Accuracy of freeform manufacturing processes

The metrology of mirrors with an off-axis aspheric or freeform shape can be based on optical testing using a Computer Generated Hologram as wavefront matching element in an interferometric setup. Since the setup can be understood as optical system consisting of multiple elements with six degrees of freedom each, the accuracy strongly depends on the alignment of the surface under test with respect to the transmission element of the interferometer and the micro optics of the CGH. A novel alignment approach for the relative positioning of the mirror and CGH in six degrees of freedom is reported. In the presented work, a proper alignment is achieved by illuminating alignment elements outside the Clear Aperture (CA) of the optical surface with the help of auxiliary holograms next to the test CGH on the substrate. The peripheral holograms on the CGH substrate are used to generate additional phase maps in the interferogram, that indicate positioning errors. Since the reference spheres represent the coordinate system of the mirror and are measured in the same precision as the optical surface, the registration and shape has to be appropriate to embody the mirrors coordinate system. The alignment elements on the mirror body are diamond machined using freeform turning or micro milling processes in the same machine setup used for the mirror manufacturing. The differences between the turning and milling of alignment lenses is discussed. The novel approach is applied to correct the shape error of a freeform mirror using ultra precision machining. The absolute measurement of the quality of freeform mirror shapes including tilt and optical power is possible using the presented alignment concept. For a better understanding, different metrology methods for aspheres and freeforms are reviewed. To verify the novel method of alignment and the measurement results, the freeform surface is also characterized using ultra high accuracy 2½D profilometry. The results of the different techniques for the absolute measurement of freeforms are compared

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“…The technical advancements allow an accurate freeform manufacturing process. [19][20][21] Another enabler is the…”

Section: Freeform Designmentioning

confidence: 99%

Freeform mirror fabrication and metrology using a high performance test CGH and advanced alignment features

et al. 2013

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“…The optical surface has roughness values of typically below 10 nm root mean square (rms) and at best 2 to 3 nm, regarding the required precision for applications in the near-infrared and IR spectral range. 3,4,7,8 Modern telescope optics for space applications based on three-mirroranastigmat are designed of ultraprecise aluminum mirrors. The machined surface after SPDT still has the periodic groove pattern microstructures left by the cutting tool.…”

Section: Introductionmentioning

confidence: 99%

Ion beam planarization of optical aluminum surfaces

Ulitschka

Bauer

Frost

et al. 2020

J. Astron. Telesc. Instrum. Syst.

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For fabrication of high-performance mirror devices, technical aluminum alloys Al6061 or Al905 are widely used. The surface error topography after manufacturing by singlepoint diamond turning is applicable in the infrared spectral range. For increasing demands on the optical surface quality in the shortwave visible and ultraviolet spectral range, further improvement of the surface roughness is required. Hence, a promising alternative process to attain the required surface quality is evaluated. Within the ion beam planarization technique, a photoresist layer is deposited by conventional spin coating or spray coating technologies exhibiting an ultrasmooth surface. When removing the resist by reactive ion beam etch (RIBE) processing using nitrogen process gas, the ultrasmooth surface topography of the resist is transferred into the substrate. We optimized the photoresist thermal pretreatment to realize roughness preservation and a steady-state material removal rate during RIBE machining. The optimum preparation steps are explored based on roughness evaluation, chemical modification, and etch resistance of the negative photoresist. Reactive ion beam etching-based planarization is conducted on single-point diamond turned RSA Al905 and RSA Al6061 samples made of rapidly solidified aluminum (RSA) in a two-step process. The optimum process and the roughness evaluation are explored by topographic analysis applying a combination of white light interferometry and atomic force microscopy measurements. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

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“…6 However, by SPDT, a typical roughness of 2-to 3-nm rms is achievable. 1,7,8 An additional issue is the periodic turning marks with a pitch of 1 to 10 μm distance and up to a few ten nanometers in height. As a result, undesired diffraction effects appear in UV/VIS range applications.…”

Section: Introductionmentioning

confidence: 99%

Finishing of metal optics by ion beam technologies

et al. 2019

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Ultraprecise mirror devices show considerable potential with view to applications in the visible and the ultraviolet spectral ranges. Aluminum alloys gather good mechanical and excellent optical properties and thus they emerge as important mirror construction materials. However, ultraprecision machining and polishing of optical aluminum surfaces are challenging, which originates from the high chemical reactivity and the heterogeneous matrix structure. Recently, several ion beam-based techniques have been developed to qualify aluminum mirrors for short-wavelength applications. We give an overview of the state-of-the-art ion beamprocessing techniques for figure error correction and planarization, either by direct aluminum machining or with the aid of polymer or inorganic, amorphous surface films.

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Accuracy of freeform manufacturing processes

Cited by 8 publications

References 3 publications

Freeform mirror fabrication and metrology using a high performance test CGH and advanced alignment features

Freeform mirror fabrication and metrology using a high performance test CGH and advanced alignment features

Ion beam planarization of optical aluminum surfaces

Finishing of metal optics by ion beam technologies

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