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.
Quantum communication is considered to be a key feature for secure communication e.g. between government organisations or other institutions with high security requirements. Therefore, the QuNET initiative was founded. It focuses on developing a quantum-secure German governmental agency network based on quantum key distribution (QKD). Free-space optical (FSO) links are a valuable part of infrastructure because they can be deployed temporarily, such as at summits or to bridge the last miles where there is no fiber infrastructure. In particular, high-throughput telescopes are of great importance as optical antennas for terrestrial networks or links between mobile nodes. The paper describes the development and manufacturing of an unobscured, afocal four-mirror metal telescope which is already tested for ground-based quantum communication. The off-axis system, operating with a full telescope aperture of 200 mm, a magnification of 20x, and a FOV (field of view) of 3.5 mrad and is designed to yield diffraction-limited performance for an operational wavelength of 810 nm and 1550 nm. The addressed wavefront error-target of the whole system amounts to 66 nm RMS (root mean square). The use case of the telescope implied an operational temperature range of -40 °C up to +50 °C. Therefore, an athermal system is realized using an aluminum-silicon alloy substrate material combined with a nickel-phosphorus polishing layer that allows to reach the required surface quality of the mirrors. To simplify the alignment of the telescope, its mechanical concept relies on a snap-together approach using two substrates with two optical mirrors on a common substrate, each. The manufacturing chain of these two so called mirror substrates is described in detail. That includes the CNC pre-manufacturing, ultra precision diamond turning and subsequent polishing steps. The resulting quality of the mirror substrates as well as of the telescope system is demonstrated by optical measurements using interferometric setups.
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