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Spectral beam combining for security as well as terrestrial, satellite, and deep space communication
The paper describes the manufacturing, integration, and testing of this optical system. It was realized as an all reflective system using metal-based mirrors and a modular, so-called snap-together approach, which allows to simplify the integration of optical systems considerably. Measured r.m.s. wavefront errors of the complete system are in the range of 63 nm to 120 nm, which is compliant with the instrument’s requirements.
To find an optimal compromise, a thermal matched aluminum-silicon alloy (silicon contents ≈ 40 wt%) plated with NiP (AlSi40/NiP ) was investigated in a joined project of the Max Planck Institute for Astronomy MPIA and the Fraunhofer Institute for Applied Optics and Precision Engineering IOF. The main tasks of the project were the minimization of the bimetallic bending, the development of reliable stabilizing and aging procedures, and the establishment of a proven fabrication method.
This paper describes fundamental results regarding the optimization of the athermal material combination. Furthermore, the developed production chain for high quality freeform mirrors made of AlSi40/NiP is pointed out.
Optical freeforms are increasingly gaining interest for optical systems like telescopes and spectrometers. This is a key topic of discussions for many years; however, the manufacturing process of freeform optics remains a challenging task whose complexity derives from the missing symmetry in freeform surfaces.
Ultra-precise manufacturing with diamond tools is an appropriate method to realize optical freeforms. Aspherical off axis mirrors machined similar to freeform or classical freeform mirrors like anamorphic mirrors can be fabricated in a deterministic process by using reference structures and correction loops. Diamond machining offers an excellent technology to meet the requirements regarding small values of surface deviation and low tolerances of position accuracy. Nevertheless, the typical micro-roughness of approximately 5 nm rms and the periodic turning structure set the limitation for diamond machined surfaces. The surfaces fulfill requirements for application in the Near Infrared (NIR) and Infrared (IR) spectral ranges, respectively. For smoothing the periodic structure, the diamond turning is combined with post polishing techniques like MRF (Magnetorheological Finishing) or computer assisted polishing. Therefore, the aluminum mirror has to be coated with amorphous nickel-phosphorous or silicon. Thus, the specification of applications in the visible (VIS) spectral range is reached. This process chain is interesting for a growing number of multi- and hyperspectral imaging devices such as telescopes and spectrometers based on all reflective metal optics.
The paper summarizes the fabrication of an optical bench for a high resolution IR telescope, discusses the results of post polishing mirrors for VIS telescopes, and shows an efficient and easy snap-together alignment strategy. The optical function of the TMA demonstrator built is an afocal imaging for a Limb-Sounder Instrument with a magnification of 4.5:1. Besides the design and manufacturing approach, the snap-together integration of the optical bench is presented, too. The presentation is finished with a forecast of a freeform IR telescope based on anamorphic mirrors.
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