ve just reposted it there
Ah yes, nothing like an afternoon in the test lab, watching the hydraulic test rigs do their stuff… You get very interesting visuals and sounds with steel as well. Good times. These days, I’m mostly pushing paper.
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