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.
The GALA (Ganymede Laser Altimeter) is one of eleven scientific instruments of the ESA mission JUICE (Jupiter Icy Moons Explorer) with the goal of exploring the icy moons of Jupiter, with a special interest in Ganymede. By its atmosphere, magnetic field, and water abundance, Ganymede is similar to Earth [1]. GALA is a laser altimeter that generates a surface profile with a resolution of < 15 cm based on an emitted laser pulse that is reflected by the surface of the moon 500 km away [2]. The mechanical development of the receiver telescope with an extremely thin-walled primary mirror (thickness 4-8 mm; diameter ~ 300 mm) was driven by tough boundary conditions. These are a small envelope and mass budget with high mechanical loads, such as a quasi-static acceleration of 120 g during rocket launch and a temperature range from -50 °C up to 150 °C, at the same time. The athermal design is based on the use of a silicon particle reinforced aluminum compound (AlSi40) and an amorphous nickel-phosphorous plating to allow various shape correction and polishing processes. Another challenge was the high radiation load of 1012 protons/cm2 @ 10 MeV. Fraunhofer IOF developed and qualified a gold HR coating based on nanolaminate with R < 98% @ 1064 nm and high resistance. Thus, almost all process steps from development through manufacturing to integration and characterization could be carried out at Fraunhofer IOF. With a shape deviation of 27 nm RMS of the primary mirror and 8 nm RMS of the secondary mirror, a system performance of 90% encircled energy could be achieved with a pupil radius of 38 µm. The telescope was handed over to HENSOLDT in spring 2020 and will start its eight-year journey to Jupiter in 2023.
Mirrors with excellent mechanical, thermal and optical properties are suitable for a broad spectrum of modern optical application. A growing number of multi- and hyperspectral imaging devices such as telescopes and spectrometers are based on all-reflective metal optics. Optics with higher mechanical or dynamic loads are often made of ceramics; at higher thermal loads, they are made of glass-ceramics. The DLR Earth Sensing Imaging Spectrometer (DESIS) is a space-based hyperspectral instrument developed by German Aerospace Center (DLR). The optical system of the spectrometer was designed, fabricated and pre-aligned by the Fraunhofer Institute of Applied Optics and Precision Engineering (IOF). The instrument was realized as an all-reflective system using metal-based mirrors using a modular, so-called snap-together approach. Parts of the system are flat mirrors for the pointing unit of the instrument. Two flat mirrors based on a metallic substrate material (Al 42Si) and one flat mirror based on a ceramic (HB Cesic®) were realized. The cost-efficient manufacturing technology of metal mirrors has an important advantage over glass, glass-ceramic and ceramic mirrors. For the pointing mirror, a more rigid and stiff material like HB-Cesic® was used. Different and tailored process chains were applied for both kinds of mirrors. The paper summarizes the fabrication of optical mirrors by i) grinding and polishing of ceramic matrix composite substrates; and ii) diamond machining combined with post-polishing techniques, like magnetorheological finishing (MRF) and chemical mechanical polishing (CMP) for metallic substrates. The process chains are described including testing setup and results with regard to different materials and manufacturing technologies. The mirrors show an excellent quality regarding flatness (lower 15 nm rms) and roughness (lower 1 nm rms, WLI magnification 50x).
Several mirrors for the upgrade of the CRyogenic high-resulution InfraRed Echelle Sprectrograph (CRIRES) at the Very Large Telescope, were manufactured by diamond turning and polishing. These mirrors will be used in the crossdispersion unit (CDU) and the fore optics of the instrument. For background level reasons, the operational temperature of the CDU is set to 65 K. Therefore, the flat and spherical mirrors used in the CDU, which are made of melt-spun aluminum alloy Al6061, had to be artificially aged, to improve the dimensional stability at cryogenic temperatures. After diamond turning, magnetorheological finishing (MRF) was used for a deterministic shape correction and to remove the turning marks of the RSA6061 mirrors. To reduce the micro-roughness, a further smoothing step was necessary. A micro-roughness between 1 nm RMS and 5 nm RMS as well as shape deviations below 35 nm RMS were achieved. The mirrors were coated by inline magnetron sputtering with a high-reflective gold layer or protected silver, respectively.
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