The Tertiary Mirror Support System and Positioner Assembly (M3SSPA) has now successfully completed its Preliminary Design Phase after the Preliminary Design Review (PDR) was held at NOIRLab in Tucson, Arizona on August 11–12, 2025. A panel of distinguished experts in astronomical telescope design examined the requirements, interfaces, performance, and design maturity, as well as potential technical risks, hazards, and mitigation strategies. They also assessed the completeness of trade studies, modeling, analysis, and plans for testing, fabrication, integration, verification, and maintenance.
The M3SSPA is a key element of the Thirty Meter Telescope (TMT) International Observatory (TIO). Its role is to direct the optical beam to the scientific instruments located on both Nasmyth platforms while maintaining the precise figure of the mirror. The system includes a large elliptical flat mirror (3.6 m × 2.5 m) positioned near the primary mirror, articulated around two axes with extreme precision. This configuration enables rapid switching between instruments on either platform.
Rendering of the TMT Tertiary Mirror System (M3S), showing the Tertiary Mirror Support System and Positioner Assembly (M3SSPA).
Access for in-situ maintenance and inspection is built into the design: at Zenith angle = 0°, technicians can reach the mirror using a servicing ladder from inside the structure, while at Zenith angle = 90°, the telescope’s Aerial Service Platform (ASP) provides external access.
The Advanced Mechanical and Optical Systems (AMOS, Belgium) and TIO teams collaborated closely on the tertiary mirror design, carried out in parallel with the final design of the Secondary Mirror Support System and Positioner Assembly (M2SSP), which AMOS completed in June 2025.
The secondary and tertiary mirrors are both critical subsystems of the telescope: the secondary mirror converts the beam to f/15 and compensates for gravitational deformation, while the tertiary mirror redirects the corrected beam to the scientific instruments on the Nasmyth platforms.
Review meeting between AMOS and TIO at NOIRLab in Tucson, Arizona (August 2025).
The committee commended TIO and AMOS for the effectiveness of the PDR, the quality of the design and documentation, the thoroughness of the analyses demonstrating critical performance, and the prompt responsiveness to questions. It identified areas for additional attention, such as overall mass reduction and imbalance control, but concluded that the project had successfully passed review and is ready to proceed to the Final Design Phase.
This work is supported by the National Science Foundation under Cooperative Agreement No. 2331108. Any opinions, findings, and conclusions or recommendations expressed are those of the authors and do not necessarily reflect the views of the National Science Foundation.
TIO gives a well-attended talk on segmented mirrors at the Habitable Worlds Obervatory conference