# Freeform Wide Field-of-View Spaceborne Imaging Telescope: From Design to Demonstrator
## 3 Telescope Proof-of-Concept Demonstrator  
### Manufacturing and Alignment Process  
The manufacturing of freeform mirrors requires high-precision multi-axis milling, based on computer-aided design (CAD). The two mirrors are made of oxygen-free high thermal conductivity (OFHC) copper, which is the material of choice for broadband infrared applications [19; 20]. Starting from two flat OFHC copper blanks, a preliminary shape is first generated using high-precision 5-axis milling (Roders TEC), as presented in Figure 6a. Next, 5-axis ultraprecision diamond tooling (Nanotech 350 FG) is used to superimpose the freeform terms on the two mirror surfaces, while ensuring high optical surface quality (Figure 6d). This latter process takes about 30 h, for a high precision (accuracy of 10 \(\upmu\)m) of the XY polynomial freeform coefficients. The combination of both manufacturing devices enables the accurate manufacturing of the freeform surface shapes (Figure 6b,c). However, precise positioning and alignment of the mirrors with respect to each other, and with the milling tool on both manufacturing devices, is also indispensable. To ensure that minimal manufacturing errors are introduced when mounting the mirrors from the milling tool to the diamond tooling device, both copper parts were first mounted on a single mounting plate, after which the milling process took place. From the start of the manufacturing process, both copper blanks were thus monolithically mounted in a fixed position. The mounting plate was generated using the Roders 5-axis milling tool ensuring a precise diameter (runout of 8 \(\upmu\)m with respect to the XY origin) and including a reference plate. Additionally, four alignment pins were used to enable accurate positioning of the copper parts. To ensure the same alignment during the milling process as during the diamond tooling, the same references were used during the alignment. The diamond milling tool could be aligned with an accuracy of 0.5 \(\upmu\)m with respect to the rotation center. The manufacturing of the mirrors was fully realized in-house, at the VUB B-PHOT Brussels Photonics Innovation Centre. \begin{table}
\begin{tabular}{c c} \hline \hline  & **Element Tolerances** \\ \hline Decenter X & 0.01 mm \\ Decenter Y & 0.01 mm \\ Tilt X & 0.01\({}^{\circ}\) \\ Tilt Y & 0.01\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Element tolerances that can be realized using COTS optomechanics with micrometer screws. Validation of the manufacturing process was performed on the secondary mirror by a characterization of the surface shape and roughness, using in-house metrology tools in a cleanroom environment. Surface roughness of 10 nm was measured using a white-light interferometer, considering a field-of-view of 0.17 mm \(\times\) 0.22 mm and a 2 nd-order Gaussian regression filter. Using a coordinate measurement machine (CMM), the surface shape of the mirror was validated, indicating a maximum error of \(\pm\)3.3 \(\upmu\)m with respect to the simulated mirror surface (Figure 7). The surface of the primary mirror could not be reached due to the monolithic mounting but is expected to show a similar performance as the same cutting parameters were applied to both surfaces. Figure 6: Manufacturing of the freeform telescope mirrors: (**a**) both mirrors positioned on the mounting structure, after polishing and before ultraprecision diamond tooling. M1 is the primary mirror and M2 the secondary mirror; (**b**) close-up of the primary mirror; (**c**) close-up of the secondary mirror; (**d**) diamond tooling generating the freeform surface shape and optical surface quality. Figure 7: Validation of the manufactured surface shape using CMM: (**a**) 3D overview of the measurements, where the measured surface profile is indicated in red and the simulated one in grey; (**b**) cross-section along the mirror surface, enabling a comparison between the measured and simulated profile; and (**c**) absolute error achieved by subtracting the measured and simulated mirror profiles.