Spaced-based mirrors are a developing use-case for Additive Manufacturing (AM), the process that builds a part layer-by-layer. The increased geometric freedom results in novel and advantageous designs previously unachievable. Conventionally, mirror fabrication uses subtractive (milling & turning), formative (casting) and fabricative (bonding) manufacturing methods; however, an additive method can simplify an assembly by consolidating individual components into one, and incorporating lattice structures and function optimised geometries to reduce the mass of components, which are beneficial to space-based instrumentation as mass and volume are constrained. Attention must be given to the printability of the design - build orientation and powder/resin removal from lattices and internal cavities are challenges when designing for AM.
This paper will describe the design, manufacture and metrology of mirror prototypes from the Active Deployable Optical Telescope (ADOT) 6U CubeSat project. The AM mirror is 52mm in diameter, 10mm deep, with a convex 100mm radius of curvature reflective surface and deploys telescopically on three booms. The objectives of the designs were to combine the boom mounting features into the mirror and to lightweight both prototypes by 50% and 70% using internal, thin-walled lattices. Four final lattice designs were downselected through simulation and prototype validation. Prototypes were printed in the aluminium alloy AlSi10Mg using powder bed fusion and fused silica using stereolithography. Aluminium mirrors were single point diamond turned and had surface roughness measurements taken. Fused silica designs were adapted from the aluminium designs and have completed printing.
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.
Details of a programme to investigate the outgassing rate of additively manufactured (AM) aluminium alloys are presented. AM has significant potential benefits to applications in ground- and space-based instrumentation, particularly in mass optimisation, part consolidation and increased design freedom. However, its use in high-risk projects is often curtailed by lack of heritage and an imperfect understanding of the materials. The programme goal was to address one of the most significant topics preventing wider adoption of AM technology in cryogenic and space-based applications; uncertainty about material outgassing. The sensitivity of outgassing rates to various key parameters was characterised, including print method, post-processing and geometrical complexity. Correlation of outgassing rates against other measurable properties, such as sample porosity and surface roughness, was also investigated via the use of X-ray computed tomography and profilometry. Finally, the test apparatus, experimental design and implications of the findings on design and process control are discussed.
Lightweight, aluminum, freeform prototype mirrors have been designed and fabricated by a Thai led team, with UK support, for intended applications within the Thai Space Consortium (TSC) satellite series. The project motivation was to explore the different design strategies and fabrication steps enabled by both conventional (mill, drill, and lathe) and additive (3D printing) manufacture of the prototype substrates. Single Point Diamond Turning was used to convert the substrates into mirrors and optical metrology was used to evaluate the different mirror surfaces. The prototype criteria originated from the TSC-1 satellite tertiary mirror, which is designed to minimize the effect of Seidel aberrations before the beam enters the hyperspectral imager. To converge upon the prototype designs, Finite Element Analysis (FEA) was used to evaluate the different physical conditions experienced by the prototypes during manufacture and how these influence the optical performance. The selected designs satisfied the mass and surface displacement criteria of the prototype and were adapted to either the conventional or additive manufacturing process. This paper will present the prototype design process, substrate manufacture, optical fabrication, and an interferometric evaluation of the optical surfaces comparing the conventional and additive manufacturing processes.
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