Polishing has traditionally been a process of mechanical abrasion with each iteration removing the damage from the
previous iteration. Modern sub-aperture techniques such as CCOS, MRF polishing etc. have added a considerable
amount of determinism to this iterative approach. However, such approaches suffer from one significant flaw, i.e., the
algorithms are completely guided by figure error. This approach fails when there is a considerable amount of strain
energy stored in the substrate and becomes very evident when the aspect ratio of the mirror increases significantly
causing relaxation of strain energy to have deleterious and unpredictable effects on figure between iterations. This is
particularly pronounced when the substrate is made of a hard ceramic such as silicon carbide requiring a considerable
amount of pressure to obtain any appreciable material removal rate. This paper presents an alternate approach involving
a stress-free figuring step and a buffing step intended to recover the surface roughness.
The next generation of 30-100 metre diameter extremely large telescopes (ELTs) requires large numbers of hexagonal
primary mirror segments. As part of the Basic Technology programme run jointly by UCL and Cranfield University, a
reactive atomic plasma technology (RAP(tm)) emerged from the US Lawrence Livermore National Laboratory (LLNL), is
employed for the finishing of these surfaces. Results are presented on this novel etching technology. The Inductively
Coupled Plasma (ICP) operated at atmospheric pressure using argon, activates the chemical species injected through its
centre and promotes the fluorine-based chemical reactions at the surface. Process assessment trials on Ultra Low
Expansion (ULE(tm)) plates, previously ground at high material removal rates, have been conducted. The quality of the
surfaces produced on these samples using the RAP process are discussed. Substantial volumetric material removal rates
of up to 0.446(21) mm <sup>3</sup>/s at the highest process speed (1,200 mm/min) were found to be possible without pre-heating the
substrate. The influences of power transfer, process speed and gas concentration on the removal rates have been
determined. The suitability of the RAP process for revealing and removing sub-surface damage induced by high removal
rate grinding is discussed. The results on SiC samples are reported elsewhere in this conference.
Mechanical grinding and shaping of optical materials imparts damage that manifests itself as defects and cracks that can propagate well below the surface of the optic. Mitigation of damage is necessary to preserve the integrity of the optic and relieve residual stress that can be detrimental to its performance. Typically, a sequence of subsequent polishing steps with finer and finer grit sizes is used to remove damage, but the process can be painfully slow especially for hard materials such as silicon carbide and often fails to remove all the damage. Reactive Atom Plasma (RAP<sup>TM</sup>) processing, a non-contact, atmospheric pressure plasma-based process, has been shown to reveal and mitigate sub-surface damage in optical materials. Twyman stress tests on thin glass and SiC substrates demonstrate RAP's ability to relieve the stress while at the same time improving surface form.