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CGH for ESO’s ELT M2 reference plate: fabrication of high precision CGHs

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.