The Salto demonstrator is a complete 1-m class telescope with a single-conjugated Rayleigh laser guide star adaptive optics (AO) system. The project aims to benchmark robust AO operations for astronomy giving an opportunity to upgrade medium size telescopes (1-4 m diameter) around the world and boost their scientific yield. But it is also a benchmark for optical communications and space debris tracking under mediocre seeing conditions, far worse than astronomical standards. Indeed, the foreseen location of the telescope is at the premises of Redu Space Services in the Belgian countryside. In our contribution, we review the overall design of the AO instrument from the optical definition to the real-time computer implementation. We discuss the integration, the calibration, and operational aspects of the instrument. Finally, we present the successful first on-sky operations, reaching the diffraction limit at 1.55μm under 2-3” seeing.
In the field of ground-based telescopes, laser guide stars (LGS) are artificial stars formed in the sky to serve as a reference for the adaptive optics. The artificial star should have a small lateral extent as this is an important factor for how well the adaptive optics can compensate for atmospheric turbulence. The laser launch telescope (LLT) is a key component of the LGS facility. It uses an afocal system to increase the waist of the laser, therefore reducing the beam divergence and limiting the size of the star. We describe the design process of LLTs and show how a combination of two afocals with controlled defocus can be used to optimize the LGS. First, the impact of defocusing a single afocal and tuning the position of the input beam waist is presented. We then demonstrate how an intermediary afocal system can be used to vary the properties of the beam at the input of the second afocal. With such a configuration, a controlled defocus of both afocals can be performed to tune the artificial star size. Moreover, the two afocals configuration can be used to adapt the system to the amount of atmospheric perturbations affecting the beam during the upward propagation.
Laser guide stars (LGSs) are fundamental elements of adaptive optics. They indeed allow to form an artificial star in the sky, which is used as a reference for the compensation of the effect of atmospheric perturbation on wave fronts. This paper describes the design principle of laser guide stars and in particular show the advantage of using a combination of two afocal systems. Indeed, the first afocal can be used, with controlled defocus, to tune the size and position of the beam waist at the entrance of the second afocal. This allows, for example, to decrease the ultimate size of the artificial star which can be achieved. Moreover, it also allows to set parameters so that tolerances of the system are released.
Shearography can be used for full-field strain measurements in the field of vibration analysis. It provides the spatial derivative of the optical phase difference of the vibration modes amplitude along the so-called shear direction. The shearographic setup considered here is based on the phase-shifting time-averaged technique. It can easily be applied experimentally, but its drawback is that binary phase patterns are obtained. These phase changes are related to the zeroes of a Bessel function. Retrieving the corresponding displacement maps is not straightforward. Moreover, integration of shearographic results need to be performed, and this step is sensitive to noise in the patterns. In this paper, different processing stages are described, from fringe denoising to integration of the displacement maps. The application on data acquired in industrial environment illustrates the good performances of the proposed method.
Numerical modeling tools are widely used in space science, but are usually limited to the thermomechanical steps. However, many payloads are equipped with high performance optical systems with tight tolerances. Therefore, experimental testing of space optics in very realistic conditions is a mandatory process. This experimental step is both time consuming and expensive. A multiphysics modeling tool that also takes into account the optical performances would therefore be an elegant solution to avoid these drawbacks. In this paper we compare some experimental results with numerical results obtained from a multiphysics suite. The local displacements of two space mirrors have been measured by use of electronic speckle pattern interferometry (ESPI) and the deformation itself has been calculated by subtracting the rigid body motion. After validation of the thermo-mechanical model, experimental and numerical wavefront errors are compared.
SALTO is a Belgian project aiming to build a complete 1 m telescope demonstrator including a single-conjugated adaptive optics (AO) system together with a Rayleigh laser guide star system. The underlying objective of SALTO consists in developing the Belgian expertise regarding AO systems for medium size telescopes (i.e. diameter from 1 to 4m), for application in astronomy, optical communication or detection of low-Earth orbit objects. The project approach is to base the design on COTS components, in order to reduce complexity, and to favor both robustness and automation of the system over performance. The SALTO demonstrator will be located at Redu Space Services in the Belgian countryside. Therefore the major challenge of the project will be to deal with poor seeing, far worst than astronomical standards, while preserving robust and reasonable correction in the red-visible and near-infrared wavelength range. Here, we present our system baseline, the expected performance, and the preliminary design of the AO system. We conclude with the current prospects for the project.
Observations from space are almost exclusively performed by means of mirrors. To achieve higher performance, larger and larger mirrors are manufacture usually in aluminum alloy in order to be cost-effective. However from the optical performance point of view, the coefficient of thermal expansion (CTE) of aluminum is an important drawback.
The manufacture of mirrors for space application is expensive and the requirements on the optical performance increase
over years. To achieve higher performance, larger mirrors are manufactured but the larger the mirror the higher the
sensitivity to temperature variation and therefore the higher the degradation of optical performances. To avoid the use of
an expensive thermal regulation, we need to develop tools able to predict how optics behaves with thermal constraints.
