Clélia Robert

Retrieving parameters of the anisotropic refractive index fluctuations spectrum in the stratosphere from balloon-borne observations of stellar scintillation

Scintillation effects are not negligible in the stratosphere. We present a model based on a 3D model of anisotropic and isotropic refractive index fluctuations spectra that predicts scintillation rates within the so-called small perturbation approximation. Atmospheric observations of stellar scintillation made from the AMON-RA (AMON, Absorption par les Minoritaires Ozone et NO(x); RA, rapid) balloon-borne spectrometer allows us to remotely probe wave-turbulence characteristics in the stratosphere. Data reduction from these observations brings out values of the inner scale of the anisotropic spectrum. We find metric values of the inner scale that are compatible with space-based measurements. We find a major contribution of the anisotropic spectrum relative to the isotropic contribution. When the sight line plunges into the atmosphere, strong scintillation occurs as well as coupled chromatic refraction effects.

Myopic deconvolution from wave-front sensing

Deconvolution from wave-front sensing is a powerful and low-cost high-resolution imaging technique designed to compensate for the image degradation due to atmospheric turbulence. It is based on a simultaneous recording of short-exposure images and wave-front sensor (WFS) data. Conventional data processing consists of a sequential estimation of the wave fronts given the WFS data and then of the object given the reconstructed wave fronts and the images. However, the object estimation does not take into account the wave-front reconstruction errors. A joint estimation of the object and the respective wave fronts has therefore been proposed to overcome this limitation. The aim of our study is to derive and validate a robust joint estimation approach, called myopic deconvolution from wave-front sensing. Our estimator uses all data simultaneously in a coherent Bayesian framework. It takes into account the noise in the images and in the WFS measurements and the available a priori information on the object to be restored as well as on the wave fronts. Regarding the a priori information on the object, an edge-preserving prior is implemented and validated. This method is validated on simulations and on experimental astronomical data.

Scintillation and phase anisoplanatism in Shack-Hartmann wavefront sensing

Adaptive optics provides a real-time compensation for atmospheric turbulence that severely limits the resolution of ground-based observation systems. The correction quality relies on a key component, that is, the wavefront sensor (WFS). When observing extended sources, WFS precision is limited by anisoplanatism effects. Anisoplanatism induces a variation of the turbulent phase and of the collected flux in the field of view. We study the effect of this phase and scintillation anisoplanatism on wavefront analysis. An analytical expression of the error induced is given in the Rytov regime. The formalism is applied to a solar and an endoatmospheric observation. Scintillation effects are generally disregarded, especially in astronomical conditions. We shall prove that this approximation is not valid with extended objects.

ATLAS: the E-ELT laser tomographic adaptive optics system

ATLAS is a generic Laser Tomographic AO (LTAO) system for the E-ELT. Based on modular, relatively simple, and yet innovative concepts, it aims at providing diffraction limited images in the near infra-red for a close to 100 percent sky coverage.

A preliminary overview of the multiconjugate adaptive optics module for the E-ELT

The multi-conjugate adaptive optics module for the European Extremely Large Telescope has to provide a corrected field of medium to large size (up to 2 arcmin), over the baseline wavelength range 0.8-2.4 μm. The current design is characterized by two post-focal deformable mirrors, that complement the correction provided by the adaptive telescope; the wavefront sensing is performed by means of a high-order multiple laser guide star wavefront sensor and by a loworder natural guide star wavefront sensor. The present status of a two years study for the advanced conceptual design of this module is reported.

C n 2 profile measurement from Shack-Hartmann data

C(n)(2) profile monitoring usually relies on the exploitation of wavefront slope correlations or of scintillation pattern correlations. Scintillation is rather sensitive to high turbulence layers whereas wavefront slope correlations are mainly due to layers close to the receiving plane. Wavefront slope and scintillation correlations are therefore complementary. A Shack-Hartmann wavefront sensor (SHWFS) is currently used to measure wavefront slopes only. But it could also be sensitive to scintillation as the average intensity in a given subaperture can be obtained by adding pixel intensities in the subaperture focal plane up. With slopes and scintillation being recorded simultaneously, their correlation is also theoretically available. We propose to exploit wavefront slope and scintillation correlations recorded with a SHWFS to retrieve the C(n)(2) profile. Two measurement methods are exposed. In CO-SLIDAR (Coupled SLODAR SCIDAR), correlations of SHWFS data recorded on two separated stars are exploited. SCO-SLIDAR (Single CO-SLIDAR) relies on the same principle as CO-SLIDAR, but SHWFS data are recorded on a single star. Results of C(n)(2) estimation from simulated SHWFS data are presented.

Shack-Hartmann wavefront estimation with extended sources: anisoplanatism influence.

Anisoplanatism limits the correction field of adaptive optics (AO). In the case of Shack-Hartmann measurement performed on extended sources it may also strongly affect wavefront estimation accuracy. An analytical formalism has been previously proposed to quantify anisoplanatism slope measurement error. It is exploited here to derive the most relevant quantity in AO, the wavefront error. Analytical and end-to-end simulation results are compared in three cases: solar observation, weakly perturbed near-to-ground observation, and strongly perturbed near-to-ground observation. In every case, anisoplanatism wavefront error takes significant values. The accuracy of the analytical model is investigated in detail. Three contributions to the slope error previously identified are considered: phase anisoplanatism, scintillation anisoplanatism, and coupling between scintillation and phase anisoplanatism. The influence of both scintillation and coupling contributions to the wavefront error is confirmed here.

