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In high-precision refractive optical systems, e.g. telescopes and spectrometers for space applications, temperature differences in influence the optical performance in the form of aberrations, especially thermal defocus. By temperature changes not only the radii of the optical elements itself, but also the distances between the optical elements are changing in dependency of the used mount materials. This causes a thermal defocus of the system and needs to be compensated. There are multiple approaches for compensation, generally divided into active and passive compensation. Active compensation techniques, e.g. piezo actuators, are highly accurate but require a control system, which is a cost and complexity driver. Passive compensation techniques often use combinations of several materials with different coeffcients of thermal expansion. This increases the overall complexity and mass. The focus of this research is to develop a mechanical structure that passively compensates for thermal defocus over a wide temperature range. Auxetic unit cells are the basis of this structure. Auxetic structures exhibit a negative Poisson´s ratio. By three-dimensional combination of auxetic unit cells, these can be arranged in a way that a transformation from radial deformation into longitudinal deformation is realized. The geometrical complexity requires additive manufacturing as manufacturing process. This concept is used to develop a mounting structure that passively compensates for thermal defocus by opposing longitudinal deformation, which is triggered by the thermally induced radial deformation of the structure itself under temperature changes. By analytical and numerical investigations, the mechanical behavior of the auxetic cells is studied in dependency of geometrical parameters. With these results, an auxetic mounting structure is developed. The aim is to compensate 5 μmK-1 axial spacing over a temperature range of 80 K. For proof of concept, a demonstrator system is numerically investigated under thermal loads and on its structural-dynamic properties.
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