Large & active mirrors

Safran Reosc paved the way to large monolithic mirrors and reached their practical limit size with the 8-m primaries of the ESO Very Large Telescope (VLT) and Gemini. At the same time these optics have become more and more active with up to 150 actuators maintaining perfect optical shape of the optics despite changing environment caused by telescope orientation and temperature within the dome.  

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Large & active mirrors

VLT

Astronomers are continuously looking for larger telescope aperture either for gaining more photons for spectroscopic observations or for gaining in resolution with the help of Active and Adaptive Optics. Safran Reosc is proud to have followed this roadmap of monolithic optics peaking with the giant 8-m aperture of the ESO Very Large Telescope (VLT).

Let's highlight some famous optics we polished through the years :

1-m: the THEMIS solar telescope installed at Teide observatory in the Canarias islands and the Laser-Moon telescope at CERGA Observatory in France,

2-m: the Pic du Midi observatory installed in the Pyrénées mountain and the Nishi Harima Astronomical Observatory in Hyogo prefecture in Japan,

4-m: the ESO 3.6-m telescope installed at La Silla observatory in Chili

8-m: the four 8-m primary mirrors for the ESO VLT polished on a Zerodur substrate and the two optics of similar size but with Ultra Low Expansion (ULE) material from Corning for the Gemini telescopes of the Association of Universities for Research in Astronomy (AURA) in the US. 

Presentation

Polishing skill: During the year the F/# of the main mirrors for astronomical telescopes has strongly reduced from F/2 to F/1.5 or less thanks to the significant progresses in polishing technology. We started with large flexible polishing tools made from plywood to be operated by skilled technicians in a very slow converging process. Today, robotic polishing combined with proprietary smoothing processes and accurate metrology through CGH allows us to process large optics down to F/1 relative aperture, off axis or freeform optics profiles.

As an example we would like to highlight the 2.7-m main mirror of the Stratospheric Observatory for IR astronomy (SOFIA). This large telescope optics has been lightweighted by machining (another world record for Safran Reosc) to be embarqued on-board a Boeing aircraft and packed with a low F/# value of F/1.19 in order to fit with the structure and minimize inertia of the whole system.

Active shape control: Active optics consists in controlling the shape of the optic with several actuators exercising forces on the main mirror rear surface to keep the image of the stars to its smallest size. This technology has progressively become the obvious only way to allow the increase of precision astronomical telescopes aperture larger than 4-m. Safran Reosc is proud to have performed validation of Active Optics for ESO with a technology validation mirror of 1-m diameter and 10 mm thickness only. This successful experiment allowed ESO to decide the construction of the New Technology Telescope (NTT) which then gave full confidence to initiate construction of the VLT.

At the time of VLT Reosc parent company Sfim (today Safran Electronics & Defense) supplied ESO with the 150 actuator units and their local control software, installed in the primary mirror cell mirror Active Optics. Six hundred such actuator combining hydraulic passive support for taking the mirror main mass and electro-mechanical actuators to ensure the accurate force control were delivered.

Durinig polishing of the 8-m mirror, Safran Reosc equipped the polishing table with 150 pneumatic actuators, less accurate but fully sufficient for performing the optical verifications in our test tower in a quasi 0-g environment. So, another world premiere is that first 8-m class active optics system has been operating since 1993 in our VLT polishing shop.

Today, Safran Reosc has adapted active mirror technology for large Fizeau interferometer systems. The technology is perfectly adapted for large laser beam shape control and Reosc can offers it combined with advanced ns and fs coating of the mirror optical surface. 

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