AO4ELT 2

Phase-Sorting Interferometry for High-Contrast Imaging with the GMT

Johanan CODONA — 2011-07-11T22:24:12Z

Authors

Johanan L. Codona

Affiliations

Univ of Arizona, Steward Observatory

Abstract

High-contrast detection and characterization are plagued by subtle telescope imperfections and non-common-path (NCP) aberrations, leading to many long-lived speckles in the search area near a star. These can be corrected using the AO DM, but the complex halo of the star must be known in the science camera focal plane in order to compute the DM corrections. Phase-sorting Interferometry (PSI) is a technique to measure the science path halo on-sky and has been demonstrated at the MMT in the L and M band. PSI makes an interferometric measurement of the halo including NCP aberrations without the use of any extra hardware or sensing light introduced into the focal plane. The only requirement is that the science camera be read out fast enough to image the varying speckle intensities over the region of interest near the star. The PSI method uses the AO system's wavefront sensor telemetry to estimate the residual wavefront error over a series of synchronized science camera exposures, and a mathematical model of the telescope to compute the phase and amplitude of the dynamic halo speckles. The computed speckle phases are used to analyze the science frames into four statistical interferograms which are combined in the usual way to derive the complex halo in the science focal plane. This allows calculation of a DM correction update, enabling a halo-suppression closed-loop to be implemented. We explore the potential for using this method with the GMT AO system and simulate a GMT anti-halo servo for various near-to-mid infrared bands combined with a 6-decade high-contrast phase-apodization coronagraph implemented using machined phase plates. The result is a robust, low-overhead, easy-to-use, high-contrast imaging technique suitable for both search and spectral characterization very near stars.

The Pyramid WFS with extended reference source

Enrico PINNA — 2011-05-19T21:19:09Z

Submitted by S. Esposito

Authors

Enrico Pinna, Alfio Puglisi, Javier Argomedo, Fernando Quiros-Pacheco, Armando Riccardi, Simone Esposito

Affiliations

Osservatorio di Arcetri

Abstract

During the LBT FLAO commissioning, the Pyramid Wave-Front Sensor (PWFS) demonstrated on sky its potential as component of a NGS SCAO system. These results confirmed the expected PWFS sensitivity in presence of a point-like guide source. The performances of the PWFS using an extended object as reference in astronomical applications have not yet been deeply investigated. In this work we show some preliminary laboratory result obtained on the LBT FLAO system #2 operated using an extended reference sources of diameter up to 1.6”. During the tests, as expected, the PWFS showed a sensitivity reduction due to the reference source extension. The sensitivity reduction only affects modes below a given radial order, leaving the sensitivity of the higher order modes unaffected. The limiting radial order increases with the reference source diameter. Our result suggests that AO systems, with an actuator pitch FLAO-like (30cm) and using a PWFS, can deliver high contrast images in H and K bands using reference object with diameter less than 1” and integrated Rmag < 9.5. The achieved result will be discussed in the framework of the ELTs AO systems and in context of LGS wavefront sensing.

SPHERE non-common path aberrations measurement and pre-compensation with optimized phase diversity processes: experimental results

Jean-Francois SAUVAGE — 2011-05-16T20:00:15Z

Submitted by J.-F. Sauvage

Authors

J-F Sauvage (1), T Fusco (1,2), D. LeMignant (2), C. Petit (1), A. Sevin (3), K. Dohlen (2), C. Robert (1), L Mugnier (1)

Affiliations

(1) ONERA, (2) Laboratoire d'Astrophysique de Marseille, (3) LESIA

Abstract

The SPHERE instrument is the 2nd generation instrument dedicated to exoplanet direct imaging and characterization. The extremely high imaging performance required by these observation mode calls for a high performance AO system. Particularly, this system has to provide a wavefront corrected from turbulent and internal defects. We present here the experimental results for the complete focal-plane calibration procedure of the SPHERE instrument internal defects. An optimized phase diversity method is applied allowing to deal with model uncertainties in the image formation (noise, residual background, amplitude fluctuation, sampling factor, défocalisation distance, object size and SH-model for reference slope modifications) The full procedure includes both Non-Common Path Aberrations (NCPA) compensation at the level of the coronagraphic mask using the eXtreme AO system itself (by the mean of modification of the filtered SH WFS reference slopes during close loop operations), but also additional measurements of IRDIS differential optical path aberrations for post processing of dual-band images.

