Mirror grinding
My parents run a business making telescope mirrors, and I grew up around this exact process. Measurement is the biggest difference between this and what they do. Take a look at <a href="https://en.wikipedia.org/wiki/Interferometry#Engineering_and_applied_science" rel="nofollow">https://en.wikipedia.org/wiki/Interferometry#Engineering_and...</a> if you want to know more about how it's done in industry.<p>The best metaphor I got from them about how precise a shape they make in glass is that if you took a typical 1 meter mirror and scaled it up to the size of the United States, the biggest deformity from a perfect curve would be less than you are tall - far less than your ability to perceive.<p>The other thing that was impressed me is that up until recently, the last stage of high-precision mirror making was literally done by hand. My dad would literally rub on a mirror with very, very fine grit to take out bumps on the order of microns. Recently, they've switched to machines in that last step to make it faster and more accurate, but for many applications the traditional way worked just fine.