Lightweight optical manufacture is no longer confined to the conventional subtractive (mill and drill), formative (casting and forging) and fabricative (bonding and fixing) manufacturing methods. Additive manufacturing (AM; 3D printing), creating a part layer-by-layer, provides new opportunities to reduce mass and combine multiple parts into one structure. Frequently, modern astronomical telescopes and instruments, ground- and space-based, are limited in mass and volume, and are complex to assemble, which are limitations that can benefit from AM. However, there are challenges to overcome before AM is considered a conventional method of manufacture, for example, upskilling engineers, increasing the technology readiness level via AM case studies, and understanding the AM build process to deliver the required material properties. This paper describes current progress within a four-year research programme that has the goal to explore these challenges towards creating a strategy for AM adoption within astronomical hardware. Working with early-career engineers, case studies have been undertaken which focus on lightweight AM aluminium mirror manufacture and optical mountings. In parallel, the aluminium AM build parameters have been investigated to understand which combination of parameters results in AM parts with consistent material properties and low defects. Metrology results from two AM case studies will be summarised: the optical characteristics of a lightweighted aluminium mirror intended for in-orbit deployment from a nanosat; and the AM build quality of wire arc additive manufacture for use in an optomechanical housing. Finally, an analysis of how surface roughness from AM mirror samples and build parameters are linked will be discussed.
Ground-based astronomical instruments have mass limits to ensure they can operate safely and accurately. Reducing the mass of optomechanical structures relieves mass budget for other components, improving the instrument’s performance. Many industries have adopted generative design (GD) and additive manufacturing (AM; 3D printing) to produce lightweight components. This is yet to be implemented in ground-based astronomical instrumentation; this paper aims to provide insight into the advantages and limitations of this approach. The project studied the Extremely Large Telescope (ELT) Mid-infrared Imager and Spectrograph (METIS) threemirror anastigmat (TMA); comparing the conventional, subtractive machined design with GD-AM designs. The TMA was selected due to its bespoke geometry constrained by an optical path, a conventional design which did not consider mass reduction, the size of the part (615mm × 530mm × 525mm) that necessitated a study of different AM methods, and the operational environment (70K & 10−6 Pa). The study created mass-optimised designs of the TMA using topology optimisation and field-driven design. The performance of these designs was analysed using finite element analysis and optical ray tracing. It was found that GD-AM designs pass the required optical, structural and modal requirements, with a greater than 30% weight reduction when compared to the conventional design. The study investigated wire arc additive manufacturing (WAAM), a viable method of manufacturing components of the TMA’s size. To commence the validation of WAAM for cryogenic environments, samples of WAAM aluminium 5356 were created and studied. The internal and external dimensions of two samples were investigated using X-ray computed tomography and the outgassing rate of two sets of three samples were evaluated to quantify the difference between machined and as-built samples.
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