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Experiment on TMT Prototype Surface Polishing by Ion Beam Figuring (IBF) Process - Timelapse video. Last month, a full-size TMT segment prototype, underwent IBF testing of its surface quality at scia Systems GmbH in Chemnitz, Germany. scia Systems’ IBF chamber is large enough to carry out a detailed study of the final polishing stage of TMT’s 1.4m segments. Video credit: TMT International Observatory View Video Page

Testing of TMT’s Primary Mirror Segment in Progress

PASADENA, CA –  The first of TMT’s Primary Mirror (M1) segment prototypes was recently tested and polished using the Ion Beam Figuring (IBF) process.

In February 2020, scia Systems GmbH invited TMT’s optics lead to perform a series of IBF runs on a TMT mirror segment prototype at its production site located in Chemnitz, Germany. The segment is made of the low-expansion glass-ceramic CLEARCERAM-Z. It was first cast by OHARA Inc. in Japan, then polished by Coherent Inc. in California and finally hexagonally cut by Harris Corp. in New York.

Valuable data were collected over 50 hours of testing to study the thermal response of segments exposed to high temperature rise, as well as to refine our thermal models. The IBF-processed segment will soon be shipped back to the U.S. for a full evaluation of its optical quality by interferometric measurements, which in turn will inform us about the characteristics of the IBF technique for producing ultra-precise optical surfaces.

“It was very nice to have the opportunity to experiment IBF on one TMT prototype,” said  Ben Gallagher, TMT M1 System Lead Engineer. “Working with the scia Systems team was very efficient as the tests were carefully thought-out.”

TMT will use IBF technologies during a final stage of segment mirror polishing for correcting any residual optical errors, providing the smoothest optical figure that meets TMT’s required specifications. Such a high optical precision is needed to avoid aberrations in the images of astronomical objects. In total, more than 550 segments (including spares) of 82 different surface shapes will need to be cast, precisely stress-polished and IBF-processed to form TMT’s primary mirror of 30 meters in diameter.


scia Systems manufactures precise surface processing equipment, based on advanced ion beam and plasma technologies. For more information on scia Systems, click here:

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