Space Telescope Mirror Research Articles

Deformation and optical aberration prediction in ultra-precision Single Point Diamond Turning of optical components

Aluminum is the material of choice for the majority of aerospace components, and, in the past few years, its application has been extended also to the mirrors of space telescopes because of the improved thermal behavior and the possibility to build the entire telescope with the same material. However, the low elastic modulus of such material, combined with the extremely tight tolerances of optical applications, make the production of these components very challenging and, usually, based on a trial-and-error approach. This paper presents a structured methodology for the prediction of the results of manufacturing in Single Point Diamond Turning of optical components, both in terms of absolute deformation as well as optical aberrations (via Zernike polynomials). All the most significant parameters acting on the workpiece have been simulated and combined. The proposed approach has been experimental validated on an actual aluminum mirror, proving its good accuracy (<5 % rms error). While some improvement can be performed to better match the experimental data in terms of Zernike coefficients, especially for non-symmetric aberrations, this paper forms the basis for an off-machine optimization of the SPDT process, drastically reducing the trial-and-error efforts.

Field-balancing alignment of the secondary mirror of space telescope using a self-calibrated model

In-orbit alignment of the secondary mirror (SM) of deployable space telescopes is crucial for high-resolution imaging. Traditional alignment methods, which typically involve dedicated wavefront sensors or numerous iterations to optimize a metric function, are not well-suited for space applications. In this paper, an improved model-based SM alignment method is proposed. The term model refers to the relationship between misalignments and a metric function. The improved method offers two advantages over the previous model-based method in [12]. Firstly, our method allows for self-calibration of the model without the need for an additional wavefront sensor, which greatly simplifies the calibration process. Secondly, our method enables field-balancing alignment of the SM with several correction cycles. The exposure number is only six in one correction cycle and is regardless of the number of FOV sampling points, making it highly efficient for telescopes with a large FOV. The feasibility and performance of our method are verified by simulation. A comparison with existing methods is also given to show the advantages of our method in convergence speed and accuracy.

Contactless active mirror for space telescopes

The SPLATT project is a technological research activity funded by INAF, in the field of primary active mirrors for next generation space telescopes. The goal is to demonstrate that active mirrors with contactless actuators are insensitive to disturbance from the telescope and can therefore be key-elements in the reduction of complexity and cost.&#x0D; Keywords: space telescopes, active mirror, active optics, LUVOIR.

Research on Active Vibration Reduction Technology for Actively Supporting Main Mirror of Space Telescope

Research on Active Vibration Reduction Technology for Actively Supporting Main Mirror of Space Telescope

Fundamental study on optical performance of low-melting-point metal mirrors for space telescopes

The installation of telescopes with larger primary mirrors in outer space is required for the study of the early universe. However, the shipping, assembling and maintenance of the primary mirror are technical challenges. The primary mirror made of low-melting-point metal (LMPM) enables to simplify these procedures. The rotating liquid metal surface is shaped into a parabola and solidified on the site. The technological feasibility depends on the dynamic transformation of the parabolic LMPM mirror and its optical reflectance. The dynamic transformation of LMPM mirror was clarified by means of experiments with liquid gallium (Ga) which has a melting point of 302.91 K. The parabolic shape of solid Ga mirror was produced by the axial rotation and the solidification. The parabolic shape agreed with the theoretical curve obtained by a physical model. The reflectance spectra on the surface of various LMPMs solidified (i.e., Ga, lead (Pb), tin (Sn), bismuth (Bi), lead-bismuth eutectic (Pb-Bi) and Wood’s metal (Bi-Pb-Sn-Cd)) were measured. The reflectance of Ga solidified was approximately 70 % and was not degraded in the near-infrared wavelength due to the favorable optical characteristics. The reflectance both for p-polarized and s-polarized light of Ga solidified was measured by the spectrophotometer with the polarizer at the angle of incidence of 30°, 45°, 60° and 75°. The trend of measured reflectance agreed with theoretical prediction.

Adaptive parabolic membrane mirrors for large deployable space telescopes.

