National Astronomical Observatory of Japan

Production of the Mirror Segment Blanks for the TMT Primary Mirror has Begun -- The First Batch of Mirror Blanks is Complete

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Two mirror blanks composing a part of TMT’s 30 meter primary mirror is complete and is ready for surface finishing. These are actual mirror blanks to be used as part of the primary mirror of the TMT telescope. The production for the mirror blanks commenced this year and the produced blanks represent the first batch of many more to come.

The TMT primary mirror consists of 492 segmented mirrors. With an additional 82 mirrors needed as replacements during the periodic mirror surface coating maintenance, a total of 574 mirrors are needed. The production of the mirror blanks commenced this year to fulfill Japan’s share of contribution to the production of the mirror blanks.*1

The mirror blanks are made from CLEARCERAM®, a glass-ceramic material produced by Ohara Inc. with virtually zero thermal expansion within the expected telescope environment. The segmented mirrors are 1.44m diagonally across and hexagonal in shape, and the mirror blanks which serve as the raw material for the segmented mirrors are circular in shape with a 1.55m diameter and 5.25cm thickness.

“We have previously succeeded in the prototype production of the segmented mirror, but to produce a full production mirror blank for an actual segmented mirror is truly a moment to celebrate,” stated Professor Takuya Yamashita, who leads development of the primary mirror for the TMT project office.

“Producing mirror blanks with such unique properties within the required specifications is a daunting task,” stated Hiroyuki Minamikawa in Ohara Inc. “But we have successfully produced quality mirror blanks within specifications.”

“Our goal is to continue producing mirror blanks that meet or exceed expectations and lead Japan in advancing its technology,” added Minamikawa.

60 mirror blanks are expected to be produced this fiscal year. Grinding and polishing of the surface and the shaping of the outer contour of the mirror blanks will follow as the next phase of production.

Illustration 1
The TMT primary mirror blanks (two pieces). The weight of each is about 200kg. In this step, it is flat plate. With grinding and polishing, it would be manufactured into the aspheric surface. (Credit: NAOJ)

*1:The mirror blank prototype with surface grinding and polishing finish was produced in 2012.