Article

# Spatially filtered wave-front sensor for high-order adaptive optics

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## Abstract

Adaptive optics (AO) systems take sampled measurements of the wave-front phase. Because in the general case the spatial-frequency content of the phase aberration is not band limited, aliasing will occur. This aliasing will cause increased residual error and increased scattered light in the point-spread function (PSF). The spatially filtered wave-front sensor (SFWFS) mitigates this phenomenon by using a field stop at a focal plane before the wave-front sensor. This stop acts as a low-pass filter on the phase, significantly reducing the high-spatial-frequency content phase seen by the wave-front sensor at moderate to high Strehl ratios. We study the properties and performance of the SFWFS for open- and closed-loop correction of atmospheric turbulence, segmented-primary-mirror errors, and sensing with broadband light. In closed loop the filter reduces high-spatial-frequency phase power by a factor of 10(3) to 10(8). In a full AO-system simulation, this translates to a reduction by up to 625 times in the residual error power due to aliasing over a specific spatial frequency range. The final PSF (generated with apodization of the pupil) has up to a 100 times reduction in intensity out to lambda/2d.

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... Particularly, the high frame rate and large subaperture numbers of the Shack-Hartmann WFS allows SAXO to optimally measure and compensate for atmospherical turbulence. Moreover, the spatial filtering [7,8] allows one to deepen the contrast curve, and is automatically adjusted on turbulence level to provide the best performance. Finally, a dedicated calibration procedure based on focal-plane wave-front sensing is optimized for NCPA compensation on the coronagraphic device, ensuring the best compensation of quasi-static speckle. ...
... SAXO WFS is based on a spatially-filtered Shack-Hartmann WFS. The role of the spatial filter is to remove (or decrease) the aliasing effect in wave-front sensing ( [7]). The aliasing effect is one of the main limitations of the extreme extinction. ...
... In order to reach the final performance of the system, the solution designed for SPHERE consists in a focal-plane estimation of these aberrations, and a compensation by the AO system itself. The focal-plane estimator chosen for estimating these aberration is the technic of Phase Diversity [7], able to retrieve aberrations from a set of two focal-plane images (One in-focus, and one out-of-focus). The compensation is done by modifying the reference slopes of the WFS accordingly, so as the closed-loop naturally compensates from the measured aberrations. ...
Conference Paper
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SPHERE instrument [1] (Spectro-Polarimetry High-contrast Exoplanet Research) is a second generation ESO instrument dedicated to high contrast imaging, and exoplanet direct detection and characterisation. The overall performance of XAO system of SPHERE, as well as the optimal control law for turbulence correction, are presented in dedicated papers [5,6]. The global performance of the instrument and of all observing modes of SPHERE is done in [4]. The strategy of Wave-front Sensing [WFS] in SPHERE relies on two faces, and is thoroughly discussed in this paper. Firstly, extreme adaptive optics (XAO) is required for both turbulence and quasi-static pattern compensation. Particularly, the high frame rate and large subaperture numbers of the Shack-Hartmann WFS allows SAXO to optimally measure and compensate for atmospherical turbulence. Moreover, the spatial filtering [7,8] allows one to deepen the contrast curve, and is automatically adjusted on turbulence level to provide the best performance. Finally, a dedicated calibration procedure based on focal-plane wave-front sensing is optimized for NCPA compensation on the coronagraphic device, ensuring the best compensation of quasi-static speckle. Secondly, a high robustness to faint magnitude guide star allows SAXO to address a large panel of targets for exoplanet detection and characterization. This is only made possible by the joint use of a dedicated Wave- Front Sensing for turbulence, EMCCD detector capability, and adaptation of the system to the star magnitude. The noise propagation has been carefully monitored and optimized. The weighted center of gravity gives an optimal trade-off between performance with respect to noise, and complexity of implementation. The use of an EMCCD detector allows a powerful noise reduction on the wave-front sensor detector. And finaly, 5 SAXO observing modes are defined in order to cover all star magnitudes up to 16, with systematic optimal performance. During the whole assembly integrations and test period, choices have been done to optimise the trade-off between performance, robustness, and simplicity of use. The self-adaptation and auto-calibration of the instrument has been a strong investment, as well as developing a great simplicity of use. We describe here the actions taken to reach this level of operation for SPHERE. Finally, perspective are withdrawn for improving the strategy of WFS in the framework of future XAO instrumentations in E-ELT.
... In this respect one should see with a different perspective the hierarchical WFS [11] where one splits the whole collected light to different kinds of samplings. A revisitation of the behavior of the Pyramid WFS, especially taking into account its inherent Fourier filtering in the focal plane introduced by the four adjacent faces of the pyramid [12,13] especially in its non-modulated version [14] , a solution well proven in the sky [15] leading to reconsider the non-modulated pyramid as a sort of Dark-WFS. While hereafter we trace the basis and we offer some suggestions on this theme, the interested reader should take a look at a more detailed paper [16] on this topic. ...
... Following the Dark-WFS recipes all the photons that do not contribute to the final AO performances are basically better to be thrown away, otherwise their presence can only introduce additional Poissonian or photon shot noise. Following the same principles used in the Shack-Hartmann [13] in the case of the pyramid this can be clearly seen by an inspection of Fig.2. In principle such a selection can sharply remove all the light outside a certain region whose only influence is to illuminate the inner side of the pupils and to contribute somehow to the photon shot noise and hence to the measurements (and the limiting magnitude) of the WFS itself. ...
Conference Paper
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We describe the concept of splitting spatial frequency perturbations into some kind of pupil planes wavefront sensors. Further to the existing approach of dropping higher spatial frequency to suppress aliasing effects (the so-called spatial filtered Shack-Hartmann), we point out that spatial frequencies splitting and mixing of these in a proper manner, could be handled in order to exhibit some practical or fundamental advantages. In this framework we describe the idea behind such class of concepts and we derive the relationship useful to determine if, by which extent, and under what kind of merit function, these devices can overperform existing conventional sensors.
... The aliasing can, to some extent, be avoided by using a spatial filter (Poyneer and Macintosh, 2004). It is an optical device built before the lenslet array to filter out the non-seen high spatial frequencies from the residual wavefront. ...
... The XAO system of the SPHERE instrument for the VLT ) is based on a spatially filtered SH (Poyneer and Macintosh, 2004). Even though the pyramid sensor was seriously considered in SPHERE conceptual design, it was considered too risky at this time, especially because of the control issues. ...
... Its propagation through the reconstruction is considered an important factor limiting the achievable contrast in high-contrast imaging (HCI) systems as SPHERE [1,21], GPI [19] ScExAO [20] and PALM-3000 [5]. The commonly used approach followed by all these instruments is to mitigate aliasing before the measurements are produced using the the spatially-filtered (SF) Shack-Hartmann (SH) wave-front sensor (WFS) [25]. ...
... High-contrast imaging (HCI) systems offer a suitable and attractive scenario of application and it is believed real-time spatial-frequency reconstructors can provide optimal or near-optimal performance -take the case of GPI [25]. Iterative extensions to the tomographic case have been pursued [13,35] as well as the Linear-Quadratic-Gaussian in the spatial-frequency domain [3]. ...
Article
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Computationally-efficient wave-front reconstruction techniques for astronomical adaptive optics systems have seen a great development in the past decade. Algorithms developed in the spatial-frequency (Fourier) domain have gathered large attention specially for high-contrast imaging systems. In this paper we present the Wiener filter (resulting in the maximization of the Strehl-ratio) and further develop formulae for the anti-aliasing Wiener filter that optimally takes into account high-order wave-front terms folded in-band during the sensing (i.e. discrete sampling) process. We employ a continuous spatial-frequency representation for the forward measurement operators and derive the Wiener filter when aliasing is explicitly taken into account. We further investigate and compare to classical estimates using least-squares filters the reconstructed wave-front, measurement noise and aliasing propagation coefficients as a function of the system order. Regarding high-contrast systems, we provide achievable performance results as a function of an ensemble of for ward models for the Shack-Hartmann wave-front sensor (using sparse and non-sparse representations) and compute point-spread function raw intensities. We find that for a 32x32 single-conjugated adaptive optics system the aliasing propagation coefficient is roughly 60% of the least-squares filters whereas the noise propagation is around 80%. Contrast improvements of factors of up to 2 are achievable across the field in H-band. For current and next generation high-contrast imagers, despite better aliasing mitigation, anti-aliasing Wiener filtering cannot be used as a stand-alone method and must therefore be used in combination with optical spatial filters deployed before image formation takes actual place.