Shack-Hartmann wavefront sensing with extended sources

Shack-Hartmann wavefront sensor is widely used for the measurement of phase perturbations induced by turbulence. Such a wavefront sensor relies on the measurement of the image displacements in the lenslet array focal plane. Different algorithms can be used to estimate this displacement. This paper is dedicated to the analysis and comparison of their performances. Special attention will be paid to correlation techniques which are well suited to extended sources.

Advances in detector technologies for visible and infrared wavefront sensing

The purpose of this paper is to give an overview of the state of the art wavefront sensor detectors developments held in Europe for the last decade. The success of the next generation of instruments for 8 to 40-m class telescopes will depend on the ability of Adaptive Optics (AO) systems to provide excellent image quality and stability. This will be achieved by increasing the sampling, wavelength range and correction quality of the wave front error in both spatial and time domains. The modern generation of AO wavefront sensor detectors development started in the late nineties with the CCD50 detector fabricated by e2v technologies under ESO contract for the ESO NACO AO system. With a 128x128 pixels format, this 8 outputs CCD offered a 500 Hz frame rate with a readout noise of 7e-. A major breakthrough has been achieved with the recent development by e2v technologies of the CCD220. This 240x240 pixels 8 outputs EMCCD (CCD with internal multiplication) has been jointly funded by ESO and Europe under the FP6 programme. The CCD220 and the OCAM2 camera that operates the detector are now the most sensitive system in the world for advanced adaptive optics systems, offering less than 0.2 e readout noise at a frame rate of 1500 Hz with negligible dark current. Extremely easy to operate, OCAM2 only needs a 24 V power supply and a modest water cooling circuit. This system, commercialized by First Light Imaging, is extensively described in this paper. An upgrade of OCAM2 is foreseen to boost its frame rate to 2 kHz, opening the window of XAO wavefront sensing for the ELT using 4 synchronized cameras and pyramid wavefront sensing. Since this major success, new developments started in Europe. One is fully dedicated to Natural and Laser Guide Star AO for the E-ELT with ESO involvement. The spot elongation from a LGS Shack Hartman wavefront sensor necessitates an increase of the pixel format. Two detectors are currently developed by e2v. The NGSD will be a 880x840 pixels CMOS detector with a readout noise of 3 e (goal 1e) at 700 Hz frame rate. The LGSD is a scaling of the NGSD with 1760x1680 pixels and 3 e readout noise (goal 1e) at 700 Hz (goal 1000 Hz) frame rate. New technologies will be developed for that purpose: advanced CMOS pixel architecture, CMOS back thinned and back illuminated device for very high QE, full digital outputs with signal digital conversion on chip. In addition, the CMOS technology is extremely robust in a telescope environment. Both detectors will be used on the European ELT but also interest potentially all giant telescopes under development. Additional developments also started for wavefront sensing in the infrared based on a new technological breakthrough using ultra low noise Avalanche Photodiode (APD) arrays within the RAPID project. Developed by the SOFRADIR and CEA/LETI manufacturers, the latter will offer a 320x240 8 outputs 30 microns IR array, sensitive from 0.4 to 3.2 microns, with 2 e readout noise at 1500 Hz frame rate. The high QE response is almost flat over this wavelength range. Advanced packaging with miniature cryostat using liquid nitrogen free pulse tube cryocoolers is currently developed for this programme in order to allow use on this detector in any type of environment. First results of this project are detailed here. These programs are held with several partners, among them are the French astronomical laboratories (LAM, OHP, IPAG), the detector manufacturers (e2v technologies, Sofradir, CEA/LETI) and other partners (ESO, ONERA, IAC, GTC). Funding is: Opticon FP6 and FP7 from European Commission, ESO, CNRS and Université de Provence, Sofradir, ONERA, CEA/LETI and the French FUI (DGCIS).

Conceptual design and performance of the multiconjugate adaptive optics module for the European Extremely Large Telescope

The Multi-conjugate Adaptive Optics RelaY (MAORY) for the European Extremely Large Telescope (E-ELT) provides a corrected field of view of up to 2 arcmin diameter over the wavelength range 0.8-2.4 μm. It is expected to achieve a correction of high quality and uniformity with high sky coverage: with a seeing of 0.8 arcsec in the visible, the expected Strehl Ratio averaged over a 1 arcmin field is approximately 50% at 2.16 μm wavelength over 50% of the sky at the Galactic Pole. Wavefront correction is obtained by means of the E-ELT adaptive mirrors M4/M5 and of two post-focal deformable mirrors conjugated at 4km and 12.7km from the telescope pupil. Wavefront sensing is performed by 6 Sodium laser guide stars and by 3 natural guide stars, used to measure atmospheric and windshake tilt and to provide a reference for the focus and for the low-order aberrations induced by the Sodium layer. MAORY is located on the E-ELT Nasmyth platform and has a gravity invariant port, feeding the high angular resolution camera MICADO, and a lateral port for a detached instrument as the infrared spectrograph SIMPLE.