The Copernicus Land Surface Temperature Monitoring, LSTM, mission will perform spatial high temporal observations of land-surface in visible, near-infrared and thermal infrared bands. LSTM payload includes a telescope feeding two optical paths: one for a visible imager (VIS) and a near-infrared imager (NIR), the other path for a thermal infrared imager (TIR). The VIS and NIR imagers contain specific freeform field lenses and shall be co-aligned with a drastic accuracy (typically +/- 5 μm). Since this co-alignment is performed before telescope availability, an on-ground testing bench (OGSE) is required to co-align VIS&NIR focal planes. This bench shall be able to emulate, field by field, the MTF performances of the LSTM telescope. OGSE main components are a spherical mirror, several beamsplitters, a deformable mirror and a wavefront sensor (WFS). The OGSE goal is to generate in closed loop representative wavefronts in VIS&NIR LSTM aperture stop plane. The particularity of this OGSE is that the output wavefront is far from being spherical since each VIS&NIR imagers field lens corrects a large WFE generated at LSTM telescope level. Furthermore, the wavefront changes significantly from one point of the field of view to another. Finally, and importantly, the OGSE needs to allow co-focussing of VNIR & SWIR detectors despite it does not illuminate both fields simultaneously. In order to simulate the OGSE achievable performances we simulate the deformable mirror on the optical design software (CodeV) with its active actuators as variable parameters.
ALPAO designs and manufactures a wide range of deformable mirrors (DM) for various applications, from large telescopes through laser or microelectronics to vision science and microscopy. In this paper we will present the recent developments towards a space use of our DMs. Thanks to the wide span of characteristics of ALPAO DMs, many applications in space telescopes can be considered: correction of surface error of a large primary mirror, correction of high spatial frequency errors for high contrast imaging, simplification of instrument design with an active surface, etc. We present the ALPAO DM technology and the performances of DMs with large number of actuators. A focus will be made on the space application with the analysis of some correction cases and of the system reliability. The optimization of the DM’s characteristics is presented, as well as the roadmap towards space qualification.
Adaptive optics (AO) systems for retinal imaging have a small field of view (FoV) of 1°-2°. To increase the FoV we developed a multi-conjugated AO-system. By choosing different scanning patterns, aberrations introduced by the eye at different locations within the FoV can be measured by a Shack Hartmann wavefront sensor. A second deformable mirror that is conjugated to a plane anterior to the retina corrects for varying aberrations across the field. Retinal imaging using spectral-domain optical coherence tomography is performed with 4º x 4º FoV and imaging data including the visualization of the cone mosaic is presented.
Adaptive Optics (AO) retinal imaging is revealing microscopic structures of the eye in a non-invasive way. Due to anisoplanatism, conventional AO systems are efficient on small 1°x1° field of view (FoV). We present a lens-based AO scanning laser ophthalmoscope (SLO) set-up with 2 deformable mirrors (DM), providing high-resolution retinal imaging on a 4°x4° FoV, for an eye pupil diameter of 7 mm. The first DM is in a pupil plane and is driven using a Shack-Hartmann (SH). The second DM is conjugated to a plane located 0.7 mm in front of the retina, to correct for aberrations varying within the FoV. Its shape is optimized using sensorless AO technique.
The performance of this set-up was characterized in-vivo by measuring the eyes of four healthy volunteers. The obtained image quality was satisfactory and uniform over the entire FoV. Foveal cones could be resolved and no image distortion was detected. Furthermore, a 10°x10° FoV image was acquired at the fovea of one volunteer, by stitching 9 images recorded at different eccentricities. Finally, different layers of the retina were imaged. In addition to the photoreceptors mosaic, small capillaries and nerve fibers were clearly identified.
The presented AO-SLO instrument provides high-resolution images of the retina on a relatively large FoV in reasonable time. With 2 DMs, one SH and no guide star, the system stays quite simple. The imaging performance of the set-up was validated on 4 healthy volunteers and we are currently imaging patients with different eye diseases.
