You can use an optical flat. Interference fringe patterns will tell you how a surface that was cut perfectly flat on a machine becomes bowl shaped when you unmo

You can use an optical flat. Interference fringe patterns will tell you how a surface that was cut perfectly flat on a machine becomes bowl shaped when you unmount the part. https://en.m.wikipedia.org/wiki/Optical_flat https://en.m.wikipedia.org/wiki/Optical_flat