I'm not sure I can explain it fully, but my understanding is that it is related to the probability distribution of the abrasive technique. The technique involv

I'm not sure I can explain it fully, but my understanding is that it is related to the probability distribution of the abrasive technique. The technique involves random rotation and random motion, using lapping blocks that are the same size or smaller than the mirror. The center of the mirror is the most likely to be contacted by any motion, so it is abraded more frequently and becomes the deepest point, while the edges are worn less. The distribution between "less" and "more" is spherical. I can at least understand the process of grinding or lapping a surface flat. Here are a couple examples: https://www.youtube.com/watch?v=jx1D0buRCOY https://www.youtube.com/watch?v=jx1D0buRCOY https://www.youtube.com/watch?v=REeGn4hN1Bg https://www.youtube.com/watch?v=REeGn4hN1Bg https://www.youtube.com/watch?v=ATG_UYYfx7w https://www.youtube.com/watch?v=ATG_UYYfx7w