... To test the performance of the MV method, nothing is done: U WFS = U AO . Nonetheless, we want to compare with the solution that consists in adding a low-pass spatial filter in the beam, as proposed by Poyneer et al.,20 to optically block the high spatial frequencies and prevent its aliasing in the command estimation. In this case, U WFS is first propagated to this pinhole, where its binary transmission, is applied before propagating the resulting wavefront towards the SH-WFS pupil. ...
Preprint
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We present the results obtained with an end-to-end simulator of an Extreme Adaptive Optics (XAO) system control loop. It is used to predict its on-sky performances and to optimise the AO loop algorithms. It was first used to validate a novel analytical model of the fitting error, a limit due to the Deformable Mirror (DM) shape. Standard analytical models assume a sharp correction under the DM cutoff frequency, disregarding the transition between the AO corrected and turbulence dominated domains. Our model account for the influence function shape in this smooth transition. Then, it is well-known that Shack-Hartmann wavefront sensors (SH-WFS) have a limited spatial bandwidth, the high frequencies of the wavefront being seen as low frequencies. We show that this aliasing error can be partially compensated (both in terms of Strehl ratio and contrast) by adding priors on the turbulence statistics in the framework of an inverse problem approach. This represents an alternative to the standard additional optical filter used in XAO systems. In parallel to this numerical work, a bench was aligned to experimentally test the AO system and these new algorithms comprising a DM192 ALPAO deformable mirror and a 15x15 SH-WFS. We present the predicted performances of the AO loop based on end-to-end simulations.
... A new Shack-Hartmann WFS concept has recently been proposed to spatially filter the input wavefront and prevent aliasing of higher-spatial frequency terms into this passband. 8 We therefore consider WFS measurement models with and without this anti-aliasing filter (AAF), defined by the pair of expressionŝ ...
Conference Paper
In addition to their essential function of providing atmospheric turbulence compensation, astronomical Adaptive Optical (AO) systems also supplement the role of active optics (aO) by providing some additional correction of the wavefront aberrations introduced by mirror mounting, alignment, thermal distortion and/or fabrication errors. This feature is particularly desirable for segmented mirror telescopes such as the Thirty Meter Telescope (TMT), but wavefront discontinuities across segment boundaries are challenging to properly sense and correct. In this paper we describe a fast, analytical, frequency domain model which may be used to study and quantify the above effects, and discuss a range of sample results obtained to support the development of the top-level requirements for the TMT primary mirror. In general, AO compensation of mirror segment piston errors is not particulary useful unless the deformable mirror (DM) interactuator spacing is equivalent to no more than one-half of a mirror segment diameter (when both of these dimensions are expressed in the same pupil plane). Effective AO compensation of mirror segment tip/tilt errors, or low order segment figure errors such as astigmatism, typically requires 3-4 DM actuators per mirror segment. These results illustrate the importance of quantifying and minimizing uncorrectable telescope wavefront errors when developing performance predictions for adaptive optical systems.
... A wide range of useful formulas for efficiently (albeit approximately) evaluating the impact of the individual error sources in an AO system have already been developed using spatial frequency domain techniques. Examples include models for anisoplanatism, 8 servo lag, 9,10 wavefront sensor (WFS) spatial aliasing, 11 and tomographic wavefront reconstruction in multi-conjugate AO (MCAO). 12,13 These results exploit the fact that the fundamental optical and control processes to be considered-wavefront propagation, sensing, reconstruction, and correction-are well modeled as spatial filtering operations. ...
Conference Paper
The limit case of an infinite aperture adaptive optics (AO) system eliminates the modeling complications associated with aperture edge effects, and thereby enables the application of simplified methods for system performance evaluation in the spatial frequency domain. We review prior work in this field and describe a new approach that enables a wider range of error sources and AO options to be evaluated with a reduced number of approximations. These errors and AO options include: Fitting error and spatial aliasing for a Shack-Hartmann wavefront sensor (WFS) and one particular deformable mirror influence function; WFS noise; servo lag for a continuous temporal filter function; anisoplanatism in either a single evaluation direction or averaged over an extended field of view; piston removal within a finite aperture; minimum variance and modal wavefront reconstruction algorithms; and multi-conjugate AO. Laser guidestars, however, are excluded. A wide range of classical results for the independent effects of individual error sources can be immediately derived from this integrated model. Performance estimates for more complex problems involving the full range of first-order AO error sources are in good agreement with the results produced by more detailed Monte Carlo simulations.
... Four selectable microlens arrays provide 8×8, 16×16, 32×32, and 64×64 subapertures, allowing the sensor to be adapted to a wide range of guidestar magnitudes. An adjustable spatial filter at input the focal plane will minimize aliasing in the fine pupil sampling modes 5 . The very fine pupil sampling modes lead to a stringent requirement on DM to microlens pupil registration of 10% of the finest subaperture, or 0.16% of the pupil. ...
Conference Paper
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Deployed as a multi-user shared facility on the 5.1 meter Hale Telescope at Palomar Observatory, the PALM-3000 highorder upgrade to the successful Palomar Adaptive Optics System will deliver extreme AO correction in the near-infrared, and diffraction-limited images down to visible wavelengths, using both natural and sodium laser guide stars. Wavefront control will be provided by two deformable mirrors, a 3368 active actuator woofer and 349 active actuator tweeter, controlled at up to 3 kHz using an innovative wavefront processor based on a cluster of 17 graphics processing units. A Shack-Hartmann wavefront sensor with selectable pupil sampling will provide high-order wavefront sensing, while an infrared tip/tilt sensor and visible truth wavefront sensor will provide low-order LGS control. Four back-end instruments are planned at first light: the PHARO near-infrared camera/spectrograph, the SWIFT visible light integral field spectrograph, Project 1640, a near-infrared coronagraphic integral field spectrograph, and 888Cam, a high-resolution visible light imager.
... Thus, additional approaches and/or modifications to this setup will ultimately be required to minimize noise amplification, such as a smaller pinhole and/or an anti-aliasing spatial filter. 22 ...
Conference Paper
Exoplanet detection and characterization through extreme adaptive optics (ExAO) is a key science goal of future extremely large telescopes. This achievement, however, will be limited in sensitivity by both quasi-static wavefront errors and residual AO-corrected atmospheric wavefront errors. A solution to both of these problems is to use the science camera of an ExAO system as a wavefront sensor to perform a fast measurement and correction method to remove these aberrations as soon as they are detected. We have developed the framework for one such method, using the self-coherent camera (SCC), to be applied to ground-based telescopes, called Fast Atmospheric SCC Technique (FAST; Gerard et al., submitted). Our FAST solution requires an optimally designed coronagraph (the SCC FPM) and post-processing algorithm and is in principle able to reach a "raw" contrast of a few times the photon noise limit, continually improving with integration time. In this paper, we present new ongoing work in exploring the manufacturing limitations of the SCC FPM as well as a general framework to implement and optimize a FAST deformable mirror control loop.
... Many previous studies have shown how implementation of a spatial filter in a focal plane of the wavefront sensing path can be used to significantly reduce the performance-limiting effects of aliasing on the wavefront sensor, thus causing the spatially filtered Shack-Hartmann wavefront sensor to become the most popular type of wavefront sensor at the moment. 1 Simulations show that spatial filtering could slightly improve the performance of a PWS, but to a lesser extent as with the Shack-Hartmann wavefront sensor . 2 ...
