Just recently the topic of glass polishing came up and I remembered a project that I was working on way back when I was still in high school. The project concerned the manufacturing of a reflector telescope. I knew I had a few pieces from that project in a box and I found the box today. I remember grinding with ever finer griot and lapping glass blanks as precisely as possible. All sorts of measuring devices were used to ensure the surface was properly ground and finished to a very tight tolerance of less than 25 nanometers. The glass was then silvered and used in the construction of the telescope. It's been over 30 years so the details are hazy.
Counterclockwise from the top right: lapping stone, glass blank, glass blank after having been finished on both sides - green glass for probably no reason, partially lapped glass blank, silvered mirror - that one was not perfect so I go to keep it You can't tell in the picture but this is not a flat but a concave mirror.