From Event: SPIE Astronomical Telescopes + Instrumentation, 2018
The first large Fabry-Perot etalon (Ø35 cm) of the VTF instrument was coated successfully using IBS technique. The High Reflective (HR) coatings need to meet the reflectivity specifications (95 +/- 1%) over the entire wavelength range 520-870 nm and the entire aperture (Ø25 cm) and also preserve the plate's flatness and airgap uniformity between the two platesto be better than 3 nm RMS. The change of the figure error of the individual faces after HR coating was exceptionally small: For plate 1 (upper) it changed from 1.7nm RMS before coating to 2.12 nm after coating, no change at all for plate 2 (lower).
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L. Pinard, C. Michel, B. Sassolas, J. Teillon, G. Cagnoli, M. Sigwarth, T. Kentischer, W. Schmidt, and B. Reichman, "High uniformity IBS coatings for the world’s largest Fabry-Perot etalon of the VTF instrument," Proc. SPIE 10706, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation III, 107061R (Presented at SPIE Astronomical Telescopes + Instrumentation: June 14, 2018; Published: 10 July 2018); https://doi.org/10.1117/12.2312007.