... The pyramid measurement has been studied [79] to explain how the slope or the phase of the wavefront measurement depends on the modulation amplitude considered, in the circular modulation scheme. When compared to a Shack-Hartmann in quad cell configuration with weighted center of gravity algorithm [80] and beam filtering [81], the pyramid was estimated to perform similarly [82] in high photon flux. ...
Thesis
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This PhD thesis presents the building of an adaptive-optics system based on a pyramid wavefront sensor applied to the imaging of the human retina (fundus) in vivo. The instrument aims to simultaneaously measure the ocular aberration, and correct it to allow the imaging of the fundus. The adaptive optics system uses a high-stroke magnetically-actuated deformable mirror with 52 elements that presents a correction range best adapted to the refraction in most non-emmetropic eyes and the appropriate surface deformation required for the correction of high-order ocular aberrations. This wavefront correction system is coupled with a sensor originally used in astronomy here selected for ophthalmic use due to its adjustable dynamic range that insures characterization of the ocular measurement and due to its robustness in adaptive optics applications. The retinal imaging is based on a green illumination (530nm) commonly used in commercial fundus cameras in clinical environments but to our knowledge not yet applied in the existing high-resolution systems imaging the retina at the cellular level. The calibration of the instrument response to the ocular aberration is performed using ophthalmic lenses and custom phase plates representing typical patterns. Adaptive optics correction is applied to these complex refractive elements and to typical test objects to estimate the improvement in retinal image quality. Using safe light levels and an experimental protocol agreed by the Research Ethics Committee of the National University of Ireland, Galway, a high-resolution image of the retina was obtained after correction of the refractive error. Use of this system for imaging at the cellular level would require additional changes. This work can be found on NUIG Library ARAN Repository at link : http://hdl.handle.net/10379/2995
... This simple consideration is at the basis of the spatial filtering that has been also used in the implementation of the spatially filtered Shack-Hartmann WFS. In that device (see also Poyneer & Machintosh 2004;Verinaud et al. 2005), a physical stop cancels out, at the level of the focal plane, spatial frequencies over the pupil above a certain limit, such that these cannot contribute to any aliasing into the sensing of lower modes. In a pyramid WFS, when the modulation is absent or negligible (a choice well tested on the sky, for example, with the MultiConjugated Adaptive Optics Demonstrator (MAD), on board the Very Large Telescope (VLT); see, for instance, Marchetti et al. 2004), a simplified model of the illumination of the pupil planes turns out to be useful in the context of this work. ...
Article
A modification of the pyramid wavefront sensor is described. In this conceptually new class of devices, the perturbations are split at the level of the focal plane depending upon their spatial frequencies, and then measured separately. The aim of this approach is to increase the accuracy in the determination of some range of spatial frequency perturbations, or a certain classes of modes, disentangling them from the noise associated to the Poissonian fluctuations of the light coming from the perturbations outside of the range of interest or from the background in the pupil planes; the latter case specifically when the pyramid wavefront sensor is used with a large modulation. While the limits and the effectiveness of this approach should be further investigated, a number of variations on the concept are shown, including a generalization of the spatial filtering in the point-diffraction wavefront sensor. The simplest application, a generalization to the pyramid of the well-known spatially filtering in wavefront sensing, is showing promise as a significant limiting magnitude advance. Applications are further speculated in the area of extreme adaptive optics and when serving spectroscopic instrumentation where "light in the bucket" rather than Strehl performance is required.
... Of particular interest for the analysis carried out here are cases where the use of an optical spatial filter [14] is limited, such as in cases where the size of the sub-apertures compared to the turbulence coherent length makes it hard to reach a suitable trade-off between aliasing rejection (small spatial filter width) and robustness to changing seeing conditions (large spatial filter width) [15]. In such cases a reconstruction process which can compensate for predicted aliasing errors is highly desirable, particularly in the case of high contrast imaging. ...
Article
Full-text available
We build on a long-standing tradition in astronomical adaptive optics (AO) of specifying performance metrics and error budgets using linear systems modeling in the spatial-frequency domain. Our goal is to provide a comprehensive tool for the calculation of error budgets in terms of residual temporally filtered phase power spectral densities and variances. In addition, the fast simulation of AO-corrected point spread functions (PSFs) provided by this method can be used as inputs for simulations of science observations with next-generation instruments and telescopes, in particular to predict post-coronagraphic contrast improvements for planet finder systems. We extend the previous results and propose the synthesis of a distributed Kalman filter to mitigate both aniso-servo-lag and aliasing errors whilst minimizing the overall residual variance. We discuss applications to (i) analytic AO-corrected PSF modeling in the spatial-frequency domain, (ii) post-coronagraphic contrast enhancement, (iii) filter optimization for real-time wavefront reconstruction, and (iv) PSF reconstruction from system telemetry. Under perfect knowledge of wind velocities, we show that $\sim$60 nm rms error reduction can be achieved with the distributed Kalman filter embodying anti- aliasing reconstructors on 10 m class high-order AO systems, leading to contrast improvement factors of up to three orders of magnitude at few ${\lambda}/D$ separations ($\sim1-5{\lambda}/D$) for a 0 magnitude star and reaching close to one order of magnitude for a 12 magnitude star.
... The reference phase constant for the plane wave component is expressed by φ o (k). Equivalently, each plane wave component can be treated as a sub-Fourier spectrum (or "Fourier sub-spectrum") of the incident wave with an amplitude envelope (slowly varying over space) [30]. ...
Article
Full-text available
The plenoptic sensor has been developed to sample complicated beam distortions produced by turbulence in the low atmosphere (deep turbulence or strong turbulence) with high density data samples. In contrast with the conventional Shack-Hartmann wavefront sensor, which utilizes all the pixels under each lenslet of a micro-lens array (MLA) to obtain one data sample indicating sub-aperture phase gradient and photon intensity, the plenoptic sensor uses each illuminated pixel (with significant pixel value) under each MLA lenslet as a data point for local phase gradient and intensity. To characterize the working principle of a plenoptic sensor, we propose the concept of plenoptic mapping and its inverse mapping to describe the imaging and reconstruction process respectively. As a result, we show that the plenoptic mapping is an efficient method to image and reconstruct the complex field amplitude of an incident beam with just one image. With a proof of concept experiment, we show that adaptive optics (AO) phase correction can be instantaneously achieved without going through a phase reconstruction process under the concept of plenoptic mapping. The plenoptic mapping technology has high potential for applications in imaging, free space optical (FSO) communication and directed energy (DE) where atmospheric turbulence distortion needs to be compensated.
... The reference phase constant for the plane wave component is expressed by φ o (k). Equivalently, each plane wave component can be treated as a sub-Fourier spectrum (or "Fourier sub-spectrum") of the incident wave with an amplitude envelope (slowly varying over space) [30]. ...
Code
Full-text available
This is a simulation code for imaging and reconstructing the complex field amplitude of a laser beam with a plenoptic sensor. The main function is "code4public.m".
... This term of error can be reduced especially by spatial filtering of high frequencies before measurement by the WFS [Poyneer and Macintosh (2004), Fusco et al. (2005)]. The term σ 2 alias is associated to this error sources. ...
Article
Refined simulation tools for wide field AO systems (such as MOAO, MCAO or LTAO) on ELTs present new challenges. Increasing the number of degrees of freedom (scales as the square of the telescope diameter) makes the standard simulation's codes useless due to the huge number of operations to be performed at each step of the Adaptive Optics (AO) loop process. This computational burden requires new approaches in the computation of the DM voltages from WFS data. The classical matrix inversion and the matrix vector multiplication have to be replaced by a cleverer iterative resolution of the Least Square or Minimum Mean Square Error criterion (based on sparse matrices approaches). Moreover, for this new generation of AO systems, concepts themselves will become more complex: data fusion coming from multiple Laser and Natural Guide Stars (LGS \ NGS) will have to be optimized, mirrors covering all the field of view associated to dedicated mirrors inside the scientific instrument itself will have to be coupled using split or integrated tomography schemes, differential pupil or/and field rotations will have to be considered, etc. All these new entries should be carefully simulated, analysed and quantified in terms of performance before any implementation in AO systems. For those reasons i developed, in collaboration with the ONERA, a full simulation code, based on iterative solution of linear systems with many parameters (use of sparse matrices). On this basis, I introduced new concepts of filtering and data fusion (LGS / NGS) to effectively manage modes such as tip, tilt and defoc in the entire process of tomographic reconstruction. The code will also eventually help to develop and test complex control laws (Multi-DM and multi-field) who have to manage a combination of adaptive telescope and post-focal instrument including dedicated deformable mirrors.\\ The first application of this simulation tool has been studied in the framework of the EAGLE multi-object spectrograph project, one of the main instrument of the future E-ELT, which, in terms of adaptive optics combine all of these issues.
