Edges in CNC polishing: from mirror-segments towards semiconductors, paper 1: edges on processing the global surface

Segment-edges for extremely large telescopes are critical for
observations requiring high contrast and SNR, e.g. detecting exo-planets. In
parallel, industrial requirements for edge-control are emerging in several
applications. This paper reports on a new approach, where edges are
controlled throughout polishing of the entire surface of a part, which has
been pre-machined to its final external dimensions. The method deploys
compliant bonnets delivering influence functions of variable diameter,
complemented by small pitch tools sized to accommodate aspheric mis-fit.
We describe results on witness hexagons in preparation for full size
prototype segments for the European Extremely Large Telescope, and
comment on wider applications of the technology.