Active Optics methods, based on elasticity theory, allow the aspherisation of optical surfaces by stress polishing but also active aspherisation in situ. Researches in this field will impact the final performance and the final cost of any telescope or instrument. The stress polishing method is well suited for the superpolishing of aspheric components for astronomy. Its principle relies on spherical polishing with a full-sized tool of a warped substrate, which becomes aspherical once unwarped. The main advantage of this technique is the very high optical quality obtained either on form or on high spatial frequency errors. Furthermore, the roughness can be decreased down to a few angstroms, thanks the classical polishing with a large pitch tool, providing a substantial gain on the final scientific performance, for instance on the contrast on coronagraphic images, but also on the polishing time and cost. Stress polishing is based on elasticity theory, and requires an optimised deformation system able to provide the right aspherical form on the optical surface during polishing. The optical quality of the deformation is validated using extensive Finite Element Analysis, allowing an estimation of residuals and an optimisation of the warping harness. We describe here the work realised on stress polishing of toric mirrors for VLT-SPHERE and then our actual work on off axis aspherics (OAA) for the ASPIICS-Proba3 mission for solar coronagraphy. The ASPIICS optical design made by Vives et al is a three mirrors anastigmat including a concave off axis hyperboloid and a convex off axis parabola (OAP). We are developing a prototype in order to demonstrate the feasibility of this type of surface, using a multi-mode warping harness (Lemaitre et al). Furthermore, we present our work on variable OAP, meaning the possibility to adjust the shape of a simple OAP in situ with a minimal number of actuators, typically one actuator per optical mode (Focus, Coma and Astigmatism). Applications for future space telescopes and instrumentation are discussed.
The goal of the MADRAS project (Mirror Active, Deformable and Regulated for Applications in Space) is to highlight the interest of Active Optics for the next generation of space telescope and instrumentation. Wave-front errors in future space telescopes will mainly come from thermal dilatation and zero gravity, inducing large lightweight primary mirrors deformation. To compensate for these effects, a 24 actuators, 100 mm diameter deformable mirror has been designed to be inserted in a pupil relay. Within the project, such a system has been optimized, integrated and experimentally characterized. The system is designed considering wave-front errors expected in 3m-class primary mirrors, and taking into account space constraints such as compactness, low weight, low power consumption and mechanical strength. Finite Element Analysis allowed an optimization of the system in order to reach a precision of correction better than 10 nm rms. A dedicated test-bed has been designed to fully characterize the integrated mirror performance in representative conditions. The test set up is made of three main parts: a telescope aberrations generator, a correction loop with the MADRAS mirror and a Shack-Hartman wave-front sensor, and PSF imaging. In addition, Fizeau interferometry monitors the optical surface shape. We have developed and characterized an active optics system with a limited number of actuators and a design fitting space requirements. All the conducted tests tend to demonstrate the efficiency of such a system for a real-time, in situ wave-front. It would allow a significant improvement for future space telescopes optical performance while relaxing the specifications on the others components.
The need for both high quality images and light structures is a constant concern in the conception of space telescopes. In this paper, we present an active optics system as a way to fulfill those two objectives. Indeed, active optics consists in controlling mirrors’ deformations in order to improve the images quality [1]. The two main applications of active optics techniques are the in-situ compensation of phase errors in a wave front by using a corrector deformable mirror [2] and the manufacturing of aspherical mirrors by stress polishing or by in-situ stressing [3]. We will focus here on the wave-front correction. Indeed, the next generation of space telescopes will have lightweight primary mirrors; in consequence, they will be sensitive to the environment variations, inducing optical aberrations in the instrument.
An active optics system is principally composed of a deformable mirror, a wave front sensor, a set of actuators deforming the mirror and control/command electronics. It is used to correct the wave-front errors due to the optical design, the manufacturing imperfections, the large lightweight primary mirrors’ deflection in field gravity, the fixation devices, and the mirrors and structures’ thermal distortions due to the local turbulence [4]. Active optics is based on the elasticity theory [5]; forces and/or load are used to deform a mirror. Like in adaptive optics, actuators can simply be placed under the optical surface [1,2], but other configurations have also been studied: a system’s simplification, inducing a minimization of the number of actuators can be achieved by working on the mirror design [5]. For instance, in the so called Vase form Multimode Deformable Mirror [6], forces are applied on an external ring clamped on the pupil. With this method, there is no local effect due to the application of forces on the mirror’s back face. Furthermore, the number of actuators needed to warp the mirror does not depend on the pupil size; it is a fully scalable configuration.
The insertion of a Vase form Multimode Deformable Mirror on the design of an optical instrument will allow correcting the most common low spatial frequency aberrations. This concept could be applied in a space telescope. A Finite Element Analysis of the developed model has been conducted in order to characterize the system’s behavior and to validate the concept.
