Johannes Hartung

Development, fabrication, and testing of an anamorphic imaging snap-together freeform telescope

The fabrication chain for the development of an afocal all aluminum telescope using four anamorphic aspherical mirrors is described. The optical and mechanical design are intended to achieve an enhanced system integration with reduced alignment effort by arranging two optical surfaces monolithically on common mirror bodies. Freeform machining is carried out by a hybrid fabrication approach combining diamond turning and diamond milling in the same machine setup. A direct figure correction of diamond turned aluminum mirrors by magnetorheological finishing is presented, resulting in high-precision athermal mirror modules with excellent figure properties. The interferometric system test highlights the diffraction limited telescope performance and the feasibility of the chosen approaches for freeform machining and mechanical integration.

Description and reimplementation of real freeform surfaces

Freeform surfaces are becoming an increasingly exciting opportunity in optical design, in particular when correcting systems with off-axis geometries. Nevertheless, especially when coming to commercial use, the challenges for manufacturing are difficult to handle. The optical quality of a system is perturbed by typical deformations, such as localized figure errors and regular mid-spatial-frequency ripples, that come from the diamond-turning process. In this proposal, we investigated a workflow for analyzing the impact of real optical surfaces on the optical performance for even complex systems. Based on a simple and robust description, the surface is implemented back into the design. While the more localized deviations are analytically described by radial basis functions, the residual ripple structures are covered by a new approach based on the power spectral density. The reimport of optical surfaces back into the design software allows simple estimations for the requirements on manufacturing and the analysis of the realistic impact on system performance.

Precision manufacturing of a lightweight mirror body made by selective laser melting

Abstract This article presents a new and individual way to generate opto-mechanical components by additive manufacturing, embedded in an established process chain for the fabrication of metal optics. The freedom of design offered by additive techniques gives the opportunity to produce more lightweight parts with improved mechanical stability. The latter is demonstrated by simulations of several models of metal mirrors with a constant outer shape but varying mass reduction factors. The optimized lightweight mirror exhibits 63.5% of mass reduction and a higher stiffness compared to conventional designs, but it is not manufacturable by cutting techniques. Utilizing selective laser melting instead, a demonstrator of the mentioned topological non-trivial design is manufactured out of AlSi12 alloy powder. It is further shown that – like in case of a traditional manufactured mirror substrate – optical quality can be achieved by diamond turning, electroless nickel plating, and polishing techniques, which finally results in

Large aperture freeform VIS telescope with smart alignment approach

The development of smart alignment and integration strategies for imaging mirror systems to be used within astronomical instrumentation are especially important with regard to the increasing impact of non-rotationally symmetric optics. In the present work, well-known assembly approaches preferentially applied in the course of infrared instrumentation are transferred to visible applications and are verified during the integration of an anamorphic imaging telescope breadboard. The four mirror imaging system is based on a modular concept using mechanically fixed arrangements of each two freeform surfaces, generated by servo assisted diamond machining and corrected using Magnetorheological Finishing as a figuring and smoothing step. Surface testing include optical CGH interferometry as well as tactile profilometry and is conducted with respect to diamond milled fiducials at the mirror bodies. A strict compliance of surface referencing during all significant fabrication steps allow for an easy integration and direct measurement of the systems wave aberration after initial assembly. The achievable imaging performance, as well as influences of the tight tolerance budget and mid-spatial frequency errors, are discussed and experimentally evaluated.

Fabrication of metal mirror modules for snap-together VIS telescopes

The assembly effort of an optical system naturally relies on the degrees of freedom and the maximum allowable tolerances each optical surface introduces into the overall budget. Snap-together approaches traditionally can be regarded as attractive solutions for IR systems having moderate tolerances, where the required precision is achieved by simultaneously machining optical surfaces and mounting interfaces in a single machine setup. Recent improvements in manufacturing and metrology enable a transfer of the assembly approach to shorter wavelength applications, where sub-aperture figuring techniques are used in combination with suitable amorphous polishing layers to achieve the increased requirements on figure and finish. A further decrease of the assembly effort is gained by machining several optical surfaces on common mechanical substrates and fixing the relative position with uncertainties as low as the machine precision. The article presents the fabrication of large electroless nickel coated aluminum mirror modules having two functional freeform surfaces and references for metrology and system integration. The modules are part of an all metal anamorphic imaging telescope operating in the visual spectral range. Presented methods open up a rapid and reliable assembly of metal mirror based VIS telescopes to be used in ground and space based astronomy or remote sensing applications.

Measuring position and figure deviation of freeform mirrors with computer generated holograms

A novel interferometric metrology using a multi-zone computer generated hologram for determining position and figure deviations of two anamorphic aspherical mirrors arranged on a common substrate is presented.

Novel applications based on freeform technologies

Metal mirrors have different advantages in comparison to mirrors made of glassy ceramics and glass elements as they are inexpensive, easy to manufacture and it is possible to integrate reference structures directly into the mirror material. During the last years diamond turning manufacturing of multi mirror freeform substrates became state of the art to reduce the alignment and integration effort of freeform mirror systems. The article gives an overview over the process chain, data analysis, and results from a mirror system for the visual spectral band and provides some theoretical insights necessary for achieving appropriate surface form and roughness values.

Additive manufacturing of metal mirrors for TMA telescope

Additive manufacturing enables enhanced designs for metal mirrors and housings of optical systems like telescopes. Internal lightweight structures are used for the mirror modules to reduce the weight of the system while keeping the mechanical stability. Internal structures can be produced by selective laser melting, which cannot be realized by conventional machining. Using an aluminum silicon alloy, the thermal mismatch of the mirror base body to the necessary polishing layer is minimized. Resulting thermal induced deformations are greatly reduced. The additive manufacturing of a mirror module with two optical surfaces is described in detail. Using a adapted process chain for the application in the visible range, first results of the additive manufacturing as well as subsequent machining steps like diamond turning of the optical surfaces are presented.

Irradiance and phase control with two freeform surfaces using partial differential equations

A design method of two coupled freeform surfaces for the control of the irradiance and phase of the input and output wavefronts without the restriction to paraxiality or spherical/planar wavefronts is presented. It can be applied to the design of coupled lens surfaces, coupled mirrors or the combination of lens and mirror surfaces. The method is based on the description of the freeform surfaces through a system of coupled partial differential equations (PDE) for the first freeform surface and a ray-mapping projection. By calculating the required output wavefront between two predefined complex illumination patterns, we demonstrate that the presented algorithm can be applied directly to the calculation of a single optical element for the generation of two different irradiance distributions on separated target planes. Additionally, a manufacturing analysis of the corresponding freeform surfaces of a double lens system is provided. Furthermore, an extension of the design approach for a single freeform lenses with a predefined entrance surface from [J. Opt. Soc. Am. A 34, 1490-1499 (2017)] to single freeform lenses with a predefined exit surface is presentend. Limitations of the design method and possible improvements are discussed.

Theoretical compensation of static deformations of freeform multimirror substrates

Varying temperatures influence the figure errors of freeform metal mirrors by thermal expansion. Furthermore, different materials lead to thermo-elastic bending effects. The paper presents a derivation of a compensation approach for general static loads. Utilizing perturbation theory, this approach works for shape compensation of substrates that operate in various temperature environments. Verification is made using a finite element analysis, which is further used to produce manufacturable CAD models. The remaining low spatial frequency errors are deterministically correctable using diamond turning or polishing techniques.