Abstract
The 4 m diameter SiC aspheric mirror emerges due to a series of technological breakthroughs in blank mirror preparation, asphere fabrication, and testing, as well as cladding and coating, laying the groundwork for future research into large SiC mirrors for astronomical observation.
“Facing upwards to the blue sky, we behold the vast immensity of the universe; when bowing our heads towards the ground, we again satisfy ourselves with the diversity of species.” Human beings have shown great curiosity about the universe and ourselves since ancient times. Telescope is one of the most essential tools for universal research. Since a larger aperture benefits higher resolution and weaker signal detection, the size of telescopes has kept increasing since the first telescope was invented over 400 years ago. To achieve scientific goals for the coming decade, high-performance telescopes with a larger aperture and better wavefront error are urgently demanded1,2.
To build a large-size telescopic system, primary mirror manufacture3 is one essential work. Researchers pursue more free design and manufacture variables to improve imaging quality and reduce size and weight. Even though subwavelength optics4 and freeform optics5 show great potential in this aim, aspherical primary mirror was still the most common choice in recent systems. Meanwhile, to increase the size of mirrors and ensure high imaging quality in complex conditions, the areal density must be reduced while maintaining stiffness and stability6. Therefore, silicon carbide (SiC) has recently drawn significant attention in the telescope community worldwide because of its high specific stiffness and dimensional stability7.
However, the stringent requirements on material, surface type, full-spatial frequency (FSF) surface shape error, and surface roughness make manufacturing large SiC aspheric mirrors faces tough challenges due to the lack of systematic technology and equipment. Previously, the most giant SiC mirror was the one used as the primary mirror of the Herschel telescope, with a diameter of 3.5 m. The rough shape accuracy (3 μm) limited its working spectral range in far infrared and sub-millimeter8. In 2014, a large SiC mirror (i.e., 1.54 m × 0.49 m) available for the visible light field was reported by the French companies Reosc and Boostec9.
Recently, Xuejun Zhang and colleagues at Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences (CIOMP, CAS) have successfully manufactured a 4 m diameter SiC aspheric mirror, which is the largest reported worldwide (Fig. 1a). They summarized challenges in the fabrication process and their complementary strategies in the paper published on Light: Science & Applications10.
Several steps should be managed carefully in large-size mirror manufacturing, including blank preparation, lapping, polishing, cladding, etc. During the manufacturing process, accurate measurement across such a large area is essential as well. Xuejun Zhang and co-authors completed a comprehensive job among all these processes. To form lightweight SiC green bodies, water-soluble room temperature vanishing mold and gel casting technology was applied. Then the whole Φ4.03 m SiC blank was stitched by 12 segments of green bodies using vacuum reaction sintering11, and SiC powder was mixed in a binder to ensure the homogeneous joining12. Afterward, they developed an efficient converging algorithm based on frequency-domain correlation of surface error and a new optimized processing chain combining the CNC generating, stressed lap grinding/polishing, computer-controlled optical surfacing (CCOS), magnetorheological finishing (MRF) polishing and ion beam finishing. This combination enables high efficiency and accuracy fabrication with a 30% increase in error convergence rate. A low-temperature magnetron sputtering physical vapor deposition Si cladding process was proposed to replace the high-temperature chemical vapor deposition cladding process. The surface roughness of the Si cladding layer could be polished to 0.8 nm RMS. Meanwhile, to ensure the fabrication accuracy of the aspherical surface, a precision measurement toolset was developed, which seamlessly combines data from different test instrument. 6 nm RMS measurement accuracy and 2.6 nm RMS reproducibility were achieved, respectively. The errors caused by mirror self-gravity deflection and air turbulence13 were decoupled by gravity unloading and time-averaging techniques.
This aspheric mirror verified the large SiC mirror production capability. The aerial density of the SiC blank is less than 120 kg/m2, the thickness inhomogeneity of the cladding layer is less than 5%, and the final surface figure error and roughness are 15.2 nm RMS and 0.8 nm RMS, respectively (Fig. 1b). This mirror was delivered to customers in 2019, and applied in astronomical observation, earth exploration, and other areas. The key characteristics of this mirror ensure its great performance in those applications.
The high-precision manufacturing of a 4 m SiC mirror has overcome the difficulties of large-aperture lightweight silicon carbide material preparation technology, aspheric processing, modified coating technology, and measurement technology. We can expect that the performance of the proposed technique could be further promoted, pushing its application to a higher level. This technique, along with the rapid developments of large-size and lightweight planar optical elements4 and mirror segmented strategies14,15, is promising to promote the next generation of engineering optics.
References
Zhu, R. Z. et al. Global high-resolution optical satellite overview (2): europe. Spacecr. Eng. 25, 95–118 (2016).
Kim, D. et al. Advances in optical engineering for future telescopes. Opto-Electron. Adv. 4, 210040 (2021).
Kim, D. W. et al. Advanced technology solar telescope 4.2 m off-axis primary mirror fabrication. Proceedings of the Optical Fabrication and Testing 2014. Optica Publishing Group, Kohala Coast, Hawaii, United States, 2014, OTh2B.3).
Luo, X. G. Subwavelength artificial structures: opening a new era for engineering optics. Adv. Mater. 31, 1804680 (2019).
Bauer, A., Schiesser, E. M. & Rolland, J. P. Starting geometry creation and design method for freeform optics. Nat. Commun. 9, 1756 (2018).
Zhang, H. D. et al. Modified surface testing method for large convex aspheric surfaces based on diffraction optics. Appl. Opt. 56, 9398–9405 (2017).
Jiang, F. et al. Research Progress of Optical Fabrication and Surface-Microstructure Modification of SiC. J. Nanomaterials 2012, 984048 (2012).
Pilbratt, G. L. Herschel mission overview and key programmes. Proceedings of SPIE 7010, Space Telescopes and Instrumentation 2008: Optical, Infrared, and Millimeter. 701002 (SPIE, Marseille, France, 2008).
Rodolfo, J. et al. SIC mirrors polishing. Proceedings of SPIE 10563, International Conference on Space Optics—ICSO 2014. 105631Z. (SPIE, Tenerife, Canary Islands, Spain, 2014).
Zhang, X. J. et al. Challenges and strategies in high-accuracy manufacturing of the world’s largest SiC aspheric mirror. Light Sci. Appl. 11, 310 (2022).
Zhang, G. Gelcasting process of 1.5m SiC ceramic green body. Opt. Precis. Eng. 21, 2989–2993 (2013).
Zhang, G. Study on join method of reaction bonded silicon carbide green body. Infrared Laser Eng. 43, 193–196 (2014).
Hu, H. X. et al. Air flow turbulence orthogonality and surface error estimation in large aperture optical testing. Proceedings of SPIE 12166, Seventh Asia Pacific Conference on Optics Manufacture and 2021 International Forum of Young Scientists on Advanced Optical Manufacturing. 121666B (SPIE, Hong Kong, China, 2021).
Cui, X. Q. et al. The large sky area multi-object fiber spectroscopic telescope (LAMOST). Res. Astron. Astrophys. 12, 1197–1242 (2012).
Gardner, J. P. et al. The James webb space telescope. Space Sci. Rev. 123, 485–606 (2006).
Author information
Authors and Affiliations
Corresponding author
Ethics declarations
Conflict of interest
The author declares no competing interests.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Luo, X. High-precision fabrication of 4m SiC aspheric mirror. Light Sci Appl 12, 4 (2023). https://doi.org/10.1038/s41377-022-01050-w
Published:
DOI: https://doi.org/10.1038/s41377-022-01050-w