# Design and Fabrication Technology of Metal Mirrors Based on Additive Manufacturing: A Review
## 3 Design Technology of Additively Manufactured Metal Mirrors
### Conventional Design  
The so-called conventional design is a lightweight design that can be manufactured by conventional machining and modeled by standard CAD software programs. Figure 13 and Table 5 show quintessential lightweight holes for mirrors and the performance comparison of each hole, respectively. Generally, quadrilateral holes and regular triangle holes are preferred. However, the triangle holes are the best choice, given their highest stiffness with the same weight [53; 54]. Figure 12: Schematic diagram of connectivity constraints [37]. Corning's researchers designed and fabricated a four-inch circular flat mirror with a honeycombed lightweight structure in 2016. They prefabricated the mirror blank by printing \(\text{ASI}_{7}\text{Mg}_{0.3}\) powder via DMLS. After SPDT and polishing, the surface roughness could reach RMS 1.5 nm (see Figure 14). Therefore, [PERSON]'s team proposed and preliminarily demonstrated the feasibility of manufacturing metal mirrors utilizing AM by comparing them with various other materials and methods. They thought the layer-by-layer SLS process and the resulting long cycle times, which are a function of the volume of material rather than complexity, are what drive the costs [17]. Finally, they concluded that the AM metal mirror has advantages in fabricating complex structures, reducing cost [29]. The round and square mirrors shown in Figure 15 were manufactured by Optimax Systems in 2017 [32]. The material used was \(\text{FeNi}_{36}\), whose CTE is near zero. Furthermore, it is suitable for the application of metal mirrors in cryogenic and space applications. However, the finished mirrors were not entirely dense when measuring them with a scanning white-light interferometer. The test result shows that the finished optical surface  
Figure 14: DMLS Mirror with Corning enhanced performance surfacing process [17]. Figure 13: Quintessential lightweight holes for mirrors. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Performance** & **Best** & **Better** & **Good** \\ \hline Lightweight rate & Quadrilateral & Hexagon & Triangle \\ Stiffness & Triangle & Quadrilateral & Hexagon \\ Deformation resistance of axial temperature gradient & Quadrilateral & Hexagon & Triangle \\ Deformation resistance of radial temperature gradient & Triangle & Quadrilateral & Hexagon \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Performance comparison of lightweight holes. exhibited a 9 \(\upmu\)m void. The mirrors were in the experimental stage and had not been put into practice. Further surface modification and polishing are needed for applications at wavelengths shorter than the infrared. In 2018, [PERSON]'s team from IOF in Germany designed and compared the performance of five lightweight metal mirrors: solid mirror, drill mirror, open-back mirror with honeycomb, sandwich mirror with honeycomb, and empty mirror (Figure 16) [33]. For the sandwich mirror, the stiff and strong face sheets carry the bending loads, while the core resists shear loads. The results of the modal analysis show that the sandwich mirror with honeycomb has the best mechanical properties (Figure 17). It has the second lightest weight, only after the open-back structure, which precisely verifies the sandwich mirror's advantages in many aspects given in Section 1. Moreover, this experiment has laid a theoretical and practical foundation for the team's next-generation AM metal mirrors. Figure 16: Several structural diagrams: (**a**) CAD model of the “drill”; (**b**) CAD model of the honeycomb mirror with sectioning planes added to demonstrate the hollow structure; (**c**) Honeycomb mirror with open backside [33]. Figure 15: Round and square metal mirrors [32]. By summarizing the experience, [PERSON] et al. from Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) fabricated an Al mirror based on SLM technology in 2020 [37]. The mirror is a flat metal mirror with the dimensions of 67 mm \(\times\) 50 mm used as a fast-steering mirror (FSM) in the infrared optical system.