The Giant Magellan Telescope is a marvel of engineering, and Dr. Patrick McCarthy explains the years-long process to make an optic mirror that costs over $20 million.
Via IEEE Spectrum:
To begin, more than 17,000 kilograms of special glass are ordered and inspected for flaws. Next, a crew must build a 15-metric ton ceramic structure to serve as a mold for the glass, which they carefully place one chunk at a time. The glass is slowly melted and continuously spun in a furnace to create a parabolic shape, then cooled by fractions of degrees over the course of three months. And that's only the beginning.
Once cooled, massive machinery lifts the mirror and tilts it to a vertical position. Engineers purge the ceramic mold from the mirror, wait for it to dry, and then rotate it again. They grind and refine the back of the mirror with exacting precision. Then they reposition the mirror in order to shape and polish the front face to within 20 nanometers of perfection—a process that takes about 18 months. Along the way, it undergoes four optical tests, some of which were engineered specifically for this project.