Jan Kinast

A novel athermal approach for high-performance cryogenic metal optics

This paper describes a new athermal approach for high performance metal optics, particularly with regard to extreme environmental conditions as they usually may occur in terrestrial as well as in space applications. Whereas for mid infrared applications diamond turned aluminium is the preferred mirror substrate, it is insufficient for the visual range. For applications at near infrared wavelengths (0.8 μm - 2.4 μm) as well as at on cryogenic temperatures (-200°C) requirements exist, which are only partially met for diamond turned substrates. In this context athermal concepts such as optical surfaces with high shape accuracy and small surface micro-roughness without diffraction effect and marginal loss of stray light, are of enormous interest. The novel, patented material combination matches the Coefficient of Thermal Expansion (CTE) of an aluminium alloy with high silicon content (AlSi, Si ≥ 40 %) as mirror substrate with the CTE of the electroless nickel plating (NiP). Besides the harmonization of the CTE (~ 13 * 10-6 K-1), considerable advantages are achieved due to the high specific stiffness of these materials. Hence, this alloy also fulfils an additional requirement: it is ideal for the manufacturing of very stable light weight metal mirrors. To achieve minimal form deviations occurring due to the bimetallic effect, a detailed knowledge of the thermal expansion behavior of both, the substrate and the NiP layer is essential. The paper describes the reduction of the bimetallic bending by the use of expansion controlled aluminium-silicon alloys and NiP as a polishing layer. The acquisition of CTE-measurement data, the finite elements simulations of light weight mirrors as well as planned interferometrical experiments under cryogenic conditions are pointed out. The use of the new athermal approach is described exemplary.

Minimizing the bimetallic bending for cryogenic metal optics based on electroless nickel

Ultra-precise metal optics are key components of sophisticated scientific instruments in astronomy and space applications. Especially for cryogenic applications, a detailed knowledge and the control of the coefficient of thermal expansion (CTE) of the used materials are essential. Reflective optical components in IR- and NIR-instruments primarily consist of the aluminum alloy Al6061. The achievable micro-roughness of diamond machined and directly polished Al6061 does not fulfill the requirements for applications in the visible spectral range. Electroless nickel enables the reduction of the mirror surface roughness to the sub-nm range by polishing. To minimize the associated disadvantageous bimetallic effect, a novel material combination for cryogenic mirrors based on electroless nickel and hypereutectic aluminum-silicon is investigated. An increasing silicon content of the aluminum material decreases the CTE in the temperature range to be considered. This paper shows the CTE for aluminum materials containing about 42 wt% silicon (AlSi42) and for electroless nickel with a phosphorous content ranging from 10.5 to 13 %. The CTE differ to about 0.5 × 10-6 K-1 in a temperature range from -185 °C (LN2) to 100 °C. Besides, the correlations between the chemical compositions of aluminum-silicon materials and electroless nickel are shown. A metrology setup for cryo-interferometry was developed to analyze the remaining and reversible shape deviation at cryogenic temperatures. Changes could be caused by different CTE, mounting forces and residual stress conditions. In the electroless nickel layer, the resulting shape deviation can be preshaped by deterministic correction processes such as magnetorheological finishing (MRF) at room temperature.

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.

Cryogenic testing of a unimorph-type deformable mirror and theoretical material optimization

Abstract. The testing of a lightweight unimorph-type deformable mirror (DM) for wavefront correction in cryogenic instruments is reported. The presented mirror manufactured from the titanium alloy TiAl6V4 with a piezoelectric disk actuator was cooled to 86 K and characterized for thermally induced deformation and the achievable piezoelectric stroke between room temperature and 86 K. Through a finite element analysis, we obtained a first approximation in determining the exact temperature-dependent coefficient of thermal expansion (CTE) of the piezo material PIC151. Simulations were based on dilatometer measurements of the CTE of the TiAl6V4 mirror base between room temperature and 86 K. These investigations will enable the improvement of the athermal design of a unimorph-type DM.

