Thierry Fusco

Zero Noise Wavefront Sensor Development within the Opticon European Network

This activity, funded by ESO and the European Commission through the Opticon Network will attempt to define, fabricate and fully characterize the best possible detector working at visible wavelengths suitable for wavefront sensors in Adaptive Optics (AO) systems. The detector will be a split frame transfer array built by e2v technologies and called CCD220. The frame rate will be very fast (up to 1.2 kHz) while the readout noise will be kept extremely low (typically below 1 e - ). The goal of this paper is to justify the choice of detector: an EMCCD with 240×240 pixels and 8 outputs that will provide sub- electron readout noise at 1-1.2 kHz frame rate. This paper shows that, despite the fact that EMCDDs have an excess noise factor of 1.4 due to the charge multiplication process; their virtually zero read noise should allow them to outperform the classical CCD. Such detectors do not yet exist and must be developed. Moreover, this paper explains how the OPTICON European network is organized.

A Dedicated L3Vision CCD for Adaptive Optics Applications

ESO and JRA2 OPTICON have funded the development of a compact packaged Peltier cooled 24 µm square 240×240 pixel split frame transfer 8-output back illuminated L3Vision CCD, L3CCD, by e2v technologies. The device will achieve sub-electron (goal 0.1e - ) read noise at frame rates from 25 Hz to 1.5 kHz and low dark current of 0.01 e - /pixel/frame. The development has many unique features. To obtain high frame rates, multi-output EMCCD gain registers and metal buttressing of parallel clocks will be used. To minimize risk, the baseline device will be built in standard silicon. In addition, a split wafer run will enable two speculative variants to be built; deep depletion silicon devices to improve red response and devices with an electronic shutter to extend use to Rayleigh Laser Guide Star (RLGS) applications. These are all unprecedented advancements for L3CCDs. This paper will describe requirements and outline the design established after careful consideration of the application, detector architecture, compact Peltier package, technology trade-offs, schedule and proposed test plan.

Multiconjugate adaptive optics: comparison of phase reconstruction approaches for large field of view

We present, in this article, an analytical study of the phase reconstruction error for MCAO systems. Two approaches are considered; a classical estimator based on a Least Square (LS) approach and a Maximum A Posteriori (MAP) estimator which uses the prior knowledge we have both on the measurement noise and the turbulence volume statistics. The effects of these modes both on the phase reconstruction and on the phase correction error are studied and quantified. T is shown that, using a MAP approach, a large part of the unseen modes can be extrapolated using correlations between unseen eigenmode coefficients and well-measured coefficients. We observe that the use of a MAP estimator allows a significant gain in terms of correction quality in the whole field of view.

Noise propagation for multiconjugate adaptive optics system

We study the noise propagation for MultiConjugate Adaptive Optics (MCA)) systems working with Shack-Hartmann wavefront sensors (WFS). Two approaches are considered to control the deformable mirrors using WFS data: a classical Least Square approach and a Maximum A Posteriori (MAP) estimator. Analytical results, based on a Fourier transform analysis, allow us to study and quantify the reconstruction phase error for the two estimators. Consequences of the noise propagation on system design and performance is analyzed.

Optimization of the Pre-Compensation of Non-Common Path Aberrations for Adaptive Optics Systems

We present experiental results of a new procedure of measurement and pre-compensation of the AO non-common path aberrations. A significant Strehl ratio increase (from 70 to 90 % in R band) is demonstrated.

MCAO for astronomical and near-to-ground applications

MCAO is a very promising technique to increase the AO corrected field of view. By now, this method was mainly studied for astronomical purposes. In case of horizontal or slant path propagation, the effects of anisoplanatism and scintillation are quite stronger than for astronomy: MCAO seems specially well-suited in this context. Therefore, many authors propose to use MCAO for laser beam control. Imaging is another potential applications: we have studied the theoretical performance of MCAO for extended source observation. We will present the results of this study.

Wave-front sensing for space active optics: Rascasse project

The payloads for Earth Observation and Universe Science are currently based on very stiff opto-mechanical structures with very tight tolerances. The introduction of active optics in such an instrument would relax the constraints on the thermo-mechanical architecture and on the mirrors polishing. A reduction of the global mass/cost of the telescope is therefore expected. Active optics is based on two key-components: the wave-front sensor and the wave-front corrector.

Deconvolution of adaptive optics images using object autocorrelation and positivity

Adaptive Optics systems provide a real time compensation for atmospheric turbulence, which severely limits the resolution of large telescopes. However, the correction is often only partial and a deconvolution is required to reach the telescope diffraction limit. The need for a regularized deconvolution is discussed, and a Maximum A Posteriori based deconvolution technique is presented. This technique incorporates a positivity constraint and the knowledge of the object Power Spectral Density. This method is then extended to the case of an unknown PSF. Deconvolution results are presented for both simulated and experimental data.

Validating the phase diversity approach for sensing NCPA in SHARK-NIR, the second-generation high-contrast imager for the Large Binocular Telescope

Phase diversity is a focal plane wavefront sensing technique that allows to retrieve the phase aberration introduced by a camera starting from two images of whatever object, one of which (the diverse image) is intentionally corrupted by a known aberration. We present here the results of a simulation campaign aimed at assessing the validity of this approach for sensing non-common path aberrations (NCPA) in SHARK-NIR, the new-generation high-contrast imager for the Large Binocular Telescope (LBT). The aberrations to be retrieved has been modeled on a realistic error budget of the instrument, while images are generated with an end-to-end Fresnel simulator which makes use of atmospheric phase screens to simulate realistic closed-loop observations. A wide parameter space is explored in order to identify the critical parameters and to estimate the expected level of correction.

Assembly, integration, test, and verification scenarios for the ELT MOSAIC instrument

Assembly, Integration, Test and Validation (AIT/V) phases for AO instruments, in laboratory as in the telescope, represent numerous technical challenges. The Laboratoire d’Astrophysique de Marseille (LAM) is in charge of the AIT/V preparation and planning for the MOSAIC (ELT-MOS) instrument, from identification of needs, challenges, risks, to defining the optimal AIT strategy for this highly modular and serialized instrument. In this paper, we present the status of this study and describe several AIT/V scenarios as well as a planning for AIT phases in Europe and in Chile. We also show our capabilities, experience and expertise to lead the instrument MOSAIC AIT/V activities.