AO4ELT 2

Laser-Guide Star Point-Spread Function Reconstruction for ELTs

Carlos CORREIA — 2011-05-17T15:01:41Z

Submitted by C. Correia

Authors

C. Correia (a) , J.-P. Véran (a), B. Ellerbroek (b), L. Gilles (b) and L. Wang (b)

Affiliations

ONERA

Abstract

To exploit the maximum potential of Extremely Large Telescopes (ELT), adaptive-optics (AO)-corrected images can be further enhanced by using image restoration techniques. Such techniques rely on accurate knowledge of the point-spread function (PSF) anywhere in the field.

To increase sky-coverage ELTs use laser beacons to probe the three-dimensional atmosphere and multi-conjugate AO to increase correction above the isoplanatic patch. These features translate into three sources of anisoplanatism: 1) focal anisoplanatism known as cone effect 2) angular anisoplanatism due to the difference of the wave-fronts in the LGS and science directions and 3) tip/tilt angular anisoplanatism, on account of the LGS being blind to TT, the latter being estimated from natural GS measurements in different locations in the field.

In our approach, the long-exposure science optical transfer function (OTF) (Fourier transform of the PSF) is estimated as a product of 3 terms: (i) the OTF of a point-source LGS, estimated from system telemetry using Véran's method, (ii) a model anisoplanatism filter, computed from a high-fidelity numerical simulation to account for the difference between the OTFs for a point-source at the location of the LGS and the science target, and (iii) a tip/tilt/tilt anisoplanatism filter obtained from system and model telemetry, and expressed as a system-to-model OTF ratio.

We present the first stage of the reconstruction, (ii) and (iii) being presented by Gilles in this same conference. Based on [Véran et al 97], we show how to reconstruct the LGS-PSF by de-noising telemetry-accumulated measurements and removing AO-loop specific terms, namely the measurement noise and aliasing components; we present the modifications needed on account of the system size and the optimisations required to accurately reconstruct the PSF. Furthermore, we compare our estimates to those of a high-fidelity Monte-Carlo simulator (MAOS) that can accurately model the PSF in those locations.

Aberrations induced by side-projected laser guide stars in laser tomography adaptive optics systems

Marcos VAN DAM — 2009-02-28T23:00:00Z

Submitted by Marcos VAN DAM

Authors

Marcos A. van Dam, Rodolphe Conan, Antonin H. Bouchez and Brady Espeland

Affiliations

(1) Flat Wavefronts (2) and (4) Australian National University (3) Giant Magellan Telescope Observatory

Abstract

A laser tomography adaptive optics (LTAO) system is currently under design for the Giant Magellan Telescope (GMT). For systems engineering reasons, it is preferable to project the laser guide stars (LGSs) from the side of the telescope. Experience with the Keck II adaptive optics system and analytical modeling have shown that side-launched lasers result in aberrations, called LGS aberrations, with a lot of power at low-spatial frequencies. This is caused by the elongation of the LGS due to the finite thickness of the sodium layer.

In this paper, we model the LGS aberrations for the GMT's LTAO system, which has three launch telescopes projecting six LGSs in a regular hexagon. When the wavefront is reconstructed tomographically, the aberrations largely cancel on-axis. However, an off-axis truth sensor with a dedicated DM on the truth sensing path will see completely different aberrations, introducing an unacceptably large wavefront error. Working with the assumption that the LGS aberrations affect all of the wavefront sensors in the sane way, we propose a method that filters the LGS aberrations directly from the Shack-Hartmann centroids. This method is shown to be very effective at filtering the LGS aberrations while having a negligible effect on the turbulence estimation.

Practical experience with AO PSF reconstruction at the Keck and Gemini telescopes

Laurent JOLISSAINT — 2009-02-28T23:00:00Z

Submitted by Laurent JOLISSAINT

Authors

Laurent Jolissaint Chris Neyman Peter Wizinowich Julian Christou

Affiliations

aquilAOptics W.M. Keck Foundation W.M. Keck Foundation Gemini Observatory

Abstract

Estimating the point spread function across the imaged field, for a given AO run, is critical for AO data reduction. In this talk I will describe our recent progress (Summer 2011) on PSF reconstruction for the Gemini North (ALTAIR) and Keck NGS based AO systems. I will shortly re-introduce the basic theory, but will put the emphasize on practical implementation issues we are facing at this two facilities, in particular how we handle and determine/calibrate the non-turbulent aberrations (telescope, instrument optics) whose amplitude can be as large as the turbulent aberrations.

