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.
The wish for smaller and lighter telescopes, better image quality, and shorter wavelength applications sets ever increasing demands towards the quality of optical surfaces. In metal mirror fabrication with diamond turning, mid spatial frequency errors become the most limiting factor in achieving a certain surface quality, and reducing them puts high requirements on the manufacturing. A first step in improving the surface quality is to utilize appropriate analysis methods that account for the highly anisotropic surfaces developing at diamond turning. In this article, a novel, two-dimensional representation of the power spectral density is demonstrated, which takes up the many benefits of the established PSD and extends it to meet the demands of anisotropic surfaces.
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