Additive manufacturing (AM), more commonly known as 3D printing, is a commercially established technology for rapid prototyping and fabrication of bespoke intricate parts. To date, research quality mirror prototypes are being trialled using additive manufacturing, where a high quality reflective surface is created in a post-processing step. One advantage of additive manufacturing for mirror fabrication is the ease to lightweight the structure: the design is no longer confined by traditional machining (mill, drill and lathe) and optimised/innovative structures can be used. The end applications of lightweight AM mirrors are broad; the motivation behind this research is low mass mirrors for space-based astronomical or Earth Observation imaging. An example of a potential application could be within nano-satellites, where volume and mass limits are critical. The research presented in this paper highlights the early stage experimental development in AM mirrors and the future innovative designs which could be applied using AM.<p> </p> The surface roughness on a diamond-turned AM aluminium (AlSi<sub>10</sub>Mg) mirror is presented which demonstrates the ability to achieve an average roughness of ~3.6nm root mean square (RMS) measured over a 3 x 3 grid. A Fourier transform of the roughness data is shown which deconvolves the roughness into contributions from the diamond-turning tooling and the AM build layers. In addition, two nickel phosphorus (NiP) coated AlSi<sub>10</sub>Mg AM mirrors are compared in terms of surface form error; one mirror has a generic sandwich lightweight design at 44% the mass of a solid equivalent, prior to coating and the second mirror was lightweighted further using the finite element analysis tool topology optimisation. The surface form error indicates an improvement in peak-to-valley (PV) from 323nm to 204nm and in RMS from 83nm to 31nm for the generic and optimised lightweighting respectively while demonstrating a weight reduction between the samples of 18%. The paper concludes with a discussion of the breadth of AM design that could be applied to mirror lightweighting in the future, in particular, topology optimisation, tessellating polyhedrons and Voronoi cells are presented.
Deformable, piezo bimorph mirrors are often used to expand X-ray beams to a continuous range of sizes. However,
optical polishing errors present on all X-ray mirrors introduce striations into the reflected beam. To counteract them, reentrant
surface modifications with alternating concave and convex curvature have been proposed and applied to mirrors
of fixed shape or bimorph mirrors. For the latter, a new method of constructing re-entrant surface modifications on
segments of unequal length is described. This allows the re-entrant modification required for a desired beam size at the
focal point to be matched to the bimorph mirror’s polishing errors, thus reducing the voltage variations. Optical
profilometry using the Diamond-NOM showed that a 5-segment and a 7-segment modification could be suitably applied
to a deformable bimorph mirror. X-ray tests showed that striations caused by the 5-segment modification in the beam at
the focus are concentrated at the beam edges, while the beam center is left clear. This is in contrast to simple defocusing,
in which a strong side shoulder appears. The 7-segment modification produces a pattern of evenly spaced striations. The
intensity spikes seen with the re-entrant modifications are caused chiefly by the finite curvature of the mirror at the
turning points. The question of whether deformable bimorph mirrors with different piezo response functions could
sharpen the curvature changes will be investigated. Optimal modifications of continuous curvature, which could more
realistically be applied, will be sought.