Today, large telescope mirrors are finished using ion figuring: neutral atoms are shot at the surface of the mirror blank, for a precision ablation of a few ang

Today, large telescope mirrors are finished using ion figuring: neutral atoms are shot at the surface of the mirror blank, for a precision ablation of a few angstroms. This works especially well for hexagonal mirror segments, because you can control the surface shape right to the edge of the mirror. Mechanical figuring (using ceramic or glass blanks) has troubles near the edge.