A key element for the development of extremely large telescopes in space or balloon-borne observatories will be a reduction in the areal weight of the primary mirror. Large membrane mirrors offer a very low areal weight but are difficult to manufacture with the optical quality needed for astronomical telescopes. This paper demonstrates a practical method to overcome this limitation. In a test chamber we have successfully grown optical quality parabolic membrane mirrors on a rotating liquid in a test chamber. These polymer mirror prototypes of up to 30cm in diameter show a sufficiently low surface roughness and can be coated with reflective layers. By manipulating the parabolic shape locally using radiative adaptive optics methods, it is shown that imperfections or changes in the shape can be corrected. With only tiny local temperature changes induced by the radiation, many micrometers of stroke have been achieved. Scaling the method investigated to produce mirrors with diameters of many meters is possible using available technology. This approach opens the possibility to produce affordable extremely large primary mirrors for space telescopes. With the flexibility of the membrane material, this type of mirror can be compactly rolled up when stored in the launch vehicle, and then be deployed in space.

A demisability analysis based on materials properties for space telescope mirrors

The increasing menace of space debris, stimulated by the current explosion of the small satellite commercial sector, urges the implementation of effective strategies for debris mitigation. Among these, substituting critical materials with more demisable ones represents an appropriate action to reduce the survivability of the component. Yet, improvements in demisability are often associated to performance penalties. In this contribution, the important topic of design for demise in the framework of material selection for optical elements has been addressed. In accordance with the typical mechanical and thermal stability requirements applied to optical systems, relevant figures of merit for the selection of optical materials have been identified. These have been exploited to optimize the design, aiming for the concurrent minimization of mass and heat required for the ablation of the component. The analysis has been applied to the most common materials currently used to fabricate space mirrors, which constitute one of the fundamental components in optical instrumentation. The proposed approach represents a valuable asset for implementing effective and innovative solutions for the sustainable use of space, aiming to concurrently reduce the casualty risk while avoiding performance penalties.

Increasing Structural Performance of Space Telescope Mirrors through Simultaneous Shape and Size Optimization

In this paper, a numeric optimization approach for designing space telescope mirrors will be presented. It is fundamental to space telescopes that each element—including their mirrors—are as lightweight as possible. Moreover, the performance of space telescopes is driven by how strongly these mirrors are distorted upon removal of gravitational load. These distortions result in a deterioration in the optical performance, which is also known as the wavefront error. This error can best be described via Zernike polynomials. To increase the optical performance, along with making the mirror lightweight, the overall root mean square (RMS) of the deformation is used as the optimization objective. An approach utilizing size and shape variables is used to define the feasible design space for the optimization. Lastly, general findings will be discussed, as well as numerical advantages of deploying structural optimization (e.g., robustness evaluation).

Femtosecond laser micromachining for stress-based figure correction of thin mirrors

The fabrication of a large number of high-resolution thin-shell mirrors for future space telescopes remains challenging, especially for revolutionary mission concepts such as NASA’s Lynx X-ray Surveyor. It is generally harder to fabricate thin mirrors to the exact shape than thicker ones, and the coatings deposited onto mirror surfaces to increase the reflectivity typically have high intrinsic stress that deforms the mirrors further. Since the rapid development of femtosecond laser technologies over the last few decades has triggered wide applications in materials processing, we have developed a mirror figure correction and stress compensation method using a femtosecond laser micromachining technique for stress-based surface shaping of thin-shell x-ray optics. We employ a femtosecond laser to selectively remove regions of a stressed film that is grown onto the back surface of the mirror, to modify the stress states of the mirror. In this paper, we present experimental results to create both isotropic and anisotropic stress states on thin flat silicon mirrors with thermal oxide ( S i O 2 ) films using femtosecond lasers. We show that equibiaxial stress can be generated through uniformly micromachined holes, while non-equibiaxial stress arises from the ablation of equally spaced troughs. We also present results from strength tests to show how this process minimally affects the strength of mirrors. These developments are beneficial to the high-throughput correction of thin-shell mirrors for future space-based x-ray telescopes.