... Considering the orders of magnitude involved here (especially pupil motion sensor integration time), the previous analysis has shown that the residual uncorrected turbulence effects are negligible in the global pupil motion sensor error budget. In addition, one can note that the quantification presented here is probably pessimistics since the SPHERE system will integrate a spatial filter device in front of its wave front sensor [17]. This device will cancel out all the high spatial frequencies before the wave front sensor measurement. ...
Article
In the last decade, the field of the search and study of cold companions around solar neighborhood stars has greatly improved. The study of low mass stellar companions and brown dwarf companions brings important constraints in the understanding of stellar formation. The study of planetary systems allows us to understand the formation of our own solar system. Up to now, most systems were discovered with the radial velocities technique. This technique is an indirect method that prevents from analysing the photons of a cold companion. The next step consists in detecting and analysing the photons of such companions. Many projects of high contrast imaging instruments have been recently developed in order to fulfil this task. This thesis was inspired by the scientific preparation and the development of such a project: the SPHERE instrument that will be installed at the Very Large Telescope in Chile in 2012. In the first part, I introduce the astrophysical questions that motivate the search for star companions and describe the two observation techniques that I used during my PhD: high contrast imaging and radial velocities techniques. A state of art about the status of the search for low mass stellar companions, brown dwarf companions and planetary companions is also done. In the second part, I describe the observation, data reduction and analysis techniques I used. The third part presents a study on the brown dwarf desert around around solar type stars that were selected from a sample of stars with radial velocity drift. In the fourth part, I develop an observation work that consists in detecting the planetary or brown dwarf companions around red dwarfs. The last part is devoted to the description of the SPHERE instrument and to my own contribution into this instrumental project.
... la XAO). Pour minimiser cette erreur, on peut recourirà des méthodes optiques (Poyneer et Macintosh, 2004;Fusco et al., 2005), ou utiliser une fréquence de coupure ASO plus grande que celle du DM si la magnitude de la source le permet. Nous proposons ici une méthode originale qui consiste simplementà inclure la connaissance a priori de cette erreur dans le reconstructeur MMSE (voir aussi Fales et al. (1988) qui propose une méthode similaire en déconvolution d'image). ...
Article
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The first part of this manuscript is devoted to the study of the formation and evolution of galaxies. I will present the results obtained with the GIRAFFE instrument as part of the IMAGES observational large program. These results show that the fraction of kinematically unrelaxed galaxies at z~0.6 is important. In addition, the detailed morphological analysis of the IMAGES sample shows that the fraction of rotating spiral galaxies were about twice as low at z~0.6 compared to the local Universe. By studying in more detail this population, we are able for the first time to explain the formation of a portion of local spiral galaxies. In the coming years, these studies will be extended to galaxies at z>> 1 thanks to the implementation of the future ELTs. To that end, these giant telescopes will have to integrate Adaptive Optics systems to compensate in real-time the degradation induced by atmospheric turbulence. Due to the low luminosity of distant galaxies, combined with the size of the corrected fields, these AO systems require artificial sources and tomography. The second part of this thesis is therefore devoted to the study of tomographic AO systems for the future ELTs. This work is particularly focused on the study and analysis of the fundamental limitations of the tomographic techniques. Based on theoretical developments in a Fourier basis, we give several ways to optimise these future systems.
... Technology wise, several kinds of WFSs and DMs are currently used in AO. As for WFSs, common choices include Shack-Hartmann, pyramid, and curvature sensors [1], [2]. As for DMs, a main distinction can be made among segmented continuous facesheet and bimorph mirrors [3]- [6]. ...
Article
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Deformable mirrors (DMs) are electromechanical devices used in ground-based telescopes to compensate for the distortions caused by the atmospheric turbulence, the main factor limiting the resolution of astronomical imaging. Adaptive secondary mirrors (ASMs) represent a new type of DMs; two of them have been recently installed on the 8-m-class large binocular telescope (LBT). ASMs are able to jointly correct rigid and nonrigid wave-front distortions thanks to the use of force actuators distributed on the overall mirror surface. As an offset, each actuator needs to be piloted by a dedicated controller, whose parameters must be accurately tuned to obtain the desired mirror shape. At the present time, the calibration of the controller parameters is executed manually. This paper presents a novel automatic controller tuning procedure that does not rely on the modeling of the mirror dynamics. The experimental validation on a prototype reproducing the three innermost rings of the LBT ASM is reported.
... We based the wave-front sensing on a filtered Shack-Hartmann (SH) approach, where we use a state-of-the-art electron multiplying CCD (EMCCD) 10 that can operate at up to 1200Hz with less than 0.1e of equivalent readout noise. We designed the filtering pinhole to remove the aliasing effect 11,12 and to be adjusted as a function of atmospheric conditions. The high-performance characteristics of the EMCCD, combined with advanced centroiding techniques (e.g., weighted center of gravity) 13 allow us to achieve impressive limit magnitudes with SAXO. ...
... The AO system is composed of a low spatial frequency, high stroke, 11x11 actuator deformable mirror, and a 64x64 Microelectromechanical System (MEMS) deformable mirror from Boston Micromachines (BMC). The AO system is innovative in that it includes a spatial filter to prevent aliasing and produce a s q u a r e , dark region very close to the star [13] . This paper presents the integral field spectrograph (IFS). ...
Article
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The Gemini Planet Imager (GPI) is a complex optical system designed to directly detect the self-emission of young planets within two arcseconds of their host stars. After suppressing the starlight with an advanced AO system and apodized coronagraph, the dominant residual contamination in the focal plane are speckles from the atmosphere and optical surfaces. Since speckles are diffractive in nature their positions in the field are strongly wavelength dependent, while an actual companion planet will remain at fixed separation. By comparing multiple images at different wavelengths taken simultaneously, we can freeze the speckle pattern and extract the planet light adding an order of magnitude of contrast. To achieve a bandpass of 20%, sufficient to perform speckle suppression, and to observe the entire two arcsecond field of view at diffraction limited sampling, we designed and built an integral field spectrograph with extremely low wavefront error and almost no chromatic aberration. The spectrograph is fully cryogenic and operates in the wavelength range 1 to 2.4 microns with five selectable filters. A prism is used to produce a spectral resolution of 45 in the primary detection band and maintain high throughput. Based on the OSIRIS spectrograph at Keck, we selected to use a lenslet-based spectrograph to achieve an rms wavefront error of approximately 25 nm. Over 36,000 spectra are taken simultaneously and reassembled into image cubes that have roughly 192x192 spatial elements and contain between 11 and 20 spectral channels. The primary dispersion prism can be replaced with a Wollaston prism for dual polarization measurements. The spectrograph also has a pupil-viewing mode for alignment and calibration.
... To prevent the aliasing error, we has proposed what we termed the spatially-filtered wavefront sensor (SFWFS). 26 For a Shack-Hartmann sensor, this is implemented as a hard-edged field stop in a focal plane of the WFS before the lenslet array. In the case of a high-strehl system, the filter will reject phase errors that scatter light beyond the size of the field stop. ...
Article
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The Gemini Planet Imager instrument's adaptive optics (AO) subsystem was designed specifically to facilitate high-contrast imaging. It features several new technologies, including computationally efficient wavefront reconstruction with the Fourier transform, modal gain optimization every 8 seconds, and the spatially filtered wavefront sensor. It also uses a Linear-Quadratic-Gaussian (LQG) controller (aka Kalman filter) for both pointing and focus. We present on-sky performance results from verification and commissioning runs from December 2013 through May 2014. The efficient reconstruction and modal gain optimization are working as designed. The LQG controllers effectively notch out vibrations. The spatial filter can remove aliases, but we typically use it oversized by about 60% due to stability problems.