We present the optimization of an adaptive optics loop for retinal imaging. Generally, the wave-front is overdetermined compared to the number of corrector elements. The sampling of the sensor can be reduced while maintaining an efficient correction, leading to higher sensitivity, faster correction and larger dynamic range. An analytical model was developed to characterize the link between number of actuators, number of micro-lenses and correction performance. The optimized correction loop was introduced into a scanning laser ophthalmoscope. In vivo images of foveal photoreceptors were recorded and the obtained image quality is equivalent to the state of the art in retinal AO-imaging.
A novel active mirror concept based on carbon fiber reinforced polymer (CFRP) materials is presented. A nanolaminate facesheet, active piezoelectric layer and printed electronics are implemented in order to provide the reflective surface, actuation capabilities and electrical wiring for the mirror. Mirrors of this design are extremely thin (500-850 µm), lightweight (~ 2 kg/m2) and have large actuation capabilities (~ 100 µm peak- to-valley deformation per channel). Replication techniques along with simple bonding/transferring processes are implemented eliminating the need for grinding and polishing steps. An outline of the overall design, component materials and fabrication processes is presented. A method to size the active layer for a given mirror design, along with simulation predictions on the correction capabilities of the mirror are also outlined. A custom metrology system used to capture the highly deformable nature of the mirrors is demonstrated along with preliminary prototype measurements.
After two years of research and development under ESO support, LAM and Thales SESO present the results of their experiment for the fast and accurate polishing under stress of ELT 1.5 meter segments as well as the industrialization approach for mass production. Based on stress polishing, this manufacturing method requires the conception of a warping harness able to generate extremely accurate bending of the optical surface of the segments during the polishing. The conception of the warping harness is based on finite element analysis and allowed a fine tuning of each geometrical parameter of the system in order to fit an error budget of 25nm RMS over 300μm of bending peak to valley. The optimisation approach uses the simulated influence functions to extract the system eigenmodes and characterise the performance. The same approach is used for the full characterisation of the system itself. The warping harness has been manufactured, integrated and assembled with the Zerodur 1.5 meter segment on the LAM 2.5meter POLARIS polishing facility. The experiment consists in a cross check of optical and mechanical measurements of the mirrors bending in order to develop a blind process, ie to bypass the optical measurement during the final industrial process. This article describes the optical and mechanical measurements, the influence functions and eigenmodes of the system and the full performance characterisation of the warping harness.
Thin, lightweight and low-cost deformable mirrors have been recently proposed, providing a pertinent device for wavefront error correction. We present different approaches to optimize actuator arrangement. The design is optimized according to a given correction requirement, through the number of electrodes, their shape and location. A first method focuses on the compensation of a given optical aberration (astigmatism). A second method directly optimizes the correction of a set of optical modes, taking into account the voltage limitation. We will describe the optimization techniques and give some examples of applications and design performance.
KEYWORDS: Mirrors, Actuators, Space telescopes, Space mirrors, Finite element methods, Wavefronts, Telescopes, Active optics, Precision calibration, Disk lasers
The need for both high quality images and lightweight structures is one of the main drivers in space telescope design. An efficient wavefront control system will become mandatory in future large observatories, retaining performance while relaxing specifications in the global system’s stability. We present the mirror actively deformed and regulated for applications in space project, which aims to demonstrate the applicability of active optics for future space instrumentation. It has led to the development of a 24-actuator, 90-mm-diameter active mirror, able to compensate for large lightweight primary mirror deformations in the telescope’s exit pupil. The correcting system has been designed for expected wavefront errors from 3-m-class lightweight primary mirrors, while also taking into account constraints for space use. Finite element analysis allowed an optimization of the system in order to achieve a precision of correction better than 10 nm rms. A dedicated testbed has been designed to fully characterize the integrated system performance in representative operating conditions. It is composed of: a telescope simulator, an active correction loop, a point spread function imager, and a Fizeau interferometer. All conducted tests demonstrated the correcting mirror performance and has improved this technology maturity to a TRL4.
KEYWORDS: Mirrors, Actuators, Space telescopes, Space mirrors, Active optics, Finite element methods, Telescopes, Deformable mirrors, Precision calibration, Astronomical imaging
MADRAS (Mirror Active, Deformable and Regulated for Applications in Space) project aims at demonstrating
the interest of Active Optics for space applications. We present the prototype of a 24 actuators, 100 mm diameter
deformable mirror to be included in a space telescope's pupil relay to compensate for large lightweight primary
mirror deformation. The mirror design has been optimized with Finite Element Analysis and its experimental
performance characterized in representative conditions. The developed deformable mirror provides an efficient
wave-front correction with a limited number of actuators and a design fitting space requirements.
