Reflective optical systems typically combine precise aligned mirrors, housings, interface structures et cetera. For MICADO (Multi-AO Imaging Camera for Deep Observations), a first-light instrument for ELT, Fraunhofer IOF realize several optical sub-systems, including ten freeform shaped metal optics for the Collimator, the high-resolution Imager, the low-resolution Imager and the Camera.
In this paper, the challenges of freeform manufacturing and metrology will be described. For manufacturing of those mirror substrates suitable technologies, as slow tool servo (STS) and fast tool servo (FTS) diamond turning and for further correction (e.g., magnetorheological finishing) and smoothing steps (e.g., chemical-mechanical polishing), sub-aperture tools are required. For interferometry of freeform shaped optical surfaces, computer generated holograms including reference fiducials are realized. After manufacturing the mirror substrates, the optical surface will be coated with a high-reflective gold coating.
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.
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