Additive manufacturing (AM; 3D printing), which builds a structure layer-by-layer, has clear benefits in the production of lightweight mirrors for astronomy, as it can create optimised lightweight structures and combine multiple components into one. AM aluminium mirrors have been reported that demonstrate a 44% reduction in mass from an equivalent solid and the consolidation of nine parts into one. However, there is a limit on the micro-roughness that can be achieved using AM aluminium at ∼5nm RMS (root mean square; Sq), therefore, to target applications at shorter wavelengths alternative AM materials are required. New capabilities in AM ceramics, silicon carbide infiltrated with silicon (SiC + Si) and fused silica, offer the possibility to combine the design benefits of AM with a material suitable for visible, ultraviolet and x-ray applications.
This paper will introduce the different printing methods and post-processing steps to convert AM ceramic samples into reflective mirrors. The samples are flat disks, 50mm diameter and 5mm in height, with three samples printed in SiC + Si and three printed in fused silica. Early results in polishing the SiC + Si material demonstrated that a micro-roughness of ∼2nm Sq could be achieved. To build on this study, the 50mm SiC + Si samples had three different AM finishing steps to explore the best approach for abrasive lapping and polishing, the reflective surfaces achieved demonstrated micro-roughness values varied between 2nm and 5nm Sq for the different AM finishing steps. To date, the printed fused silica material has heritage in lens applications; however, its suitability for mirror fabrication was to be determined. Abrasive lapping and polishing was used to process the fused silica to reflective surface and an average micro-roughness of <1nm Sq achieved on the samples.
The next generation of ultra-precision optics requires rapid and cost effective surface figuring technology. Different manufacturing technologies exist: magnetorheological finishing, chemical mechanical polishing, and ion beam figuring; however, these technologies are slow and lead to expensive optics. Plasma figuring, operating at atmospheric pressure, is a cost effective method for figure correction of ultra-precision optical surfaces. In this presentation, fast figure correction of optical surfaces is reported using the Satisloh Plasma Polisher (SPP). The technology uses a reactive plasma jet to surface figure flats of fused silica. The plasma jet is powered by a solid state microwave generator, which operates in pulse mode to reduce the plasma temperature hence increasing the repeatability of the etched trenches. The trenches are characterised using an interferometer. Each trench follows a Gaussian function. Material removal rates can range from nm3 to a mm3 per minute, which can result in surface form error reductions of 90% in a single iteration. The surface roughness is measured using a white light interferometer and shows no degradation in the surface finish.
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