Delivering Roundels for the Thirty Meter Telescope

“We know that this project is important and is being closely monitored by our customer community as well as our own internal team."

Coherent reached a milestone this week with the completion of the first two roundels for the Thirty Meter Telescope (TMT) project. Each roundel is 1.52m (59.8″) in diameter, 45mm (1.77″) thick, and weighs 0.25 metric tons (550 pounds), precisely cut and polished to an accuracy of 2 microns.  
 
These roundels are the first of 231 we will provide for the TMT primary mirror array, nearly half the total of 492 required to complete the telescope. “This milestone has been nearly three years in the making,” noted Greg Feller, Project Manager at Coherent. “We know that this project is important and is being closely monitored by our customer community as well as our own internal team.”  

The roundels are polished to an accuracy within 2 microns

Making all these large components takes up a lot of space! We recently completed a significant factory redesign in our Richmond, CA facility to accommodate the TMT project. The new TMT manufacturing bay provides the expanded space required for producing and finishing the polished roundels. It also accommodates additional support areas for building the equipment that supports our colleagues in India through an international TMT partner organization.  

Precision manufacturing and quality are essential to the long-term success of these large optical components. TMT Telescope Optics Group Leader Ben Gallagher noted that the roundel polishing has been a great partnership between TMT and Coherent.  The TMT team and our staff jointly refined a manufacturing process called Stressed Mirror Polishing, which uses our specially-designed fixtures to apply calculated force that warps the roundels into the desired aspherical shape.  We also designed a two-dimensional profilometer (2DP) to precisely measure and polish the desired surface figure. Additional 2DP instruments are also used in other countries manufacturing the remaining roundels: Japan and India. 

“We are happy and excited to have the production mirrors,” said Gallagher.  When asked what’s next, Gallagher replied, “Coherent is gearing up for what we call the ‘Hexing’ process, where the roundels are fabricated into hexagonal segments.”  

That’s right! Next, we will be cutting all our newly made roundels into the polished hexagonal mirror segments for the optic array. The TMT uses a hexagonal array of mirrors, but there are slight differences in the final perimeter of each component to ensure a precise mounted alignment with a mere 2.5mm (0.1″) between each mirror. In total, 82 slightly different hexagonal shapes will need to be made, each serialized for quality tracing and alignment identification. Small pockets will be carved out behind the mirrors to allow room for mounting hardware and the mounting of electronic alignment sensors. 
 
The 492 polished mirror segments are slated to be assembled in the late 2020s with the Thirty Meter Telescope expected to be fully operational early 2030’s.