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CGH for ESO's ELT M2 reference plate: Fabrication of high precision CGHs

 
: Burmeister, F.; Ehrhardt, S.; Benkenstein, T.; Lammers, T.; Klein, A.C.; Schleicher, P.; Flügel-Paul, T.; Zeitner, U.; Giraud, P.; Makhlouf, H.; Mercier-Ythier, R.; Cluzel, R.; Harel, E.

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Navarro, R. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation IV : 14-22 December 2020, Online Only
Bellingham, WA: SPIE, 2020 (Proceedings of SPIE 11451)
ISBN: 978-1-5106-3689-7
ISBN: 978-1-5106-3690-3
Paper 114510W, 10 S.
Conference "Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation" <4, 2020, Online>
Englisch
Konferenzbeitrag
Fraunhofer IOF ()
Computer Generated Hologram (CGH); electron-beam lithography; error analysis; ESOs ELT

Abstract
The highly aspheric secondary mirror M2 of ESO's Extremely Large Telescope (ELT) is the largest convex mirror ever polished. We report on manufacturing and error analysis of the high accuracy computer-generated hologram (CGH) used as part of the test concept for the M2 mirror. In order to comply with the required measurement accuracies in the single nanometer range (RMS), existing error sources along the entire process chain need to be considered. Available characterization methods for measurement of fabrication errors are described, as well as modelling of wavefront errors resulting from the CGH manufacturing process itself. Additionally, a general approach to improve the transmission of CGHs applying an effective multi-level patterning realized by binary sub-wavelength structures is introduced.

: http://publica.fraunhofer.de/dokumente/N-625052.html