Eigenvectors-guided topology optimization to control the mode shape and suppress the vibration of the multi-material plate

Although the structural vibration can be suppressed by adjusting eigenvalues outside the excitation frequency band, it becomes increasingly difficult to suppress vibration as the width of the excitation frequency band increases. Besides, for some high-precision equipment, e.g. space telescope mirror substrate and rocket motor casings, their complex and diverse modals directly affect the performance. Thus, a novel optimization algorithm was proposed, which applies to achieve eigenvector-based modal control and vibration suppression. The eigenvectors were defined as the objective function, and Nelson’s method without truncation error was used to calculate the sensitivity information. Due to the introduction of dynamic equations, the non-linearity of the objective function is prominent. Then, an improved solver that can handle this non-linear topology optimization problem was proposed. The optimization was performed under a multi-material framework, and the extended multi-material interpolation scheme was proposed to readily realize the optimization with three or more materials. In addition, the consistent mass matrix constructed by multi-material interpolation was used to describe the mass matrix without concentrated mass in the dynamic equation. Moreover, the modal assurance criterion was used to track jumping modals. Finally, the modal controllability was achieved through several numerical examples, which verify the effectiveness of the proposed method.

Mathematical simulation of a 3D scanner for controlling the mirror system of the Millimetron Observatory

We develop an original system for controlling the mirror geometry of the Millimetron observatory as a part of the on-board scientific equipment. The system is designed to monitor the quality of the space telescope's mirror system and use the data received as feedback signals for presetting and adjusting the telescope's optical system in outer space. The system aims to determine a multi-dimensional vector of unknown parameters that define the state of the telescope's mirror system by indirect measurements of the telescope with a 3D scanner. An unparalleled mathematical model has been created, numerically describing a process of pre-measurement of the mirror system of the Millimetron Observatory using optical control marks on the surface of the mirror system. Using the mathematical model created and the geometric optics approximation, we numerically simulate the performance of the on-board 3D scanner in the course of preliminary measurements of the mirror system of the Millimetron Observatory using optical control marks applied on the mirror surfaces. A new effective method of pre-estimation of the displacement of elements of the AP telescope by indirect (implicit) measurements performed with the 3D scanner has been created. The method is based on the mathematical transformation of indirect measurements of deviations of the position of the telescope's mirror control marks from their reference position, which provides an easy-to-use list of estimates of the offsets of the unknown parameters of the mirror system elements. A possibility to measure the telescope's mirror system with the aim to pre-configure it using a 3D scanner on board the spacecraft is shown. Estimates of acceptable deviations of the mirror system component needed to ensure the telescope's functionality are given.

临近空间望远镜次镜优化设计

临近空间望远镜次镜优化设计

临近空间望远镜次镜优化设计

临近空间望远镜次镜优化设计

Deployment dynamics for flexible deployable primary mirror of space telescope with paraboloidal and laminated structure by using absolute node coordinate method

A nonlinear dynamic modeling method for primary mirror of Flower-like Deployable Space Telescope (F-DST) undergoing large deployment motion is proposed in this paper. To ensure pointing accuracy and attitude stability of the paraboloidal primary mirror, the mirror is discretized into equal thickness shell elements by considering shell curvature. And the material nonlinear constitutive relation of flexible mirror is acquired using Absolute Nodal Coordinate Formulation (ANCF). Furthermore, the primary mirror of F-DST can be regarded as a clustered multi-body system, and its dynamic equations of elastic deformation and deployment motion are established by virtual work principle. Finally, the deployment motion of primary mirror by different driving conditions are simulated, the results show that the vibrations of mirrors that driven by elastic hinge device are more than that driven by servo motor. In addition, single sub-mirror deployment process will perturb the pointing accuracy of primary mirror, and the multiple sub-mirrors simultaneously deploying will seriously affect all the sub-mirrors surface accuracy because of the coupling effect. Thus, there are theoretical value and practical significance for the controlling surface accuracy and attitude accuracy of space telescope.

Development of space active optics for a whiffletree supported mirror.

The requirements of a lightweight primary mirror for large-aperture space telescopes include a precise mirror figure and high reliability. However, lightweight mirrors are easily affected by environmental disturbances, as they lack structural stability and rigidity. Active optics can be used to compensate for the gravity-induced deformation and correct low-order aberrations due to thermal changes and gravity relief during observing periods. Due to their complexity, active optics have been rarely used in space. To validate the technology of space active optics, an active optics system based on a passive, whiffletree-supported mirror is developed. During integration and testing on ground and under normal conditions in space, the surface accuracy is guaranteed by passive support. Within this hybrid support, the active optics system only serves to assist support. This paper focuses on the compatibility between a passive multisupporting system and active optics. We present the prototype of a 0.676m diameter passive supported lightweight mirror and active support with nine axial force actuators. The passive support includes a 9-point axial support and three A-frame lateral support. The active actuator distribution has been optimized with finite element analysis and its experimental performance characterized in representative conditions. The effectiveness of the hybrid passive-active support developed has been verified.