This paper presents the comparison between experimental surface mirror deformation and theoretical results from a
multiphysics model. The local displacements of the mirror surface have been measured with the use of electronic speckle
pattern interferometry (ESPI) and the deformation itself has been calculated by subtracting the rigid body motion. After
validation of the mechanical model, experimental and numerical wave front errors are compared.
Laser ultrasonics is a technique currently studied for nondestructive inspection of aerospace composite structures based on carbon fibers. It combines a pulsed laser impacting the surface generates an ultrasound inside the material, through the nondestructive thermoelastic effect. Second a detection interferometer probes the impacted point in order to measure the displacement of the surface resulting from the emitted ultrasound wave and the echo coming back from the different interfaces of the structure. Laser ultrasonics is of interest for inspecting complex shaped composites. We have studied the possibility of using frequency doubled YAG laser for the generation and which is fiber-coupled, together with a fibercoupled interferometric probe using a YAG laser in the NIR. Our final system is a lightweight probe attached to a robot arm and which is able to scan complex shapes. The performances of the system are compared for different wavelengths of generations. Also we have studied some experimental parameters of interest such as tolerance to angle and focus distance, and different geometries of generation beams. We show some examples of inspection of reference parts with known defects. In particular C-scans of curved composites structures are presented.
Locating defects in CFRP composite materials is a hot topic in nondestructive inspection (NDI). Beside classical NDI technique, such as ultrasound testing (UT), contactless techniques are actively studied. Generally manufacturers of CFRP structure incorporate artificial defects in the bulk, with different extents and depths, in order to study the performance of a specific NDI technique to detect the defect. One of the most common defects in CFRP is delamination between two layers. This is simulated by inserting teflon sheets which, like air, acts as ultrasound blocker in UT. When such reference part is used to assess NDI performance of thermography or shearography, we only observe respectively the thermal or mechanical response of teflon with respect to external loading used with these techniques. In this work, we assess other possibilities for artificial defects in CFRP matrix. For that a CFRP structure was developed and which incorporates teflon, flat-bottom holes and delamination obtained by the pull-out method. We experimentally studied the signals and we discuss the difference between the various artificial defects methods.
We review some full-field interferometric techniques which have been successfully applied in different applications
related to the aerospace industry. The first part of the paper concerns the long-wave infrared (LWIR) digital holographic
interferometry which allows the measurement large displacements that occur when space structures undergo large
temperature excursions. A second part of the paper concerns different developments in interferometric nondestructive
testing (NDT) techniques intended to improve their usability in aerospace industrial environments. Among others, we
discuss LWIR speckle interferometry for simultaneous deformation and temperature variation measurements and new
post-processing techniques applied to shearography for an easier detection of flaws in composite structures.
We present our investigations on two interferometric methods suitable for industrial conditions dedicated to the visualization of vibration modes of aeronautic blades. First, we consider long-wave infrared (LWIR) electronic speckle pattern interferometry (ESPI). The use of long wavelength allows measuring larger amplitudes of vibrations compared with what can be achieved with visible light. Also longer wavelengths allow lower sensitivity to external perturbations. Second, shearography at 532 nm is used as an alternative to LWIR ESPI. Both methods are used in time-averaged mode with the use of phase-stepping. This allows transforming Bessel fringes, typical to time averaging, into phase values that provide higher contrast and improve the visualization of vibration mode shapes. Laboratory experimental results with both techniques allowed comparison of techniques, leading to selection of shearography. Finally a vibration test on electrodynamic shaker is performed in an industrial environment and mode shapes are obtained with good quality by shearography.
This experimental study was carried out within the context of high concentration photovoltaics. The paper presents the results of an experimental investigation relating to the quantification of the impacts of the chromatic effect on the performance of a double junction GaInP/GaAs solar cell. Chromatic effects are the result of material dispersion caused by the refractive optics component. This study aims to evaluate the effect of the spectral modification of the incident beam on the whole solar concentrator system performance. Such considerations are fundamental in producing a highly accurate design, with which to achieve the best possible system performance. Efficiency is evaluated within the vicinity of the focus of a Fresnel lens designed for concentration. On the optical axis, rays with different wavelengths are not focalized at the same points. The spectral content of the beam depends, therefore, upon the position of the cell along the optical axis. It is assumed that spectral content modification may have an impact on cell performance and, as a consequence, on system efficiency as a whole. Efficiency of the optical Fresnel lens and of the cell were evaluated in relation to spectral content modification.
We present a new solar concentrator concept. This concept is based on spectral splitting. It implies reflective, refractive and diffractive elements that allow two spectrally differentiated beams to reach different and/or unmatched lattice solar cells. The aimed geometrical concentration factor is 5× and the theoretical optical efficiency of that concentrator concept reaches theoretically 82%. The following study will discuss the concept of such a solar concentrator. A practical application to dye sensitized solar cells is given. The manufacturing and design of the element is then exposed. Those elements have been tested in the laboratory. Good agreements with theoretical simulations are demonstrated.
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