Rapid optical surface figuring using reactive atom plasma

Abstract

In the context of large optics figure correction, the reactive atom plasma (RAP) process constitutes a fast and unique solution for ultra-precise surface figuring. The RAP process combines high material removal rates with the advantages of non-contact machining methods. The RAP technology is based on an inductively coupled plasma torch that produces a sub-aperture near-Gaussian etching footprint, with material removal repeatability at nanometre level. A large-scale RAP figuring facility, Helios 1200, has been developed in the Precision Engineering Centre at Cranfield University to optimize and apply the RAP technology on metre-sized optical surfaces. In this paper, the first examples of figure correction carried out with Helios 1200 are reported. These experimental results were achieved over 100 and 140 mm diameter areas by means of in-house developed timedwell figuring techniques and a dedicated tool motion path. In particular, classical de-convolution techniques were adapted to the non-linear nature of the etching rate in order to derive velocity maps, while the tool-path algorithm was designed to induce a homogeneous temperature distribution on the process surface. Still, surface temperature raises are known to increase the rate of material chemical etching. Therefore, as a form of heat effects compensation, the adapted tool-path algorithm was combined with an iterative figuring procedure in order to assure faster process convergence. This promptly enabled the realization of figure error corrections down to λ/40 rms. High overall rates of convergence between 78 and 89% were attained within a maximum of three iterations. The mean processing times per iterative step were 6.6 minutes over the 140 mm diameter areas, thus confirming the potentiality of RAP as large surface figuring technique. Applied to metre-class optics, the method should then deliver comparable levels of form accuracy within less than ten hours processing time. The scope of the work presented included both investigation and verification of the effects of tool-path parameters, as well as of the modified de-convolution method. The validated figuring procedure can be progressively scaled up to medium and large optical surfaces. The surface texture after plasma machining was characterized. Some deterioration of surface roughness was observed.