... Such sensors are susceptible to aliasing of errors outside the Nyquist range, which can inject significant midfrequency wavefront errors into the final image. A spatial filter (27) was installed to mitigate this. The spatial filter is adjustable with size set by the Nyquist criterion. ...
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Significance Direct detection—spatially resolving the light of a planet from the light of its parent star—is an important technique for characterizing exoplanets. It allows observations of giant exoplanets in locations like those in our solar system, inaccessible by other methods. The Gemini Planet Imager (GPI) is a new instrument for the Gemini South telescope. Designed and optimized only for high-contrast imaging, it incorporates advanced adaptive optics, diffraction control, a near-infrared spectrograph, and an imaging polarimeter. During first-light scientific observations in November 2013, GPI achieved contrast performance that is an order of magnitude better than conventional adaptive optics imagers.
... Such sensors are susceptible to aliasing of errors outside the Nyquist range, which can inject significant midfrequency wavefront errors into the final image. A spatial filter [26] was installed to mitigate this. The spatial filter is adjustable with size set by the Nyquist criterion. ...
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The Gemini Planet Imager (GPI) is a dedicated facility for directly imaging and spectroscopically characterizing extrasolar planets. It combines a very high-order adaptive optics system, a diffraction-suppressing coronagraph, and an integral field spectrograph with low spectral resolution but high spatial resolution. Every aspect of GPI has been tuned for maximum sensitivity to faint planets near bright stars. During first light observations, we achieved an estimated H band Strehl ratio of 0.89 and a 5-sigma contrast of $10^6$ at 0.75 arcseconds and $10^5$ at 0.35 arcseconds. Observations of Beta Pictoris clearly detect the planet, Beta Pictoris b, in a single 60-second exposure with minimal post-processing. Beta Pictoris b is observed at a separation of $434 \pm 6$ milli-arcseconds and position angle $211.8 \pm 0.5$ deg. Fitting the Keplerian orbit of Beta Pic b using the new position together with previous astrometry gives a factor of three improvement in most parameters over previous solutions. The planet orbits at a semi-major axis of $9.0^{+0.8}_{-0.4}$ AU near the 3:2 resonance with the previously-known 6 AU asteroidal belt and is aligned with the inner warped disk. The observations give a 4% posterior probability of a transit of the planet in late 2017.
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Context. The direct imaging of potentially habitable exoplanets is one prime science case for the next generation of high contrast imaging instruments on ground-based, extremely large telescopes. To reach this demanding science goal, the instruments are equipped with eXtreme Adaptive Optics (XAO) systems which will control thousands of actuators at a framerate of kilohertz to several kilohertz. Most of the habitable exoplanets are located at small angular separations from their host stars, where the current control laws of XAO systems leave strong residuals. Aims. Current AO control strategies such as static matrix-based wavefront reconstruction and integrator control suffer from a temporal delay error and are sensitive to mis-registration, that is, to dynamic variations of the control system geometry. We aim to produce control methods that cope with these limitations, provide a significantly improved AO correction, and, therefore, reduce the residual flux in the coronagraphic point spread function (PSF). Methods. We extend previous work in reinforcement learning for AO. The improved method, called the Policy Optimization for Adaptive Optics (PO4AO), learns a dynamics model and optimizes a control neural network, called a policy. We introduce the method and study it through numerical simulations of XAO with Pyramid wavefront sensor (PWFS) for the 8-m and 40-m telescope aperture cases. We further implemented PO4AO and carried out experiments in a laboratory environment using Magellan Adaptive Optics eXtreme system (MagAO-X) at the Steward laboratory. Results. PO4AO provides the desired performance by improving the coronagraphic contrast in numerical simulations by factors of 3–5 within the control region of deformable mirror and PWFS, both in simulation and in the laboratory. The presented method is also quick to train, that is, on timescales of typically 5–10 s, and the inference time is sufficiently small (<ms) to be used in real-time control for XAO with currently available hardware even for extremely large telescopes.
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Conference Paper
High dynamic range coronagraphy targeted at discovering planets around nearby stars is often associated with monolithic, unobstructed aperture space telescopes. With the advent of extreme adaptive optics (ExAO) systems with thousands of sensing and correcting channels, the benefits of placing a near-infrared coronagraph on a large segmented mirror telescope become scientifically interesting. This is because increased aperture size produces a tremendous gain in achievable contrast at the same angular distance from a point source at Strehl ratios in excess of 90\% (and at lower Strehl ratios on future giant telescopes such as the Thirty Meter Telescope). We outline some of the design issues facing such a coronagraph, and model a band-limited coronagraph on an aperture with a Keck-like pupil. We examine the purely diffractive challenges facing the eXtreme AO Planetary Imager (XAOPI) given the Keck pupil geometry, notably its inter-segment gap spacing of 6~mm. Classical Lyot coronagraphs, with hard-edged occulting stops, are not efficient enough at suppressing diffracted light, given XAOPI's scientific goal of imaging a young Jupiter at a separation as close as 0.15 arcseconds (4λD at H on Keck) from its parent star. With a 4000 channel ExAO system using an anti-aliased spatially-filtered wavefront sensor planned for XAOPI, we wish to keep diffracted light due to coronagraphic design at least as low as the noise floor set by AO system limitations. We study the band-limited Lyot coronagraph (BLC) as a baseline design instead of the classical design because of its efficient light suppression, as well as its analytical simplicity. We also develop ways of investigating tolerancing coronagraphic mask fabrication by utilizing the BLC design's mathematical tractability.
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The Gemini Planet Imager Exoplanet Survey (GPIES) is a direct imaging campaign designed to search for young, self-luminous, giant exoplanets. To date, GPIES has observed nearly 500 targets, and generated over 30,000 individual exposures using its integral field spectrograph (IFS) instrument. The GPIES team has developed a campaign data system with a database incorporating all of the metadata for all individual raw data products, including environmental conditions and instrument performance metrics. The same database also indexes metadata associated with multiple levels of reduced data products, including contrast measures for individual images and combined image sequences, which serve as the primary metric of performance for the final science products. The database is also used to track telemetry products from the adaptive optics subsystem, and associate these with corresponding IFS data. Here, we discuss several data exploration and visualization projects enabled by the GPIES database. Of particular interest are any correlations between instrument performance and environmental or operating conditions. We show single and multiple-parameter fits of single-image and observing sequence contrast as functions of various seeing measures, and discuss automated outlier rejection and other fitting concerns. Supervised learning techniques are employed in order to partition the space of raw (single image) to final (full sequence) contrast in order to better predict the value of the final data set from the first few completed observations. Finally, we discuss the particular features of the database design that aid in performing these analyses, and suggest potential future upgrades and refinements.
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Time-series photometry taken from ground-based facilities is improved with the use of comparison stars due to the short timescales of atmospheric-induced variability. However, the sky is bright in the thermal infrared (3-5 um), and the correspondingly small fields-of-view of available detectors make it highly unusual to have a calibration star in the same field as a science target. Here we present a new method of obtaining differential photometry by simultaneously imaging a science target and a calibrator star, separated by <2 amin, onto a 10x10 asec field-of-view detector. We do this by taking advantage of the LBT's unique binocular design to point the two co-mounted telescopes apart and simultaneously obtain both targets in three sets of observations. Results indicate that the achievable scatter in Ls-band (3.3 um) is at the percent level for bright targets, and possibly better with heavier sampling and characterization of the systematics.
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The improvement of the optical devices in this decade, such as the MEMS-SLM ( Micro Electro Mechanical Systems- Spatial Light Modulator ) and wave front sensor with micro lens device, is making adaptive optics commonly available. It also gives the new basis of the design of adaptive optics with the improved accuracy and the compactness. We have developed an adaptive optics bench from such a point of view, and the application to the optical microscope has attained effective results in the observation of the live cell samples. In this presentation, our recent results will be shown. The result includes analysis of blur by the fine structures in biological sample and result of the image correction by the adaptive optics.