Numerical simulation of the adaptive control system of the composite main mirror of a large-size space telescope

Numerical simulation of the adaptive control system of the composite main mirror of a large-size space telescope

Strength behaviour of etched Zerodur®

Zerodur® is a well-known glass-ceramic used for optical components because of its unequalled dimensional stability under changing temperature. For example, it is used for mirrors of space telescopes. Due to the large amount of material that has to be removed during fabrication of the mirrors a rather coarse grit is used for grinding. Strength limiting defects introduced by this procedure are eliminated by removing the damaged surface layer by etching to achieve a high strength.Through a large test campaign comprising a high number of strength specimens (>160) of various types and sizes, the validity of the Weibull strength statistics was investigated. The purpose was to accurately determine the parameters of the Weibull law for Zerodur® when treated in the same conditions as mirrors.As a rule of thumb the average strength of Zerodur® is assumed to be around 10MPa. The measured strength is much higher. The strength distributions showed unexpected results, especially the absence of the expected size effect. Through a thorough fractographic analysis it was shown what kind of defects were responsible for failure of etched Zerodur® and why the prerequisites for the validity of the Weibull theory were not fulfilled in the investigated specimens.

Damage-free finishing of CVD-SiC by a combination of dry plasma etching and plasma-assisted polishing

To realize the damage-free finishing of CVD-SiC substrates, which are used as materials for space telescope mirrors and glass lens molds, plasma chemical vaporization machining (PCVM) and plasma-assisted polishing (PAP) were combined. In this study, the properties of such CVD-SiC substrates, including their surface morphology, composition and crystalline orientation, were investigated. Lapping using diamond abrasives and conventional chemical mechanical polishing (CMP) using CeO2 slurry were conducted for comparison with the proposed atmospheric-pressure-plasma-based finishing process. Many scratches and a subsurface damage (SSD) layer were formed by the diamond lapping of CVD-SiC. Conventional CMP using CeO2 slurry was conducted for the damage-free finishing of CVD-SiC. However, the polishing efficiency was very low. In the proposed process, PCVM, which is a noncontact dry etching process, was performed to remove the SSD layer while PAP, which combines plasma modification and soft abrasive polishing, was performed for damage-free surface finishing. PCVM was conducted on a diamond-lapped CVD-SiC surface. After PCVM for a short duration of 5min, the scratches and SSD layer formed by lapping were completely removed, although the surface roughness was slightly increased. PAP using a resin-bonded CeO2 grindstone was conducted to decrease the surface roughness of CVD-SiC processed by diamond lapping and PCVM for 5min, for which a loose-held-type grindstone was demonstrated to be very useful. A flat and scratch-free surface with an rms roughness of 0.6nm was obtained after PAP finishing.

In-focus wavefront sensing using non-redundant mask-induced pupil diversity.

Wavefront estimation using in-focus image data is critical to many applications. This data is invariant to a sign flip with complex conjugation of the complex amplitude in the pupil, making for a non-unique solution. Information from an in-focus image taken through a non-redundant pupil mask (NRM) can break this ambiguity, enabling the true aberration to be determined. We demonstrate this by priming a full pupil Gerchberg-Saxton phase retrieval with NRM fringe phase information. We apply our method to measure simulated aberrations on the segmented James Webb space telescope (JWST) mirror using full pupil and NRM data from its near infrared imager and slitless spectrograph (NIRISS).

Unimorph deformable mirror for space telescopes: environmental testing.

We have developed and manufactured a unimorph deformable mirror for space telescopes based on piezoelectric actuation. The mirror features 44 actuators, has an aperture of 50 mm, and is designed to reproduce low-order Zernike modes with a stroke of several tens of μm. We assessed the space compliance by operating the mirror in thermal vacuum, and exposing it to random and sinusoidal vibrations, as well as to ionizing irradiation. Additionally, the operational life time and the laser power handling capability were tested. The mirror was successfully operated in thermal vacuum at 100 K. We report on the conducted tests and the methods used to evaluate the mirror's performance, and discuss the compliance with the demanded requirements.