Point Spread Function Reconstruction for Laser Guide Star Multi Conjugate Adaptive Optics Systems on Extremely Large Telescopes

Luc GILLES — 2009-02-28T23:00:00Z

Submitted by Luc GILLES

Authors

L. Gilles (1), B. L. Ellerbroek (1), L. Wang (1), J-P. Veran (2) and C.Correia (2)

Affiliations

(1) Thirty Meter Telescope Observatory Corporation, 1111 S. Arroyo Parkway, Suite 200, Pasadena, California 91105, USA (2) NRC Herzberg Institute of Astrophysics, 5171 W. Saanich Road, Victoria, BC, V9E 2E7, Canada

Abstract

We discuss a point spread function (PSF) reconstruction strategy for laser guide star (LGS) multi conjugate adaptive optics (MCAO) systems on Extremely Large Telescopes (ELTs). The approach is based on system telemetry data, combined with model data obtained from a high-speed highly-parallelized Monte Carlo numerical simulation of the full system. Such a simulation is fed by critical system data, including the Cn2 turbulence profile estimated in real-time during LGS tomography, wavefront sensor (WFS) signal levels, control loop parameters, and calibration data to account for non-common-path (NCP) aberrations and sodium layer tracking errors. The long-exposure science optical transfer function (OTF) (Fourier transfrom of the PSF) is estimated as as a product of 3 terms: (i) the OTF of a point-source LGS, estimated from system telemetry using Veran's method, (ii) a model anisoplanatism filter, computed from the numerical simulation to account for the difference between the OTFs for a point-source at the location of the LGS and the science target, and (iii) a tip/tilt/tilt anisoplanatism filter obtained from system and model telemetry, and expressed as a system-tomodel OTF ratio.A Strehl ratio error sensitivity analysis is given for the Thirty Meter Telescope (TMT) Narrow- Field Infrared AO System (NFIRAOS), for the case of uncertainties in the turbulence seeing parameter r0, and for the case of un-modeled tip/tilt disturbances simulating system vibrations.

Numerical simulations of an Extreme AO system for an ELT

2009-02-28T23:00:00Z

Submitted by Miska LE LOUARN

Authors

R. Clare, M. Le Louarn

Affiliations

ESO, ESO

Abstract

We present the first full end to end numerical simulation results for a 200x200 sub-aperture modulated Pyramid WFS-based extreme AO, on a 42m telescope. The Pyramid wavefront sensor with modulation is used to reduce the effects of noise propagation (like aliasing and photon-noise). We present performance (Strehl and an estimate of contrast) as a function of system parameters like modulation, guide star magnitude and to explore the sensitivity of the system to for example loop gain and atmospheric turbulence profiles.

Myopic exoplanet detection algorithm based on an analytical model of AO-corrected coronagraphic multi-spectral imaging.

Marie YGOUF — 2009-02-28T23:00:00Z

Submitted by Marie YGOUF

Authors

Marie Ygouf (1,2,3) - Laurent M. Mugnier (1,3) - David Mouillet (2,3)- Thierry Fusco (1,3) - Jean-Luc Beuzit (2,3)

Affiliations

(1) ONERA (2) IPAG (3) GIS PHASE

Abstract

High contrast imaging for the detection and characterisation of exoplanets rests upon the instrument's capability to cancel the light of the host star. Unfortunately the combination of adaptive optics and coronagraphy is not sufficient: the residual starlight, or speckle noise, may be relatively bright compared to the signal of the planet and limits the detection sensitivity. These speckles find their origin in wavefront errors created by imperfections in the optical components. As they evolve on various time scales, calibrating these speckles out is very tricky and the suppression of the unavoidable residual speckle noise must be done by post-processing methods. The current empirical post-processing methods for calibrating out the residual speckles and detecting the potential exoplanets are not sufficient with respect to the specifications to be reached by the new and future generations of instruments. In this communication, we develop, in a bayesian framework, an inversion method that is based on an analytical imaging model. The model links the instrumental aberrations to the speckle pattern on the image focal plane, distinguishing between aberrations upstream and downstream of the coronagraph. This approach allows us to estimate both the speckles and the object map using the fact that the object does not scale with the wavelength as the speckle pattern does. We validate this method on realistic images with simulation conditions typical of a SPHERE-like instrument. We assess the performance of the method for different contrasts between the star and the planet flux.

