In the last few years the concept of an active space telescope has been greatly developed, to meet demanding requirements with a substantial reduction of tolerances, risks and costs. This is the frame of the LATT project (an ESA TRP) and its follow-up SPLATT (an INAF funded R&D project). Within the SPLATT activities, we outline a novel approach and investigate, both via simulations and in the optical laboratory, two main elements: an active segmented primary with contactless actuators and a pyramid wavefront sensor (PWFS) to drive the correction chain. The key point is the synergy between them: the sensitivity of the PWFS and the intrinsic stability of a contactless-actuated mirror segment. Voice-coil, contactless actuators are in facts a natural decoupling layer between the payload and the optical surface and can suppress the high frequency vibration as we verified in the lab. We subjected a 40 cm diameter prototype with 19 actuators to an externally injected vibration spectrum; we then measured optically the reduction of vibrations when the optical surface is floating controlled by the actuators, thus validating the concept at the first stage of the design. The PWFS, which is largely adopted on ground-based telescope, is a pupil-conjugated sensor and offers a user-selectable sampling and capture range, in order to match different use cases; it is also more sensitive than Shack-Hartmann sensor especially at the low-mid spatial scales. We run a set of numerical simulations with the PWFS measuring the misalignment and phase steps of a JWST-like primary mirrors: we investigated the PWFS sensitivity in the sub-nanometer regime in presence of photon and detector noise, and with guide star magnitudes in the range 8 to 14. In the paper we discuss the outcomes of the project and present a possible roadmap for further developments.
Large format deformable mirrors have been proposed in the last few years as key elements to implement active wave front correction for future space telescopes. Active optics is, in fact, an enabling technology for high stability, high contrast and high resolution systems. We present in this work a 40 cm diameter prototype, together with its laboratory characterization, based on voice-coil actuators. When the mirror is operated, such contact-less actuation allows the optical surface to float at a given distance from its support and the mirror is virtually decoupled from the mechanics; such condition offers an intrinsic isolation from external vibrations with no need for further damping devices. We demonstrated experimentally this concept in the laboratory on a dedicated interferometric setup, registering a substantial rejection of the vibrations injected. We will present in this work the test results and a roadmap for future developments.
Although adaptive optics (AO) systems have been developed to compensate for atmospheric turbulence, their application is mostly limited to large size (1.5 to 8 m) telescopes due to high cost and complexity. In particular, for small to medium aperture telescopes for which the need for AO correction is critical, AO systems have different requirements such as compactness and transportability. We report on the realization and the on-sky test of an extremely small adaptive optics module (XSAO) based on a multiactuator adaptive lens. Observations were performed on an 11-in. commercial telescope and a 1-m scientific telescope with minimal modifications. The results show a strong improvement of the acquired images for different types of observed objects, such as bright stars, double stars, planets, and deep sky objects.
Adaptive Optics (AO) is a key technology for ground-based astronomical telescopes, allowing to overcome the limits imposed by atmospheric turbulence and obtain high resolution images. This technique however, has not been developed for small size telescopes, because of its high cost and complexity. We realized an AO system based on a Multi-actuator Adaptive Lens and a Shack-Hartmann wavefront sensor (WFS), allowing for a great compactness and simplification of the optical design. The system was integrated on a 11” telescope and controlled by a consumer-grade laptop allowing to perform Closed-Loop AO correction up to 400 Hz.
Adaptive Optics (AO) is a key technology for ground-based astronomical telescopes, allowing to overcome the limits imposed by atmospheric turbulence and obtain high resolution images. This technique however, has not been developed for small size telescopes, because of its high cost and complexity. We realized an AO system based on a Multi-actuator Adaptive Lens and a Shack-Hartmann wavefront sensor (WFS), allowing for a great compactness and simplification of the optical design. The system was integrated on a 11” telescope and controlled by a consumer-grade laptop allowing to perform Closed-Loop AO correction up to 400 Hz.
The use of adaptive lenses instead of deformable mirrors can simplify the implementation of an adaptive optics system. The recently introduced Multi-actuator Adaptive Lens (MAL) can be used in closed loop with a wavefront sensor to correct for time-variant wavefront aberrations. The MAL can guarantee a level of correction and a response time similar to the ones obtained with deformable mirrors. The adaptive lens is based on the use of piezoelectric actuators and, without any obstruction or electrodes in the clear aperture, can guarantee a fast response time, less than ~10ms. Our tests show that the MAL can be used both in combination with a wavefront sensor in a “classical” adaptive optics closed loop, or in a wavefront sensorless configuration. The latter has allowed us to design more compact and simple imaging systems for different microscopy platforms. We will show that the Multi-actuator Adaptive Lens has been successfully used for in-vivo OCT ophthalmic imaging in both mice and humans, as well as confocal and two photon microscopy. We tested and compared different optimization strategies such as coordinate search and the DONE algorithm. The results suggest that the MAL optimization can correct for eye aberrations with a pupil of 5mm or sample induced aberrations in microscopy.
In this paper, modern subspace identification methods are applied to a Multi-conjugate Adaptive optics Demonstrator, MAD, developed at ESO. The identified multi-input multi-output systems mapping voltages into slopes can be obtained on data taken in open/closed loop on beacon and in both SCAO and GLAO configurations. The advantages of the proposed approach is twofold: on the one hand the experiment to collect the data takes only few minutes during day time, it can be done on beacon, and all the computational effort is moved off-line. On the other hand, subspace identification provides the mathematical model necessary to design modern model-based controllers (e.g. linear quadratic control).
