Membrane mirror technology offers the prospect for future extremely large space telescopes. Utilizing a thin parabolic membrane as primary mirror base, very low aerial weights can be achieved. The flexible nature of those membranes allows to roll the mirror and compactly store them upon launch. In this presentation the evaluation of a mounting structure for the membrane mirror will be presented. As the mirror is a thin membrane only, a stress free mounting need to be realized taking the gravity release into account. The mounting structure must take the membrane unfolding and radiatively controlled surface optimization into account, requiring special attend on the radiation management. Regarding a telescope structure to finalize a complete optical system has to be discussed in that respect.
Most optical instruments for space applications in the areas of earth observation, science and optical communication require high performance optical mirrors. Driving requirements are typically low Surface Form Error (SFE), high structural stiffness over a wide temperature range, a low micro roughness and low mass. A promising material combination for optical mirrors is the usage of the aluminium alloy AlSi40 as mirror substrate and electroless Nickel phosphorous (NiP) as coating for the optical surface. The main advantage of this combination is the CTE compatibility of AlSi40 and NiP over a large temperature range. In addition, NiP coated surfaces can be easily diamond turned and polished. This material combination thus enables relatively fast production cycles even for a mirror surface roughness in the few-nm range. Within the ESA GSTP SME4ALM program, the feasibility to produce optical mirrors from AlSi40 by additive manufacturing (AM) was demonstrated. Main objectives were the determination of AlSi40 AM parameters and material properties as well as the demonstration of advantages of AM (e.g. topology optimization, monolithic design, lattice structures). By consequent improvement of the Laser Powder Bed Fusion (LPBF) manufacturing process, similar material properties to bulk AlSi40 were obtained. Up to 40% improvement were achieved for relevant material properties such as stiffness and SFE. Based on the good results of the first project, a follow up program was launched by ESA (also in the context of ESA-GSTP) with the objective to design and print an optical mirror with TRL ≥ 5. This paper will summarize the development program and results of the follow-up project, in particular: · Design and testing of lattice structures · Definition of a cleaning process · Transfer of AlSi40 processing strategies to industrial-scale machines · Design and testing of a mirror demonstrator, which is representative for space applications.
Advanced Manufacturing (AM) has the potential to improve existing technologies and applications in terms of performance, light-weighting and costs. In the context of the SME4ALM initiative, launched by DLR and ESA, the company Kampf Telescope Optics GmbH (KTO) in cooperation with the Fraunhofer Institute for Material and Beam Technology (IWS) have assessed the feasibility of AM to build a high-performance optical mirror for space applications.
For the assessment of the AM potentials, a mirror design concept for cryogenic instruments for observations in the IR and NIR range was baselined. In a second step, Nickel-Phosphorus (NiP) was selected as optical coating. The combination of coating and mirror material is a primary design driver for optical performance. Both materials must have a very similar CTE as well as be compliant to modern optical manufacturing (diamond turning, polishing). As a promising candidate for NiP coating the AlSi40 was selected for the mirror structure.
The potential advantages of AM for optical mirrors in terms of mechanical performance, cost, and manufacturing time were exploited. The achievement of those objectives was / will be demonstrated by:
1. verifying AM material properties and manufacturability of AM mirrors by material sample tests and subcomponent tests
2. designing AM mirror demonstrator by structural, thermal, and optical performance analysis
3. applying and elaborating AM specific design methods (topology optimization, sandwich structures with internal microstructures, monolithic design, etc.)
4. manufacturing, assembling, and testing AM mirror demonstrator to verify manufacturability and optical performance
5. comparing optical and mechanical performance of the AM mirror demonstrator to a conventional mirror by numerical analysis to exploit potential advantages of AM
The Environmental Mapping and Analysis Program (EnMAP) is a German hyperspectral satellite mission that aims at monitoring and characterizing the Earth’s environment on a global scale. Its hyperspectral imager (HSI) is capable of measuring the solar radiance reflected from the Earth’s surface as a continuous spectrum in the spectral range of 420 nm to 2450 nm.
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