Pasadena, CA – The Thirty Meter Telescope Optics Group recently reached a critical production milestone, with the approval of the first polished roundel (# SN023) for TMT’s Primary Mirror (M1). Teams at TIO, NAOJ, and Canon reviewed the manufacturing operations required to turn a meniscus mirror blank into a polished roundel, and the roundel was confirmed to meet all requirements set by the TMT Project. This approval obtained in December 2020 validates years of effort by Canon Inc. to develop a successful mirror polishing process.
Each roundel will eventually become a finished segment of the 30m diameter M1 primary mirror. To ensure that each segment performs satisfactorily, each roundel must be produced in accordance with the strictest technical specifications. Segment number “SN023” was the first of the 492 mirror segments to have been polished in accordance with its full optical prescription.
The “Stressed Mirror Polishing” (SMP) technique, employed by TMT-Japan, and refined jointly with TMT, has successfully transformed a meniscus mirror blank into an off-axis aspheric roundel. The glass segment is primarily “bent” using controlled shear forces and its surface is polished to a concave sphere. After polishing, the applied stress is released, the glass springs back into its relaxed shape, and the spherical surface deforms into the desired aspherical surface figure.
The 2DP is designed and built by Coherent Inc. in the United States. The imposing machine requires a crane to lift it up and lower it down onto the glass. Once very precisely centered and aligned over the glass, the 2DP evaluates the optical surface with respect to the specifications. The 2DP uses a contact-approach metrology system consisting of probes that are positioned in a carefully designed pattern that measures how the shape of the roundel matches its designed curvature, called surface figure.
The 2DP also provides a total measurement of the surface variation from its ideal profile, which needs to be less than two microns Peak-to-Valley at this phase of processing.
TMT quality assurance assists in improving work process and efficiency, emphasizing the importance of catching any potential defects or issues early on, before completion of segment production. As part of this process, a complete Data Package is produced for each roundel and includes all surface figure measurements, the thickness and diameter of the roundel, as well as information about the instrumentation, tooling and calibration methods used for inspecting and measuring the roundel. This technical record becomes available as part of the TMT database for the lifetime of the Observatory.
“The conformance review process is a really significant team effort between TMT’s Systems Engineering, Optics, Quality and NAOJ-Canon teams,” said Josh Church, Associate Mechanical Systems Engineer at TMT. “The approval of the first Roundel Data Package paves the way for two exciting next steps: SN023 can be queued for its next manufacturing phase, and Canon can begin the process of submitting additional roundels for conformance review.”
Mr. Tetsuji Ota, Canon team lead engineer, also voiced his gratitude for achieving this significant milestone. “My colleagues and I are grateful for the continued support we have received from NAOJ and TIO during these past years. We are pleased that SN023 performance has met all the rigorous specifications and inspection established by TIO. Canon Inc. is committed to continue providing TMT polished roundels. In fact, we are preparing the review of 10 additional roundels and look forward to many future roundel acceptances.”
Hence, TMT and NAOJ plan to continue with the production and technical conformance reviews of additional roundels within the next months. The exact dates of these future roundel inspections will be confirmed as soon as international travel can safely resume after the Covid-19 pandemic.
“TMT congratulates TMT Japan and Canon Inc. for the successful technical conformance of the first roundel SN023,” said John Rogers, TMT Deputy Systems Engineering Group Leader. “The review showed that TMT specifications are achievable using TMT’s unique fabrication and metrology processes. This milestone demonstrates the manufacturing efficiency and performance provided by NAOJ/Canon.”
TMT partners in China, India and California are also involved in polishing M1 segments and a similar conformance review of roundel production is expected to happen early 2021 at Coherent Inc. in the United States.
TMT’s primary mirror has a six-fold symmetry, with 82 unique segments each with a slightly different surface shape. A seventh set of 82 segments will be manufactured as spare, which is needed to allow segments to be swapped out for periodic recoating. As a result, a total of 574 Segments will be manufactured, and Japan will handle the processing of approximately 30% of the total segments needed.
The fabrication of TMT’s Primary Mirror System (M1S) is an international effort with TIO members from China, India, Japan, and USA making significant contributions to the M1S. Work on M1S is actively happening in all four countries.
Overview of TMT primary mirror segment production:
The primary mirror segment fabrication starts with the casting of Clearceram glass-ceramic at the Ohara Inc. factory in Japan. Each large circular casting or boule is sliced into three blanks, flat on both sides.
The blanks go through a first grinding and polishing process to become Meniscus blanks. In Japan, the company Okamoto Optics, grinds the optical and back surfaces to a meniscus. Then Canon Inc. is responsible for the delicate polishing process that brings the glass surface quality closer to the prescription needed by TMT, including aspheric grinding and stressed mirror polishing, to become roundels. Next come the machining steps to cut the roundels into a hexagonal shape, mount them on their support structure assemblies and execute the final step of Ion Beam Figuring to provide the precise optical figure required by TMT.