The next generation of ground-based astronomical observatories will require fabrication and maintenance of extremely
large segmented mirrors tens of meters in diameter. At present, the large production of segments required by projects like
E-ELT and TMT poses time frames and costs feasibility questions. This is principally due to a bottleneck stage in the
optical fabrication chain: the final figuring step. State-of-the-art figure correction techniques, so far, have failed to meet
the needs of the astronomical community for mass production of large, ultra-precise optical surfaces. In this context,
Reactive Atom Plasma (RAP) is proposed as a candidate figuring process that combines nanometer level accuracy with
high material removal rates. RAP is a form of plasma enhanced chemical etching at atmospheric pressure based on
Inductively Coupled Plasma technology. The rapid figuring capability of the RAP process has already been proven on
medium sized optical surfaces made of silicon based materials. In this paper, the figure correction of a 3 meters radius of
curvature, 400 mm diameter spherical ULE® mirror is presented. This work demonstrates the large scale figuring
capability of the Reactive Atom Plasma process. The figuring is carried out by applying an in-house developed
procedure that promotes rapid convergence. A 2.3 μm p-v initial figure error is removed within three iterations, for a
total processing time of 2.5 hours. The same surface is then re-polished and the residual error corrected again down to
λ/20 nm rms. These results highlight the possibility of figuring a metre-class mirror in about ten hours.
Fast figuring of large optical components is well known as a highly challenging manufacturing issue. Different
manufacturing technologies including: magnetorheological finishing, loose abrasive polishing, ion beam figuring are
presently employed. Yet, these technologies are slow and lead to expensive optics. This explains why plasma-based
processes operating at atmospheric pressure have been researched as a cost effective means for figure correction of metre
scale optical surfaces. In this paper, fast figure correction of a large optical surface is reported using the Reactive Atom
Plasma (RAP) process. Achievements are shown following the scaling-up of the RAP figuring process to a 400 mm
diameter area of a substrate made of Corning ULE®. The pre-processing spherical surface is characterized by a 3 metres
radius of curvature, 2.3 μm PVr (373nm RMS), and 1.2 nm Sq nanometre roughness. The nanometre scale correction
figuring system used for this research work is named the HELIOS 1200, and it is equipped with a unique plasma torch
which is driven by a dedicated tool path algorithm. Topography map measurements were carried out using a vertical work
station instrumented by a Zygo DynaFiz interferometer. Figuring results, together with the processing times, convergence
levels and number of iterations, are reported. The results illustrate the significant potential and advantage of plasma
processing for figuring correction of large silicon based optical components.
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