Pasadena, CA - Exciting news out of India: TMT’s primary mirror segment polishing and metrology equipment have been successfully installed in the India-TMT Optics Fabrication Facility (ITOFF). The Stress Mirror Polishing machines and metrology tools involved in the production of TMT’s aspherical polished mirrors (aka roundels) were shipped to India to be installed and tested over several months. The commissioning took place in May 2023 by teams from the India TMT Coordination Centre (ITCC), Coherent Inc., and the TMT Project Office.
“Congratulations to the India-TMT Optics & Coherent teams for this great milestone,” said Benjamin Gallagher, TMT Telescope Optics Group Leader. “The commissioning of TMT’s Primary Mirror Segment Polishing and Metrology Equipment in India is successfully completed, and the very high-tech equipment is now functional and ready to proceed with the start of fabrication of roundels in India.”
To date, the Stress Mirror Polishing (SMP) technology has only been available from a few companies around the globe. The building, testing, and providing of a complete set of equipment to polish TMT’s glass roundels and metrology tools was included in the transfer of technology between TMT partners. As a result of this agreement, ITCC and Coherent have been working together on the supply, installation, and commissioning of the SMP equipment.
TMT India optical engineers participated in a program at Coherent facility in California involving hands-on training on the essential machines fabricated by Coherent for ITCC. Three complete mirror roundels were produced in California in accordance with TMT's strict technical specifications. These three polished roundels and the complete set of polishing material were then shipped by ocean-freight to India. The metrology tools and reference sphere were shipped by air and were received last April.
Installation of polishing machine at ITOFF
Over the last few months, several crates containing the mirror polishing system were unpacked at a new world-class mirror polishing facility located in Hosakote, India. The facility offers two clean room areas with more than 2,700 m2 of space for production activity.
Two optics and electronics senior engineers from Coherent Inc. traveled to ITOFF for several weeks of intense collaboration with ITCC qualified personnel. The machines were re-assembled on site, and functionalities were tested during the successful completion of all construction and commissioning activities, which themselves offered an opportunity for training and demonstrating several mirror handling operations. The training program in India emphasized installation and testing, as well as troubleshooting and maintenance of the machines and equipment.
The work included the installation of the reference sphere equipment in the machine-room. The reference sphere is a polished glass roundel used as a metrology reference to capture the surface figure data from the polished roundel. It allows checking of the surface accuracy of the polished roundel and is used in the quality assurance stage of the polishing process. The reference sphere built by India was cross-calibrated with the Coherent reference sphere during the training program at Coherent.
The two-dimensional profilometer (2DP) tool used to measure the peak to valley of the roundel surface was positioned above the reference sphere and all the functional tests of movement, camera, thermal sensor, and probes were carried out. A total of 61 mechanical probes, mounted on a panel of the 2DP (~1500 mm diameter size), came into contact with the polished roundel to measure its surface figure and obtain the final surface figure measurement. Three metrology tools, including the 2DP, an interferometer and a phase-measuring microscope, are required to capture the roundel surface data, and are used during the polishing process.
The installation of the two draper machine structures followed. Each draper machine will hold together the mirror segment and the SMP unit to grind and polish its surface to nanometer accuracy. The two SMP fixtures were installed on their mount structures and tested. The emergency stop safety switches were also tested and verified as being in operational condition to meet TMT’s strict safety requirements.
The team additionally accomplished a crucial step of the roundel production, preparing a roundel for bonding onto the SMP machine. Bonding preparation was required before testing the SMP fixture performances and a total of 48 bond pads (~ 2kg each) were connected by epoxy glue to the edge of the roundel (#SN003P) to get it ready and installed into the polishing machine.
An intense series of testing, positioning, and measuring took place over the month of May to characterize the performances of the polishing, metrology, and software systems. The team performed several verification procedures for the functionality, repeatability, and accuracy of the different equipment.
“The commissioning process was a really significant team effort between ITCC and Coherent teams,” said S. Sriram, the work package manager of TMT’s segment polishing in India. “All equipment has been assembled and integrated into complete machines whose functionality and test metrics were successfully inspected. And we can say that all machines are in excellent working condition!”
The facility will be completely ready to start glass roundel polishing in a few months, once the new water supply infrastructure, which includes a system of pressurized and deionized water lines, is finalized.
The goal for ITCC and the Indian Institute of Astrophysics is to complete TMT’s Fabrication Readiness Review by the end of 2023 and start mirror production soon after. A full production rate of 24 roundels per year should be reached to deliver a total of 84 mirrors. These 84 roundels will be converted into hexagonal mirror segments and mounted on their respective segment support structures before being integrated among the 492 segments of TMT’s primary mirror.
 Update on TMT High Quality Optics Polishing at Coherent, https://www.tmt.org/news/559
 India TMT Coordination Centre Produces TMT Main Mirror Roundel, https://www.tmt.org/news/597