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Current wavefront sensors for high resolution imaging have either a large dynamic range or a high sensitivity. A new kind of wavefront sensor is developed which can have both: the Generalised Optical Differentiation wavefront sensor. This new wavefront sensor is based on the principles of optical differentiation by amplitude filters. We have extended the theory behind linear optical differentiation and generalised it to nonlinear filters. We used numerical simulations and laboratory experiments to investigate the properties of the generalised wavefront sensor. With this we created a new filter that can decouple the dynamic range from the sensitivity. These properties make it suitable for adaptive optic systems where a large range of phase aberrations have to be measured with high precision.
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The SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) instrument aims at detecting extremely faint sources (giant extrasolar planets) in the vicinity of bright stars¹. Such a challenging goal requires the use of a very-high-order performance Adaptive Optics [AO] system feeding the scientific instruments with a quasi-perfect flat wave front corrected from all the atmospheric turbulence and internal defects. This AO system, called SAXO (Sphere Ao for eXoplanet Observation) is the heart of the instrument, a heart beating 1200 time per second and providing unprecedented image quality for a large ground based telescope at optical/near infrared wavelength. We will present the latest results obtained on-sky, demonstrating its exceptional performance (in terms of correction quality, stability and robustness) and tremendous potentiality for high contrast imaging and more specifically for exoplanet discovery.
Conference Paper
The Gemini Planet Imager is a high-contrast near-infrared instrument specifically designed to image exoplanets and circumstellar disks over a narrow field of view. We use science data and AO telemetry taken during the first 1.5 yr of the GPI Exoplanet Survey to quantify the performance of the AO system. In a typical 60 sec H-band exposure, GPI achieves a 5σ raw contrast of 10⁻⁴ at 0.4"; typical final 5σ contrasts for full 1 hr sequences are more than 10 times better than raw contrasts. We find that contrast is limited by bandwidth wavefront error over much of the PSF. Preliminary exploratory factor analysis can explain 60{70% of the variance in raw contrasts with combinations of seeing and wavefront error metrics. We also examine the effect of higher loop gains on contrast by comparing wavefront error maps reconstructed from AO telemetry to concurrent IFS images. These results point to several ways that GPI performance could be improved in software or hardware.
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The Gemini Planet Imager's adaptive optics (AO) subsystem was designed specifically to facilitate high-contrast imaging. A definitive description of the system's algorithms and technologies as built is given. 564 AO telemetry measurements from the Gemini Planet Imager Exoplanet Survey campaign are analyzed. The modal gain optimizer tracks changes in atmospheric conditions. Science observations show that image quality can be improved with the use of both the spatially filtered wavefront sensor and linear-quadratic-Gaussian control of vibration. The error budget indicates that for all targets and atmospheric conditions AO bandwidth error is the largest term.
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Laser assisted adaptive optics systems rely on Laser Guide Star (LGS) Wave-Front Sensors (WFS) for high order aberration measurements, and rely on Natural Guide Stars (NGS) WFS to complement the measurements on low orders such as tip-tilt and focus. The sky-coverage of the whole system is therefore related to the limiting magnitude of the NGS WFS. We have recently proposed LIFT, a novel phase retrieval WFS technique, that allows a 1 magnitude gain over the usually used 2×2 Shack-Hartmann WFS. After an in-lab validation, LIFT's concept has been demonstrated on sky in open loop on GeMS (the Gemini Multiconjugate adaptive optics System at Gemini South). To complete its validation, LIFT now needs to be operated in closed loop in a laser assisted adaptive optics system. The present work gives a detailed analysis of LIFT's behavior in presence of high order residuals and how to limit aliasing effects on the tip/tilt/focus estimation. Also, we study the high orders' impact on noise propagation. For this purpose, we simulate a multiconjugate adaptive optics loop representative of a GeMS-like 5 LGS configuration. The residual high orders are derived from a Fourier based simulation. We demonstrate that LIFT keeps a high performance gain over the Shack-Hartmann 2×2 whatever the turbulence conditions. Finally, we show the first simulation of a closed loop with LIFT estimating turbulent tip/tilt and focus residuals that could be induced by sodium layer's altitude variations.
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Exoplanet direct imaging with large ground based telescopes requires eXtreme Adaptive Optics that couples high-order adaptive optics and coronagraphy. A key element of such systems is the high-order wavefront sensor. We study here several high-order wavefront sensing approaches, and more precisely compare their sensitivity to noise. Three techniques are considered: the classical Shack-Hartmann sensor, the pyramid sensor and the recently proposed LIFTed Shack-Hartmann sensor. They are compared in a unified framework based on precise diffractive models and on the Fisher information matrix, which conveys the information present in the data whatever the estimation method. The diagonal elements of the inverse of the Fisher information matrix, which we use as a figure of merit, are similar to noise propagation coefficients. With these diagonal elements, so called "Fisher coefficients", we show that the LIFTed Shack-Hartmann and pyramid sensors outperform the classical Shack-Hartmann sensor. In photon noise regime, the LIFTed Shack-Hartmann and modulated pyramid sensors obtain a similar overall noise propagation. The LIFTed Shack-Hartmann sensor however provides attractive noise properties on high orders.
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The effects of photon noise, aliasing, wave front chromaticity, and scintillation on the point-spread function (PSF) contrast achievable with ground-based adaptive optics (AO) are evaluated for different wave front sensing schemes. I show that a wave front sensor (WFS) based on the Zernike phase contrast technique offers the best sensitivity to photon noise at all spatial frequencies, while the Shack-Hartmann WFS is significantly less sensitive. In AO systems performing wave front sensing in the visible and scientific imaging in the near-IR, the PSF contrast limit is set by the scintillation chromaticity induced by Fresnel propagation through the atmosphere. On an 8 m telescope, the PSF contrast is then limited to 10-4 to 10-5 in the central arcsecond. Wave front sensing and scientific imaging should therefore be done at the same wavelength, in which case, on bright sources, PSF contrasts between 10-6 and 10-7 can be achieved within 1'' on an 8 m telescope in optical/near-IR. The impact of atmospheric turbulence parameters (seeing, wind speed, turbulence profile) on the PSF contrast is quantified. I show that a focal plane wave front sensing scheme offers unique advantages, and I discuss how to implement it. Coronagraphic options are also briefly discussed.
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The Laboratory for Adaptive Optics (LAO) has completed the third year of its six-year program to develop adaptive optics technology, concepts, and instruments for astronomy.
Conference Paper
The SPHERE (Spectro-Polarimetry High-contrast Exoplanet Research) instrument is an ESO project aiming at the direct detection of extra-solar planets. It should equip one of the four VLT 8-m telescopes in 2010. The heart of the SPHERE instrument is its eXtrem Adaptive Optics (XAO) SAXO (SPHERE AO for eXoplanet Observation) subsystem that should deal with a tight error budget. To fulfil SAXO challenging requirements a mixed control law has been designed. It includes both an optimized modal gain integrator to control the Deformable Mirror (DM) and a Linear Quadratic Gaussian (LQG) control law to manage the tip-tilt (TT) mirror and filter possible vibrations. A specific scheme has been developed to optimize the correction provided by the DM and the TT while minimizing the coupling between both control loops. Actuator saturation and wind-up effects management are described. We describe the overall control architecture and focus on these main issues. We present expectable performance and also consider the interactions of the main control loop with other subsystems. PUBLISHER'S NOTE Sept. 9, 2010: Due to a production error, SPIE Paper 70151U was inadvertently published also as SPIE Paper 70151D. This has been corrected. This record contains the correct citation, abstract, and manuscript for paper 70151D.
