Matthias Beier

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.

Fabrication of high precision metallic freeform mirrors with magnetorheological finishing (MRF)

The fabrication of complex shaped metal mirrors for optical imaging is a classical application area of diamond machining techniques. Aspherical and freeform shaped optical components up to several 100 mm in diameter can be manufactured with high precision in an acceptable amount of time. However, applications are naturally limited to the infrared spectral region due to scatter losses for shorter wavelengths as a result of the remaining periodic diamond turning structure. Achieving diffraction limited performance in the visible spectrum demands for the application of additional polishing steps. Magnetorheological Finishing (MRF) is a powerful tool to improve figure and finish of complex shaped optics at the same time in a single processing step. The application of MRF as a figuring tool for precise metal mirrors is a nontrivial task since the technology was primarily developed for figuring and finishing a variety of other optical materials, such as glasses or glass ceramics. In the presented work, MRF is used as a figuring tool for diamond turned aluminum lightweight mirrors with electroless nickel plating. It is applied as a direct follow-up process after diamond machining of the mirrors. A high precision measurement setup, composed of an interferometer and an advanced Computer Generated Hologram with additional alignment features, allows for precise metrology of the freeform shaped optics in short measuring cycles. Shape deviations less than 150 nm PV / 20 nm rms are achieved reliably for freeform mirrors with apertures of more than 300 mm. Characterization of removable and induced spatial frequencies is carried out by investigating the Power Spectral Density.

In situ and ex situ characterization of optical surfaces by light scattering techniques

Abstract. The continuous development of optical technologies and the accompanying requirements on the manufacturing process place challenging demands on metrology. In addition to highly sensitive and robust measurement techniques, the inspection tools should be fast and capable of characterizing large and complex-shaped surfaces. These aspects can be addressed by light-scattering-based characterization techniques, which also enable a large flexibility for the measurement conditions because of the noncontact data acquisition and are, thus, suited not only for ex situ but also in situ characterization scenarios. Application examples ranging from the roughness characterization of magneto-rheological finished substrates to polished extreme ultraviolet mirror substrates with diameters of more than 600 mm by compact as well as laboratory-based instruments are presented.

ATHERMAL METAL OPTICS MADE OF NICKEL PLATED ALSI40

Metal optics is an inherent part of space instrumentation for years. Diamond turned aluminum (Al6061) mirrors are widely used for application in the mid- and near-infrared (mid-IR and NIR, respectively) spectral range. Aluminum mirrors plated with electroless nickel (NiP) expand the field of application towards multispectral operating instruments down to the ultraviolet wavelengths. Due to the significant mismatch in the coefficient of thermal expansion (CTE) between aluminum and NiP, however, this advantage occurs at the cost of bimetallic bending. Challenging requirements can be met by using bare beryllium or aluminum beryllium composites (AlBeMet) as a CTE tailored substrate material and amorphous NiP as polishable layer. For health reasons, the use of beryllium causes complications in the process chain. Thus, the beryllium approach is subjected to specific applications only. Metal optics has proven to be advantageous in respect of using conventional CNC and ultra-precision fabrication methods to realize complex and light-weighted instrument structures. Moreover, the mirror designs can be effectively optimized for a deterministic system assembly and optimization. Limitations in terms of dimensional stability over temperature and time are mainly given by the inherent material properties (figures of merit) of the substrate material in interaction with the polishing layer. 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.

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.

Lens centering of aspheres for high-quality optics

Abstract The assembly effort of optical systems can be reduced immensely by the use of appropriate manufacturing technologies. For refractive optical systems with a common axis of symmetry, lens centering allows a final system assembly with high accuracy and minimal effort, but is today mostly limited to spherical optical elements only. To overcome the current restrictions, a lens centering process for one-sided aspherical lenses was investigated both in theory and practice. The deviation of the rotational aspheric axis from a reference spindle axis is measured precisely with an electronic autocollimator and an additional distance sensor at the same time. Displacements can be minimized by a combination of lateral and rotational alignment motions, aiming at a coaxiality of both axes. In order to achieve a high performance of the entire optical system, the centered lens housings are machined with a diamond tool at their outer diameter, ground, and top surfaces. The feasibility of the complete process was proved by machining several aspherical lenses and measuring the final assemblies with a coordinate measuring machine. Thereby, a residual decenter <1 μm for the aspheric vertex and a tilt <0.5 arcmin between the aspheric axis and the axis of symmetry of the lens housing could be verified. The achievable manufacturing tolerances prove the practicability of the proposed lens-centering process for a majority of high-quality optical applications.

Design of an imaging spectrometer for earth observation using freeform mirrors

In 2017 the new hyperspectral DLR Earth Sensing Imaging Spectrometer (DESIS) will be integrated in the Multi-User-System for Earth Sensing (MUSES) platform [1] installed on the International Space Station (ISS).

Anamorphotic telescope for earth observation in the mid-infrared range

In the framework of the “Earth Explorer” program, the European Space Agency had foreseen the PREMIER mission intended to monitor the three-dimensional distribution of trace gasses in the atmosphere.