194 research outputs found
Development of an ELT XAO testbed using a self referenced Mach-Zehnder wavefront sensor
Extreme adaptive optics (XAO) has severe difficulties meeting the high speed (>1kHz), accuracy and photon efficiency requirements for future extremely large telescopes. An innovative high order adaptive optics system using a self-referenced Mach-Zehnder wavefront sensor (MZWFS) allows counteracting these limitations. In addition to its very high accuracy, this WFS is the most robust alternative to segments gaps and telescope spiders which can result in strong wavefront artifacts. In particular in XAO systems when the size of these gaps in the wavefront measurement is comparable to the sub aperture size, loss in performance can be very high. The MZWFS estimates the wavefront phase by measuring intensity differences between two outputs, with a λ/4 path length difference between its two legs, but is limited in dynamic range. During the past few years, such an XAO system has been studied by our team in the framework of 8-meter class telescopes. In this paper, we report on our latest results with the XAO testbed recently installed in CRAL laboratory, and dedicated to high contrast imaging with 30m-class telescopes (such as the E-ELT or the TMT). A woofer-tweeter architecture is used in order to deliver the required high Strehl ratio (>95%). It consists of a 12x12 deformable mirror (DM) and a 512x512 Spatial Light Modulator (SLM) characterized both using monochromatic and polychromatic light. We present our latest experimental results, including components characterization, close loop performances and sensitivity to calibration errors. This work is carried out in synergy with the validation of fast iterative wavefront reconstruction algorithms and the optimal treatment of phase ambiguities in order to mitigate the dynamical range limitation of such a wavefront sensor
LITpro: a model fitting software for optical interferometry
9 pagesInternational audienceLITpro is a software for fitting models on data obtained from various stellar optical interferometers, like the VLTI. As a baseline, for modeling the object, it provides a set of elementary geometrical and center-to-limb darkening functions, all combinable together. But it is also designed to make very easy the implementation of more specific models with their own parameters, to be able to use models closer to astrophysical considerations. So LITpro only requires the modeling functions to compute the Fourier transform of the object at given spatial frequencies, and wavelengths and time if needed. From this, LITpro computes all the necessary quantities as needed (e.g. visibilities, spectral energy distribution, partial derivatives of the model, map of the object model). The fitting engine, especially designed for this kind of optimization, is based on a modified Levenberg-Marquardt algorithm and has been successfully tested on real data in a prototype version. It includes a Trust Region Method, minimizing a heterogeneous non-linear and non-convex criterion and allows the user to set boundaries on free parameters. From a robust local minimization algorithm and a starting points strategy, a global optimization solution is effectively achieved. Tools have been developped to help users to find the global minimum. LITpro is also designed for performing fitting on heterogeneous data. It will be shown, on an example, how it fits simultaneously interferometric data and spectral energy distribution, with some benefits on the reliability of the solution and a better estimation of errors and correlations on the parameters. That is indeed necessary since present interferometric data are generally multi-wavelengths
Fast minimum variance wavefront reconstruction for extremely large telescopes
We present a new algorithm, FRiM (FRactal Iterative Method), aiming at the
reconstruction of the optical wavefront from measurements provided by a
wavefront sensor. As our application is adaptive optics on extremely large
telescopes, our algorithm was designed with speed and best quality in mind. The
latter is achieved thanks to a regularization which enforces prior statistics.
To solve the regularized problem, we use the conjugate gradient method which
takes advantage of the sparsity of the wavefront sensor model matrix and avoids
the storage and inversion of a huge matrix. The prior covariance matrix is
however non-sparse and we derive a fractal approximation to the Karhunen-Loeve
basis thanks to which the regularization by Kolmogorov statistics can be
computed in O(N) operations, N being the number of phase samples to estimate.
Finally, we propose an effective preconditioning which also scales as O(N) and
yields the solution in 5-10 conjugate gradient iterations for any N. The
resulting algorithm is therefore O(N). As an example, for a 128 x 128
Shack-Hartmann wavefront sensor, FRiM appears to be more than 100 times faster
than the classical vector-matrix multiplication method.Comment: to appear in the Journal of the Optical Society of America
Simulations of Adaptive Optics Systems for the E-ELT
ABSTRACT In this paper, we present simulation work done on AO systems for the E-ELT. We study the influence of the number of Laser Guide Stars (LGS) on system performance. Then, we investigate the impact of the conjugation height of the M4 adaptive mirror on GL/LT/MC-AO. Finally, we compare the results of a Fourier code and end-to-end models on the position of the LGS in the field of view
Adaptive Optics system of the Evanescent Wave Coronagraph (EvWaCo): optimised phase plate and DM characterisation
The Evanescent Wave Coronagraph (EvWaCo) is an achromatic coronagraph mask
with adjustable size over the spectral domain [600nm, 900nm] that will be
installed at the Thai National Observatory. We present in this work the
development of a bench to characterise its Extreme Adaptive Optics system (XAO)
comprising a DM192 ALPAO deformable mirror (DM) and a 15x15 Shack-Hartmann
wavefront sensor (SH-WFS). In this bench, the turbulence is simulated using a
rotating phase plate in a pupil plane. In general, such components are designed
using a randomly generated phase screen. Such single realisation does not
necessarily provide the wanted structure function. We present a solution to
design the printed pattern to ensure that the beam sees a strict and controlled
Kolmogorov statistics with the correct 2D structure function. This is essential
to control the experimental conditions in order to compare the bench results
with the numerical simulations and predictions. This bench is further used to
deeply characterise the full 27 mm pupil of the ALPAO DM using a 54x54 ALPAO
SH-WFS. We measure the average shape of its influence functions as well as the
influence function of each single actuator to study their dispersion. We study
the linearity of the actuator amplitude with the command as well as the
linearity of the influence function profile. We also study the actuator offsets
as well as the membrane shape at 0-command. This knowledge is critical to get a
forward model of the DM for the XAO control loop
SAXO+ upgrade : second stage AO system end-to-end numerical simulations
SAXO+ is a proposed upgrade to SAXO, the AO system of the SPHERE instrument
on the ESO Very Large Telescope. It will improve the capabilities of the
instrument for the detection and characterization of young giant planets. It
includes a second stage adaptive optics system composed of a dedicated
near-infrared wavefront sensor and a deformable mirror. This second stage will
remove the residual wavefront errors left by the current primary AO loop
(SAXO). This paper focuses on the numerical simulations of the second stage
(SAXO+) and concludes on the impact of the main AO parameters used to build the
design strategy. Using an end-to-end AO simulation tool (COMPASS), we
investigate the impact of several parameters on the performance of the AO
system. We measure the performance in minimizing the star residuals in the
coronagraphic image. The parameters that we study are : the second stage
frequency, the photon flux on each WFS, the first stage gain and the DM number
of actuators of the second stage. We show that the performance is improved by a
factor 10 with respect to the current AO system (SAXO). The optimal second
stage frequency is between 1 and 2 kHz under good observing conditions. In a
red star case, the best SAXO+ performance is achieved with a low first stage
gain of 0.05, which reduces the first stage rejection.Comment: 10 pages, 8 figures. Submitted to AO4ELT7 conference proceeding
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