Nicolas Lange

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.

Temporally-stable active precision mount for large optics.

We present a temporally-stable active mount to compensate for manufacturing-induced deformations of reflective optical components. In this paper, we introduce the design of the active mount, and its evaluation results for two sample mirrors: a quarter mirror of 115 × 105 × 9 mm3, and a full mirror of 228 × 210 × 9 mm3. The quarter mirror with 20 actuators shows a best wavefront error rms of 10 nm. Its installation position depending deformations are addressed by long-time measurements over 14 weeks indicating no significance of the orientation. Size-induced differences of the mount are studied by a full mirror with 80 manual actuators arranged in the same actuator pattern as the quarter mirror. This sample shows a wavefront error rms of (27±2) nm over a measurement period of 46 days. We conclude that the developed mount is suitable to compensate for manufacturing-induced deformations of large reflective optics, and likely to be included in the overall systems alignment procedure.

Mounting with compliant cylinders for deformable mirrors.

A method is presented to mount large aperture unimorph deformable mirrors by compliant cylinders (CC). The CCs are manufactured from a soft silicone, and shear testing is performed in order to evaluate the Youngs modulus. A scale mirror model is assembled to evaluate mount-induced change of piezoelectric deformation, and its applicability for tightly focusing mirrors. Experiments do not show any decrease of piezoelectric stroke. Further it is shown that the changes of surface fidelity by the attachment of the deformable mirror to its mount are neglectable.

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.

Long-term stable active mount for reflective optics

We report on the development of an active mount with an orthogonal actuator matrix offering a stable shape optimization for gratings or mirrors. We introduce the actuator distribution and calculate the accessible Zernike polynomials from their actuator influence function. Experimental tests show the capability of the device to compensate for aberrations of grating substrates as we report measurements of a 110x105 mm2 and 220x210 mm2 device With these devices, we evaluate the position depending aberrations, long-term stability shape results, and temperature-induced shape variations. Therewith we will discuss potential applications in space telescopes and Earth-based facilities where long-term stability is mandatory.

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.

Innovative approach to high stroke electrostatic actuators

The electrostatic actuation is still a preferred principle in modern microelectromechanical systems (MEMS). Based on the Coulomb attraction between two different point charges, the electrostatic actuation is a surface effect and thus volume-independent. In addition, the efficiency of electrostatic actuation increases with a decreasing gap size between the electrodes. The relatively simple morphology of an electrostatic actuator allows low-cost wafer-level fabrication, making it a versatile and convenient principle for MEMS actuators. Although the electrostatic principle seems only to be applicable for actuators in micrometer scale, this paper presents the successful upscaling of an electrostatic actuator to the millimeter scale, still utilizing the major advantages, including wafer-level fabrication, of the said principle and providing a high stroke with low actuation voltage. For such an actuator, we replace the common silicon with the non-conducting OrmoComp, a UV-curable hybrid polymer, suitable for wafer-level fabrication. With a significantly lower elastic modulus, only a fraction of actuation voltage is necessary for a similar deflection. The electrodes are realized with additional coatings of thin metal layers. To achieve a high stroke, while maintaining a relatively low voltage, the actuator design is based on a redesigned zipper actuator. With our developed fabrication process, we are able to create a highly displaced out-of-plane actuator, while almost eliminating the initial gap between the electrodes. Experimental results of a wafer-level fabricated zipper actuator show an out-of-plane motion up to 470 μm at an actuation voltage of 374 V.

Design and wafer-level replication of a freeform curvature for polymer-based electrostatic out-of-plane actuators

Abstract. The purpose of this paper is the fabrication, replication, and wafer-level imprinting of a polynomial curvature to enable the realization of an electrostatic out-of-plane zipper actuator with considerably altered and enhanced voltage versus deflection behavior. This is achievable only by changing silicon as established main material to a UV-curable polymer, while retaining the lithography-based fabrication technology. The basic concept of this actuator is explained, and with derived design rules, a finite element analysis is established to design an actuator with an integrated micro-mirror and 10-μm deflection at 60-V driving voltage. The diamond turning of the master mold and the wafer-level fabrication process of the polynomial curvature are explained in detail and realized by unconventional wafer-level imprinting of a UV-curable, nonconducting polymer. The experimental results of the deflection measurements show a deflection of the intended 10 μm at 200 V. This deviation in necessary driving voltage can be explained by fabrication-induced intrinsic stresses, which bend the actuator beams upward. This increases the gap between the electrodes, making it possible to achieve 26-μm deflection at 300 V. This paper finalizes with an illustration about the now possible designs for polymer-based electrostatic zipper actuators.

Hypatia: a 4m active space telescope concept and capabilities

While ambitious plans are being developed for giant, segmented telescopes in space, we feel that a large monolithic mirror telescope would have several advantages in the near term. In particular, the risk involved in deploying the optics will be significantly reduced, and the telescope can provide excellent image quality without the need for precise segment alignment and phasing.

Laser-based soldering technique for hermetical sealing of the calibration target for the Exomars' Raman instrument

We propose the laser-based Solderjet Bumping as a full inorganic joining technique for the hermetical sealing of a possible calibration target container for the ExoMars Raman Laser Spectrometer. This technique allows the adhesive free bonding in a flux free and localized soldering process. We show a finite elements analysis based optimization of a soldering adapted design for the calibration target container. Current experimental results document hermetical sealing of a stainless steel tube with BK7 and D263 windows with a helium leakage rate down to 5 · 10−6 mbarls−1.