Segmented optics

Through GTC Segments and E-ELT prototype segment successful realization with advanced industrial large freeform optics technology Safran Reosc demonstrated that « segmented is as good as monolithic » 


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Segmented optics

ELT M1 Segment

After VLT and under the pioneering impulse given by Keck observatory the astronomical community is clearly moving to segmented optics technology.

The Gran Telescopio Canarias (GTC) is the European pioneering project on the subject. It consists in a 11-m segmented telescope installed at the top of La Palma island in the Canarias. Safran Reosc was selected by GTC for the polishing and integration on their support of the 36 1.8-m hexagonal segments, plus 6 spare pieces, one per segment type.

For this project Reosc revolutionized it optical manufacturing technology with the rapidly development and maturation of fully robotic aspheric and freeform large optical surface generating, polishing, smoothing and figuring. 

All new equipment was installed within the VLT optical shop, the polishing table of which was transformed in a large test bench capable to do optical verification of each segment independently as well as global test of a full set of 6 segments of different type.

After 2 years facilitization and process development, production of the segment quickly went up to the required rate of 1 segment every 2 weeks. This is to be compared with our previous state of the art with one 1-2 meter class optics per year only. What a jump in productivity ! At the same time, the optical surface mid-spatial frequency residuals were reduced and edge effect was cancelled with optical performance achieved up to the last millimeter along the hexagonal contour.

More recently, ESO designed its European Extremely Large Telescope (E-ELT) with 798 x 1.45-m size hexagonal segments constituting its 39-m aperture primary mirror (plus 133 spare pieces, one of each type).

Safran Reosc was selected to demonstrate its capability to meet the tightened optical performances, two times tighter than GTC segments one, with a contract signed in 2008 for polishing and testing 7 proto pieces and studying the serial production of the 931 segments in total.

This project was a great technical success with Safran Reosc achieving the goal performance value of 10 nm RMS WFE residuals of these segments, after integration on their wiffle tree active support.

Today Safran Reosc is actively preparing itself for the construction phase of this project with the design of a new optical facility that will produce about 1200 m² of extreme precision optical surface, nearly as much as all large telescopes installed with the last 50 years on the planet.