GALACSI is one of the Adaptive Optics (AO) systems part of the ESO Adaptive Optics Facility (AOF). It will use the
VLT 4-Laser Guide Stars system, high speed and low noise WaveFront Sensor cameras (<1e-, 1000Hz) the
Deformable Secondary Mirror (DSM) and the SPARTA Real Time Computer to sharpen images and enhance faint
object detectability of the MUSE Instrument. MUSE is an Integral Field Spectrograph working at wavelengths from
465nm to 930nm. GALACSI implements 2 different AO modes; in Wide Field Mode (WFM) it will perform Ground
Layer AO correction and enhance the collected energy in a 0.2" by 0.2" pixel by a factor 2 at 750nm over a Field of
View (FoV) of 1' by 1'. The 4 LGSs and one tip tilt reference star (R-mag <17.5) are located outside the MUSE FoV.
Key requirements are to provide this performance and a very good image stability for a 1hour long integration time. In
Narrow Field Mode (NFM) Laser Tomography AO will be used to reconstruct and correct the turbulence for the center
field using the 4 LGSs at 15" off axis and the Near Infra Red (NIR) light of one reference star on axis for tip tilt and
focus sensing. In NFM GALACSI will provide a moderate Strehl Ratio of 5% (goal 10%) at 650nm. The NFM hosts
several challenges and many subsystems will be pushed to their limits. The opto mechanical design and error budgets
of GALACSI is described here.
In modern adaptive optics systems, lightly damped sinusoidal oscillations resulting from telescope structural vibrations have a significant deleterious impact on the quality of the image collected at the detector plane. Such oscillations are often at frequencies beyond the bandwidth of the wave-front controller that therefore is either incapable of rejecting them or might even amplify their detrimental impact on the overall AO performance. A technique for the rejection of periodic disturbances acting at the output of unknown plants, which has been recently presented in literature, has been adapted to the problem of rejecting vibrations in AO loops. The proposed methodology aims at estimating phase and amplitude of the harmonic disturbance together with the response of the unknown plant at the frequency of vibration. On the basis of such estimates, a control signal is generated to cancel out the periodic perturbation. Additionally, the algorithm can be easily extended to cope with unexpected time variations of the vibrations frequency by adding a frequency tracking module based either on a simple PLL architecture or on a classical extended Kalman filter. Oversampling can be also easily introduced to efficiently correct for vibrations approaching the sampling frequency. The approach presented in this contribution is compared against a different algorithm for vibration rejection available in literature, in order to identify drawbacks and advantages. Finally, the performance of the proposed vibration cancellation technique has been tested in realistic scenarios defined exploiting tip/tilt measurements from MACAO and NACO
We present in this paper an analysis of several tip-tilt on-sky data registered on adaptive optics systems installed on different telescopes (Gemini South, William Herschel Telescope, Large Binocular Telescope, Very Large Tele scope, Subaru). Vibration peaks can be detected, and it is shown that their presence and location may vary, and that their origin is not always easy to determine. Mechanical solution that have been realized to mitigate vibrations are presented. Nevertheless, residual vibrations may still affect the instruments' performance, ranging from narrow high frequency vibration peaks to wide low frequency windshake-type perturbations. Power Spectral Densities (PSDs) of on-sky data are presented to evidence these features. When possible, indications are given regarding the gain in performance that could be achieved with adequate controllers accounting for vibration mitigation. Two examples of controller identification and design illustrate their ability to compensate for various types of disturbances (turbulence, windshake, vibration peaks, ...),showing a significant gain in performance.
A 42 meters telescope does require adaptive optics to provide few milli arcseconds resolution images. In the current
design of the E-ELT, M4 provides adaptive correction while M5 is the field stabilization mirror. Both mirrors have an
essential role in the E-ELT telescope strategy since they do not only correct for atmospheric turbulence but have also to
cancel part of telescope wind shaking and static aberrations. Both mirrors specifications have been defined to avoid
requesting over constrained requirements in term of stroke, speed and guide stars magnitude. Technical specifications
and technological issues are discussed in this article. Critical aspects and roadmap to assess the feasibility of such
mirrors are outlined.
Several designs of future Adaptive Optics (AO) systems propose to use a large Deformable Mirror (DM), regarding the size as well as the number of actuators. Most of the time, there is no focal plane upstream the DM. Therefore, the classical way of calibrating the interaction matrix on an artificial source cannot be applied. Furthermore, the requirements in terms of calibration error budget are tight and the high order modes of such DMs are stiff and hence they achieve only a small stroke. This is why novel ways to determine the system Interaction Matrix (IM) have to be investigated. Several paths have been studied. One solution would be to simulate a synthetic IM. However, calibration on sky is also an option. Different techniques were simulated, tested and optimized on real AO systems. The results are presented in this paper.
In this paper, a novel procedure to design optimal controllers for adaptive optics systems is proposed. The most important feature of this procedure is that it does not require a complex and long tuning procedure like the standard PI controllers and that it is based on real data, i.e. does not require an a priori model for the plant. For this reason, we use system identification methods which, starting from real input-output time series, are able to build a discrete-time equivalent model for the whole plant. Since the plant is Multi-Input Multi-Output (MIMO), the identification methods are the one referred in the literature as Subspace Identification Methods (SID). They provide a state-space model with a deterministic part and a stochastic part. The first one is, in our case, composed by the cascade of the discrete-time approximation of the transfer function of all subsystems, whereas the latter takes into account the measurement errors and, more important, the atmospheric turbulence. In particular the stochastic part does not provide the statistics of the atmosphere, but explains how the wave-front sensor sees the turbulence. The optimal linear-quadratic controller is based on the deterministic part of the identified model and minimizes the variance of the wave-front phase error. Of course, minimizing the error variance is the same as to maximizing the Strehl ratio. Our approach is an extension of the SISO-class controllers, based on the interaction matrix/control matrix, to the dynamical MIMO-class case. In this manner, the dynamical interaction between actuators and sub-apertures is not lost.
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