Analysis of the Improvement in Sky coverage for TMT NFIRAOS

Lianqi WANG — 2009-02-28T23:00:00Z

Submitted by Lianqi WANG

Authors

Lianqi Wang, Brent Ellerbroek, and Luc Gilles

Affiliations

TMT Corporation

Abstract

The scientific utility of laser guide star based adaptive optics (AO) systems depend upon high sky coverage. Previously we reported a high fidelity sky coverage analysis of an ad hoc split tomography (AHST) control algorithm and a post-processing simulation technique. In this paper, we will validate the post-processing sky coverage analysis using integrated simulations of both high order (Laser guide star) and low order loops (NGS guide star) with our efficient simulation software, multi-threaded AO simulator (MAOS). We will show a newly identified important term in the noise model that improves the performance by more properly regularizing the reconstructor. We Will also present the sky coverage results of a newer minimum variance split tomography (MVST) algorithm, and show that it brings a median improvement at zenith of 21 nm RMS OPD error over AHST without complicates the real time controller. The improvement is more significant for worse asterisms and/or for higher zenith angles.

Achieving High Contrasts Through Speckle Rejection With Slicer Based Integral Field Spectrographs

Graeme SALTER — 2009-02-28T23:00:00Z

Submitted by Graeme SALTER

Authors

Graeme Salter[1], Niranjan Thatte[2], Matthias Tecza[2], Fraser Clarke[2]

Affiliations

[1]University of New South Wales, [2]University of Oxford

Abstract

Speckle noise, not photon noise, remains to be the limiting factor in the direct detection of high contrast companions. With studies for the future exo-planet characterisation and imaging instruments on the ELTs under way, the correct choice of technology must be made that will enable the fitting and removal of the speckle noise that remains after AO. We conclusively demonstrate, through the use of an experimental setup producing a simulated speckle, that slicer based integral field spectrographs (IFS) and post-processing using spectral deconvolution can achieve speckle rejection factors of 1000 in broad band reconstructed images (and 100 per spectral channel). This represents an order of magnitude improvement over current state-of-the-art. Contrary to popular belief, we do not find any evidence that this choice of IFS technology limits the achievable contrast of extra solar planet direct detection instruments. Coupled with extreme adaptive optics systems and high performance coronagraphs, a slicer based integral field spectrograph can achieve contrasts exceeding 10^9, making it an attractive option for the next generation of instruments being designed for the direct detection of extra solar planets (e.g. EPICS for the E-ELT).

SPHERE: Confronting in-lab performance with system analysis predictions

Kjetil DOHLEN — 2009-02-28T23:00:00Z

Submitted by Kjetil DOHLEN

Authors

Kjetil Dohlen, François P. Wildi, Jean-Luc Beuzit, Pascal Puget, David Mouillet and teh SPHERE consortium

Affiliations

Laboratoire d'Astrophysique de Marseille; Observatory of Geneva; Institut de Planétologie et d'Astrophysique de Grenoble

Abstract

SPHERE is an extra-solar planet imager instrument for ESO's VLT telescope. Scheduled for first light in 2012, aims to detect giant extra-solar planet in the vicinity of bright stars and to characterise the objects found through spectroscopic and polarimetric observations. The observations will be done both within the Y, J, H and Ks atmospheric windows ( 0.95 – 2.32μm) by the aid of a dual imaging camera (IRDIS) and an integral field spectrograph (IFS), and in the visible using a fast-modulating polarizing camera (ZIMPOL). The instrument employs an extreme-AO turbulence compensation system, focal plane tip-tilt correction, and interferential coronagraphs.

The aim of this paper is to analyse the approach taken for system analysis and implementation in the light of forthcoming instruments of this type to be design and built for ELT-class telescopes. In particular, we review the performance analysis concept used for SPHERE, with, as far as results are available, comparison with actually obtained in-lab performance. We also consider the implementation of practical aspects related to alignment, integration, and testing activities, again comparing plans with reality. In the light of this experience, we tempt an extrapolation of our approach to next generation systems.