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Ultra-precise manufacturing of aspherical and freeform mirrors for high resolution telescopes

: Risse, S.; Scheiding, S.; Beier, M.; Gebhardt, A.; Damm, C.; Peschel, T.


Navarro, Ramón ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation : Montréal, Quebec, Canada, 22.-27.6.2014
Bellingham, WA: SPIE, 2014 (Proceedings of SPIE 9151)
ISBN: 978-0-8194-9619-5
Paper 91510M, 10 S.
Conference "Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation" <2014, Montréal>
Fraunhofer IOF ()

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.