Conference Paper
The Canary Hosted-Upgrade for High-Order Adaptive Optics, or CHOUGH, is an upgrade for the Canary Tomographic AO experiment. It aims to enable a high-order 30×30, single-conjugate AO capability on a 4m telescope. It utilizes a Shack-Hartmann WFS with a spatial filter for measurements together with a MEMS-DM and a magnetically actuated DM in tandem to provide the correction (dual-DM architecture). For analysis of the residuals from the correction, there are two separate instruments: a conventional imager operating in the visible part of the spectrum (V- to I-band), and an interferometer that directly measures the phase. At present the system is in the design stage and this paper reports progress towards developing a system that is capable of delivering the goals on-sky.
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VMM(vector-matrix-multiply) method was widely used for wave-front reconstruction in Shack-Hartmann wave-front sensor. Taking advantage of great number of calculations, VMM can give accurate enough result for most of cases, but it can not work properly in scenes which have strict requirement of frequency in time or special domain. In this paper, wave-front is intended to be reconstructed by FTR(Fourier transform reconstruction) method. A common software routine was designed and implemented based on Shack-Hartmann sensor model and FTR algorithms. With the help of these codes, the time analysis of FTR was obtained by software tests. Two kinds of data set were used for the study of accuracy performance, one was ideal slopes derived for known wave-front, the other was slopes corrupted by random noise from actual experiment. Then wave-fronts reconstructed by VMM and FTR were compared and analyzed. Results demonstrate that FTR algorithm has the capability to yield unbiased reconstructions, and shares better time performance than VMM method. Due to extra steps to maintain periodical property of slope data, the wave-front difference between FTR and VMM appears more apparently in edge area. The study and analysis make the limited factors clear, and will guide the implement of FTR in Shack-Hartmann wave-front sensor and supply clues for the improvement in future works.
Conference Paper
The extreme AO system, SAXO (SPHERE AO for eXoplanet Observation), is the heart of the SPHERE system, feeding the scientific instruments with flat wave front corrected from all the atmospheric turbulence and internal defects. We will present the final performance of SAXO obtained during the instrument AIT in Europe as well as the very first on-sky results. The main requirements and system characteristics will be recalled and the full AO loop performance will be quantified and compared to original specifications. It will be demonstrated that SAXO meets or even exceeds (especially its limit magnitude and its jitter residuals) its challenging requirements (more than 90% of SR in H band and a 3 mas residual jitter). Finally, after 10 years of AO developments, from early design to final on-sky implementations, some critical system aspects as well as some important lesson-learned will be presented in the perspective of the future generation of complex AO systems for VLTs and ELTs.
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The Gemini Planet Imager (GPI) is a next-generation, facility instrument currently being commissioned at the Gemini South observatory. GPI combines an extreme adaptive optics system and integral field spectrograph (IFS) with an apodized-pupil Lyot coronagraph (APLC) producing an unprecedented capability for directly imaging and spectroscopically characterizing extrasolar planets. GPI's operating goal of $10^{-7}$ contrast requires very precise alignments between the various elements of the coronagraph (two pupil masks and one focal plane mask) and active control of the beam path throughout the instrument. Here, we describe the techniques used to automatically align GPI and maintain the alignment throughout the course of science observations. We discuss the particular challenges of maintaining precision alignments on a Cassegrain mounted instrument and strategies that we have developed that allow GPI to achieve high contrast even in poor seeing conditions.
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The Gemini Planet Imager (GPI) is an "extreme" adaptive optics coronagraph system that is now on the Gemini South telescope in Chile. This instrument is composed of three different systems that historically have been separate instruments. These systems are the extreme Adaptive Optics system, with deformable mirrors, including a high-order 64x64 element MEMS system; the Science Instrument, which is a near-infrared integral field spectrograph; and the Calibration system, a precision IR wavefront sensor that also holds key coronagraph components. Each system coordinates actions that require precise timing. The observatory is responsible for starting these actions and has typically done this asynchronously across independent systems. Despite this complexity we strived to provide an interface that is as close to a one-button approach as possible. This paper will describe the sequencing of these systems both internally and externally through the observatory.
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In the frame of the VLT Planet-Finder project a global system study has demonstrated the feasability of an AO system for the direct detection of exoplanets. The main results of this design study are presented.
Conference Paper
This paper introduces a unified formulism to describe many of the high contrast correction methods, namely, phase conjugation, classical speckle nulling and energy minimization. This unified formalism led to the Electric Field Conjugation (EFC) algorithm.
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To achieve high resolution imaging the standard control algorithm used for classical adaptive optics (AO) is the simple but efficient proportional-integral (PI) controller. The goal is to minimize the root mean square (RMS) error of the residual wave front. However, with the PI controller one does not reach this minimum. A possibility to achieve is to use Linear Quadratic Gaussian Control (LQG). In practice, however this control algorithm still encounters one unexpected problem, leading to the divergence of control in AO. In this paper we propose a Modified LQG (MLQG) to solve this issue. The controller is analyzed explicitly. Test in the lab shows strong stability and high precision compared to the classical control.
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Ground based adaptive optics is a potentially powerful technique for direct imaging detection of extrasolar planets. Turbulence in the Earth's atmosphere imposes some fundamental limits, but the large size of ground-based telescopes compared to spacecraft can work to mitigate this. We are carrying out a design study for a dedicated ultra-high-contrast system, the eXtreme Adaptive Optics Planet Imager (XAOPI), which could be deployed on an 8-10m telescope in 2007. With a 4096-actuator MEMS deformable mirror it should achieve Strehl >0.9 in the near-IR. Using an innovative spatially filtered wavefront sensor, the system will be optimized to control scattered light over a large radius and suppress artifacts caused by static errors. We predict that it will achieve contrast levels of 107-108 at angular separations of 0.2-0.8" around a large sample of stars (R
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We summarize the theory of coronagraphic optics and identify a dimensionless fine-tuning parameter, , which we use to describe the Lyot stop size in the natural units of the coronagraphic optical train and the observing wavelength. We then present simulations of coronagraphs matched to adaptive optics (AO) systems on the Calypso 1.2 m, Palomar Hale 5 m, and Gemini 8 m telescopes under various atmospheric conditions and identify useful parameter ranges for AO coronagraphy on these telescopes. Our simulations employ a tapered, high-pass filter in spatial frequency space to mimic the action of adaptive wave front correction. We test the validity of this representation of AO correction by comparing our simulations with recent K-band data from the 241 channel Palomar Hale AO system and its dedicated Palomar High Angular Resolution Observer (PHARO) science camera in coronagraphic mode. Our choice of monochromatic modeling enables us to distinguish between underlying halo suppression and bright Airy ring suppression in the final coronagraphic images. For a given telescope-AO system combination, we find that AO systems delivering images with Strehl ratios below a threshold value are not well suited to diffraction-limited coronagraphs. When Strehl ratios are above this threshold, an optimized coronagraph with occulting image plane stops as small as 4λ/D creates a region around the AO target where dynamic range is significantly enhanced.
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We describe the symmetries present in the point-spread function (PSF) of an optical system either located in space or corrected by an adaptive o to Strehl ratios of about 70% and higher. We present a formalism for expanding the PSF to arbitrary order in terms of powers of the Fourier transform of the residual phase error, over an arbitrarily shaped and apodized entrance aperture. For traditional unapodized apertures at high Strehl ratios, bright speckles pinned to the bright Airy rings are part of an antisymmetric perturbation of the perfect PSF, arising from the term that is first order in the residual phase error. There are two symmetric second degree terms. One is negative at the center, and, like the first order term, is modulated by the perfect image's field strength -- it reduces to the Marechal approximation at the center of the PSF. The other is non-negative everywhere, zero at the image center, and can be responsible for an extended halo -- which limits the dynamic range of faint companion detection in the darkest portions of the image. In regimes where one or the other term dominates the speckles in an image, the symmetry of the dominant term can be exploited to reduce the effect of those speckles, potentially by an order of magnitude or more. We demonstrate the effects of both secondary obscuration and pupil apodization on the structure of residual speckles, and discuss how these symmetries can be exploited by appropriate telescope and instrument design, observing strategies, and filter bandwidths to improve the dynamic range of high dynamic range AO and space-based observations. Finally, we show that our analysis is relevant to high dynamic range coronagraphy. Comment: Accepted for publication in ApJ; 20 pages, 4 figures
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The useful dynamic range of an image in the diffraction limited regime is usually limited by speckles caused by residual phase errors in the optical system forming the image. The technique of speckle decorrelation involves introducing many independent realizations of additional phase error into a wavefront during one speckle lifetime, changing the instantaneous speckle pattern. A commonly held assumption is that this results in the speckles being `moved around' at the rate at which the additional phase screens are applied. The intention of this exercise is to smooth the speckles out into a more uniform background distribution during their persistence time, thereby enabling companion detection around bright stars to be photon noise limited rather than speckle-limited. We demonstrate analytically why this does not occur, and confirm this result with numerical simulations. We show that the original speckles must persist, and that the technique of speckle decorrelation merely adds more noise to the original speckle noise, thereby degrading the dynamic range of the image.
