A new type of optical component : large glass thin shells

The new trend in astronomy is to compensate for the atmospheric turbulences directly within the telescope thanks to AO secondaries or folding mirrors like the E-ELT M4. Safran Reosc has developed a leading position with the supply of the large glassy this shells, one of the critical parts of such units. 


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Safran Reosc roadmap from meniscus mirror to large thin glass shells

e-elt m4 shell sector
VLT M2 AO Unit

1985: Safran Reosc produced for ESO a 1-m diameter, 10 mm thin concave meniscus mirror used for validation of Active Optics concept. Successful results contributed to the go-ahead of the NTT and the VLT projects.

1988: Safran Reosc produced for ESA-Airbus a 800 mm diameter, 5 mm thin mirror used for similar validation and breadboard activities for earth observation from space.

1995: Safran Reosc produced a 160-cm hexagonal, 3 mm thin shell mirror that was bond on a CFRP back stiffening structure as one of the candidate technology for the JWST primary mirror segments.

2011: Safran Reosc supplied ESO with the world's largest aspheric thin glass shell for the VLT M2 Adaptive Optics Facility. The shell has the aspheric profile of the VLT M2 Mirror but with only 2 mm uniform thickness. The ESO key requirement for such thin shell are :

  • Aspheric profile within 10 µm PTV
  • Profile smoothness with < 5 nm RMS over any 30 mm sub-pupil
  • Mid-spatial frequency errors specified with < 0.1 N actuator force to correct up to 1170 Zernike deformation modes
  • Thickness uniformity < 15 µm for uniform dynamic behavior

A second spare shell was delivered in 2013.

2015: Safran Reosc is selected by ESO for the supply of two sets of 6 thin shell petals for the E-ELT M4 mirror Unit. These pieces are flat but petal shaped and shall be produced with similar specifications than the VLT M2 AO Unit. The E-ELT will be the world largest AO mirror with close to 5000 actuators controlling the shape of the shells to compensate for the atmospheric turbulences.