Dimensional stability of metal optics on nickel plated AlSi40

Ultra precise mirrors are particularly useful for space applications and scientific instrumentations for large telescopes, covering a highly variable temperature range.

Polishability of thin electrolytic and electroless NiP layers

Ultra-precise metal optics are key components of sophisticated scientific instrumentation in astronomy and space applications, covering a wide spectral range. Especially for applications in the visible or ultra-violet spectral ranges, a low roughness of the optics is required. Therefore, a polishable surface is necessary. State of the art is an amorphous nickel-phosphorus (NiP) layer, which enables several polishing techniques achieving a roughness of <1 nm RMS. Typically, these layers are approximately 30 μm to 60 μm thick. Deposited on Al6061, the bimetallic effect leads to a restricted operational temperature, caused by different coefficients of thermal expansion of Al6061 and NiP. Thinner NiP layers reduce the bimetallic effect. Hence, the possible operating temperature range. A deterministic shape correction via Magnetorheological Finishing of the substrate Al6061 leads to low shape deviations prior to the NiP deposition. This allows for depositing thin NiP-layers, which are polishable via a chemical mechanical polishing technique aiming at ultra-precise metal optics. The present article shows deposition processes and polishability of electroless and electrolytic NiP layers with thicknesses between 1 μm and 10 μm.

Unimorph-type deformable mirror for cryogenic telescopes

Deformable mirrors can be used in cryogenic instruments to compensate for temperature-induced deformations. A unimorph-type deformable mirror consists of a mirror substrate and a piezoelectric layer bonded on substrates rear surface. A challenge in the design of the deformable mirror is the lack of knowledge about material properties. Therefore, we measured the coefficient of thermal expansion (CTE) of the substrate material TiAl6V4 between 295 K and 86 K. The manufactured mirror is characterized by an adaptive optical measurement setup in front of a test cryostat. The measured mirror deformations are feedback into a finite element model to calculate the CTE of the piezoelectric layer. We compare our obtained results to other published CTE-values for the piezoelectric material PIC151.

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.

Manufacturing of aluminum mirrors for cryogenic applications

Several mirrors for the upgrade of the CRyogenic high-resulution InfraRed Echelle Sprectrograph (CRIRES) at the Very Large Telescope, were manufactured by diamond turning and polishing. These mirrors will be used in the crossdispersion unit (CDU) and the fore optics of the instrument. For background level reasons, the operational temperature of the CDU is set to 65 K. Therefore, the flat and spherical mirrors used in the CDU, which are made of melt-spun aluminum alloy Al6061, had to be artificially aged, to improve the dimensional stability at cryogenic temperatures. After diamond turning, magnetorheological finishing (MRF) was used for a deterministic shape correction and to remove the turning marks of the RSA6061 mirrors. To reduce the micro-roughness, a further smoothing step was necessary. A micro-roughness between 1 nm RMS and 5 nm RMS as well as shape deviations below 35 nm RMS were achieved. The mirrors were coated by inline magnetron sputtering with a high-reflective gold layer or protected silver, respectively.

Investigation of electroless Ni(P)/Pd/Au metallization for solder joining of optical assemblies using laser-based solderjet bumping

Solder joining is an all inorganic, adhesive free bonding technique for optical components and support structures of advanced optical systems. We established laser-based Solderjet Bumping for mounting and joining of elements with highest accuracies and stability. It has been proven for optical assemblies operating under harsh environmental conditions, high energetic or ionizing radiation, and for vacuum operation. Spaceborne instrumentation experiencing such conditions and can benefit from inorganic joining to avoid adhesives and optical cements. The metallization of components, necessary to provide solder wetting, mainly relies on well-adhering layer systems provided by physical vapor deposition (PVD). We present the investigation of electroless Ni(P)/Pd/Au plating as a cost-efficient alternative under bump metallization of complex or large components unsuitable for commercially available PVD. The electroless Ni(P)/Pd/Au plating is characterized with respect to layer adherence, solderability, and bond strength using SnAg3Cu0.5 lead-free solder alloy.