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Standard FFT-based phase screen generation methods do not accurately model low-frequency turbulence characteristics. This paper introduces a new phase screen generation technique which uses low frequency subharmonic information to correct the problem. We compare our technique to two other subharmonic methods. The structure functions for this new method match very closely the structure functions of Kolmogorov turbulence theory.
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Direct detection of photons emitted or reflected by an extrasolar planet is an extremely difficult but extremely exciting application of adaptive optics. Typical contrast levels for an extrasolar planet would be 10â¹-Jupiter is a billion times fainter than the sun. Current adaptive optics systems can only achieve contrast levels of 10â¶, but so-called ''extreme'' adaptive optics systems with 10â´-10âµ degrees of freedom could potentially detect extrasolar planets. We explore the scaling laws defining the performance of these systems, first set out by Angel (1994), and derive a different definition of an optimal system. Our sensitivity predictions are somewhat more pessimistic than the original paper, due largely to slow decorrelation timescales for some noise sources, though choosing to site an ExAO system at a location with exceptional râ (e.g. Mauna Kea) can offset this. We also explore the effects of segment aberrations in a Keck-like telescope on ExAO; although the effects are significant, they can be mitigated through Lyot coronagraphy.
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In this paper, we present a novel approach to modeling Shack-Hartmann-based adaptive optics (AO) system which allow to easily predict their performance. The idea is to start for the power spectral density (PSD) of the turbulent phase and to derive the PSD of the compensated phase by taking into account the various errors affecting the AO correction, which can be expressed analytically in the Fourier domain. Once the PSD of the compensated phase is known, the computation of the residual point spread function is straightforward.
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Direct detection of photons emitted or reflected by an extrasolar planet is an extremely difficult but extremely exciting application of adaptive optics. Typical contrast levels for an extrasolar planet would be 109 - Jupiter is a billion times fainter than the sun. Current adaptive optics systems can only achieve contrast levels of 106, but so-called extreme adaptive optics systems with 104 -105 degrees of freedom could potentially detect extrasolar planets. We explore the scaling laws defining the performance of these systems, first set out by Angel (1994), and derive a different definition of an optimal system. Our sensitivity predictions are somewhat more pessimistic than the original paper, due largely to slow decorrelation timescales for some noise sources, though choosing to site an ExAO system at a location with exceptional r0 (e.g. Mauna Kea) can offset this. We also explore the effects of segment aberrations in a Keck-like telescope on ExAO; although the effects are significant, they can be mitigated through Lyot coronagraphy.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
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This book by one of the leaders in adaptive optics covers the fundamental theory and then describes in detail how this technology can be applied to large ground-based telescopes to compensate for the effects of atmospheric turbulence. It includes information on basic adaptive optics components and technology, and has chapters devoted to atmospheric turbulence, optical image structure, laser beacons, and overall system design. The chapter on system design is particularly detailed and includes performance estimation and optimization. Combining a clear discussion of physical principles with numerous real-world examples, this book will be a valuable resource for all graduate students and researchers in astronomy and optics.
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To simulate nondiffraction-limited laser beams numerically, one needs to understand their nature better. For a phase aberration the correlation length of which is much less than the width of the beam, one can show that the far-field irradiance distribution can be written as the sum of two beams. One beam is the attenuated diffraction-limited beam; the other is a much wider beam the exact shape of which is closely related to the power spectrum of the phase fluctuations. Computer simulations of random phase aberrations are shown to agree well with the analytic predictions. Attempts at simulation of nondiffraction-limited beams for nonlinear wave propagation problems are discussed.
Article
We are currently investigating the possibilities for a high-contrast, adaptive optics assisted instrument to be placed as a 2nd-generation instrument on ESO's VLT. This instrument will consist of an 'extreme-ao' system capable of producing very high Strehl ratios, a contrast-enhancing device and two differential imaging detection systems. It will be designed to collect photons directly coming from the surface of substellar companions - ideally down to planetary masses - to bright, nearby stars and disentangle them from the stellar photons. We will present our current design study for such an instrument and discuss the various ways to tell stellar from companion photons. These ways include the use of polarimetric and/or spectroscopic information as well as making use of knowledge about photon statistics. Results of our latest simulations regarding the instrument will be presented and the expected performance discussed. Derived from the simulated performance we will also give details about the expected science impact of the planet finder. This will comprise the chances of finding different types of exo-planets - notably the dilemma of going for hot planets marginally separated from their parent stars or cold, far-away plamnets delivering very little radiation, the scientific return of such detections and follow-up examinations, as well as other topics like star-formation, debris disks, and planetary nebulae where a high-resolution, high-contrast system will trigger new break-throughs.
Article
Wave-front reconstruction with use of the Fourier transform has been validated through theory and simulation. This method provides a dramatic reduction in computational costs for large adaptive (AO) systems. Because such a reconstructor can be expressed as a matrix, it can be used as an alternative in a matrix-based AO control system. This was done with the Palomar Observatory AO system on the 200-in. Hale telescope. Results of these tests indicate that Fourier-transform wave-front reconstruction works in a real system. For both bright and dim stars, a Hudgin-geometry Fourier-transform method produced performance comparable to that of the Palomar Adaptive Optics least squares. The Fried-geometry method had a noticeable Strehl ratio performance degradation of 0.043 in the K band (165-nm rms wave-front error added in quadrature) on a dim star.
Article
In a previous paper, we described a successful technique, the broadband algorithm, for phasing the primary mirror segments of the Keck telescopes to an accuracy of 30 nm. Here we describe a complementary narrow-band algorithm. Although it has a limited dynamic range, it is much faster than the broadband algorithm and can achieve an unprecedented phasing accuracy of approximately 6 nm. Cross checks between these two independent techniques validate both methods to a high degree of confidence. Both algorithms converge to the edge-minimizing configuration of the segmented primary mirror, which is not the same as the overall wave-front-error-minimizing configuration, but we demonstrate that this distinction disappears as the segment aberrations are reduced to zero.
personal communication (agiveon@princeton.edu, Princeton University
• A Give
• J On
• Kasdin
A. Give'on and J. Kasdin, personal communication (agiveon@princeton.edu, Princeton University, Princeton, New Jersey, 2003). L. A. Poyneer and B. Macintosh Vol. 21, No. 5 / May 2004 / J. Opt. Soc. Am. A 819
personal communication (cjc@llnl.gov
• C Carrano
C. Carrano, personal communication (cjc@llnl.gov, Lawrence Livermore National Laboratory, Livermore, Cali-fornia, 2003).
personal communication (mtroy@jpl.nasa.gov
• M Troy
M. Troy, personal communication (mtroy@jpl.nasa.gov, Jet Propulsion Lab, Pasadena, California, 2003).
Extreme adaptive optics planet imager: XAOPI,'' Techniques and Instrumentation for Detection of Exoplanets
• A Severson
• A Sheinis
• M Sivaramakrishnan
• J K Troy
• Wallace
Severson, A. Sheinis, A. Sivaramakrishnan, M. Troy, and J. K. Wallace, ''Extreme adaptive optics planet imager: XAOPI,'' Techniques and Instrumentation for Detection of Exoplanets, D. R. Coulter, ed., Proc. SPIE 5170, 272–282 (2003).