In this paper, we present two original concepts of deformable mirrors to compensate for first orders optical
aberrations with a minimum number of actuators: one optical mode is generated with one actuator. The
Variable O_-Axis parabola (VOALA) concept is a 3-actuators, 3-modes system able to generate independently
Focus, Astigmatism3 and Coma3. The Correcting Optimized Mirror with a Single Actuator (COMSA) is a
1-actuator system able to generate a given combination of optical aberrations. The limited number of degrees of
freedom of such systems makes them easy to set up and monitor, which is a significant advantage, notably for
space use.
The Laboratoire d'Astrophysique de Marseille (LAM) is involved in the prototyping of a full scale demonstrator for
stress polishing of segments for the European Extremely Large Telescope (E-ELT). Stress polishing method is developed
at LAM since more than 40 years, and this mature technology has recently been used with success for VLT instruments.
Stress polishing is now considered as a promising manufacturing method for mass production of large off axis mirrors,
specifically for ELT segments. This powerful method, based on elasticity theory, allows the generation of super-smooth
off-axis aspherics with a minimal amount of high spatial frequency ripples by spherically polishing a warped blank with
a full-sized tool. Thanks to the simple spherical polishing, the operation time can be strongly reduced compared to the
time-consuming sub-aperture tool methods of grinding and polishing. The goal is to rapidly converge to less than 1
micron RMS of optical quality on a circular blank which will be finally cut hexagonally and finished using Ion Beam
Finishing. In this paper we will present the status of the demonstrator and the design of the warping harness prototype
that must be able to precisely warp the circular blank.
KEYWORDS: Actuators, Mirrors, Space telescopes, Deformable mirrors, Active optics, Finite element methods, Space mirrors, Optical aberrations, Monochromatic aberrations, Precision calibration
Wave-front correction in optical instruments is often needed, either to compensate Optical Path Differences,
off-axis aberrations or mirrors deformations. Active optics techniques are developed to allow efficient corrections
with deformable mirrors. In this paper, we will present the conception of particular deformation systems which
could be used in space telescopes and instruments in order to improve their performances while allowing relaxing
specifications on the global system stability.
A first section will be dedicated to the design and performance analysis of an active mirror specifically designed
to compensate for aberrations that might appear in future 3m-class space telescopes, due to lightweight primary
mirrors, thermal variations or weightless conditions. A second section will be dedicated to a brand new design of
active mirror, able to compensate for given combinations of aberrations with a single actuator. If the aberrations
to be corrected in an instrument and their evolutions are known in advance, an optimal system geometry can be
determined thanks to the elasticity theory and Finite Element Analysis.
KEYWORDS: Mirrors, Actuators, Space telescopes, Active optics, Deformable mirrors, Finite element methods, Space mirrors, Optical aberrations, Zernike polynomials, Precision calibration
The need for both high quality images and light structures is a constant concern in the conception of space telescopes.
The goal here is to determine how an active optics system could be embarked on a satellite in order to correct the wave
front deformations of the optical train. The optical aberrations appearing in a space environment are due to mirrors'
deformations, with three main origins: the thermal variations, the weightlessness in space with respect to the
Assemblage, Integration and Testing (AIT) conditions on ground and the use of large weightlighted primary mirrors.
We are developing a model of deformable mirror as minimalist as possible, especially in term of number of actuators,
which is able to correct the first Zernike polynomials in the specified range of amplitude and precision. Flight constraints
as weight, volume and power consumption have to be considered. Firstly, such a system is designed according to the
equations from the elasticity theory: we determine the geometrical and mechanical characteristics of the mirror, the
location of the forces to be applied and the way to apply them. The concept is validated with a Finite Element Analysis
(FEA), allowing optimizing the system by taking into account parameters absent from the theory. At the end of the
program the mirror will be realized and characterized in a representative optical configuration.
The stress polishing method is well suited for the superpolishing of aspheric components for astronomy. The main
advantage of this technique is the very high optical quality obtained either on form or on high spatial frequency
errors. Furthermore, the roughness can be decreased down to a few angstrom, thanks the classical polishing with
a large pitch tool. We describe here the results obtained on the three toric mirrors for the ESO/VLT-SPHERE
instrument, dedicated to exoplanet direct imaging. On going work on variable off axis aspherics is presented in
the frame of the ESA/ASPIICS-Proba3 mission for solar coronography and applications for Exoplanet detection
and solar observations are discussed.
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