We validate the algorithm and the pre-compensation procedure using data obtained during the first eXtreme AO bench (SAXO) integration and tests. We also applied the Diversity tool on stand-alone IRDIS data obtained at LAM during its local integration. In both cases, we demonstrate the robustness and the ultimate performance (nanometric precision of the residual quasi-static pattern) of the phase diversity approach which will allow us to obtain nanometric accuracy on the final SPHERE system.

Phase correction of segment diffraction for high-contrast imaging

Laurent PUEYO — 2009-02-28T23:00:00Z

Submitted by Bruce Macinstosh

Authors

Laurent Puyeo, Bruce Macintosh, Remi Soummer, Mitchell Troy

Affiliations

LLNL

Abstract

The exquisite angular resolution of segmented extremely large telescopes will provide astronomers with unique science opportunities in exoplanet imaging, from the ability to characterize the birth of exoplanets in star-forming regions to the direct detection of mature exoplanets in reflected light. However segmented apertures complicate the design of coronagraphic solutions for these instruments. While fill factor is a crucial figure of merit, e.g. many small segments with small gaps greatly simplify coronagraphic designs compared to a few large segments with large gaps, the static contrast is ultimately limited by optical artifacts due to the image of the segments gaps leaking through the starlight suppression system. Recent developments have shown how to accommodate segmented geometries using tailored coronagraphic designs (such as the generalized APLC and double stage Optical Vector Vortex Coronagraph). The successful implementation of such solutions at the very high contrast level can potentially degrade throughput and render the whole starlight suppression system more sensitive to both manufacturing and segments phasing errors. In this paper we propose an alternative solution that treats segment gaps can as a special case in reflectivity errors, with favorable spatial frequency properties but very high amplitude. Such reflectivity errors will have to be controlled in even a monolithic high-contrast system. We present the results of a numerical study which includes two sequential deformable mirrors as an extra degree of freedom in the design of the coronagraphic solution.

Tip-tilt wavefront sensing from a coronagraphic image: laboratory demonstration with a four-quadrant phase mask.

Marion MAS — 2009-02-28T23:00:00Z

Submitted by Marion MAS

Authors

Marion Mas, Pierre Baudoz, Gérard Rousset, Raphaël Galicher

Affiliations

Observatoire de Paris-Meudon

Herzberg Institute of Astrophysics, NRC-CNRC

Abstract

Direct detections of exoplanets at wide separations requires high contrast imaging. A coronagraph can be used to suppress the overhelming light of the hosting star and detect its faint neighborhood. Neverthless, low order wavefront errors such as tip-tilt catastrophically affect the coronagraph performance. Classical wavefront sensors unfortunately need a dedicated channel and are thus subject to non-common path errors. We propose a method to estimate the tip-tilt errors upstream a four-quadrant phase mask coronagraph with no such non-common path aberrations. To do so, we directly use the coronagraphic image and measure the intensity variations induced by tip-tilt errors. We applied this technique on our laboratory high contrast imaging bench and could stabilize the beam position on the coronagraphic mask with a 0.05L/D accuracy. In our presentation, we will develop the technique formalism and present the laboratory results in detail.

Tomographic phase diversity for phase retrieval on wide-field AO systems

Damien GRATADOUR — 2009-02-28T23:00:00Z

Submitted by Damien GRATADOUR

Authors

Damien Gratadour(1), François Rigaut(2)

Affiliations

(1) LESIA, Observatoire Paris , (2) Gemini Observatory

Abstract

Phase diversity is a commonly used technique to retrieve the wavefront at the focal plane. The usual algorithm involves two or more images of the same target with known phase changes like defocus. It has been shown to be very efficient at measuring on-axis the non-common path aberrations of classical AO systems. In this paper, we present an evolution of this algorithm towards tomographic measurements. This novel technique is dedicated to wide-field AO systems, allowing phase retrieval on multiple layers, conjugated at various altitudes. While the general grounds are very similar to classical phase diversity, the tomographic algorithm involves two or more images with known phase changes of several targets dispatched over the entire field of view. Regularization on the phase is usualy done by factorizing it on a basis of modes, traditionally Zernike polynomials. In this paper, we discuss the choice of a proper basis in the tomographic case and show that other basis such as disk harmonics are interesting alternatives in the case of real AO systems. We additionally propose two versions for this algorithm: an image-based and a Fourier-based both leading to comparable results. We finally present the results obtained on simulated data as well as on real data obtained on the Gemini MCAO system on which this algorithm has been used to estimate and compensate for non common path aberrations.

The ARGOS wavefront sensor pnCCD camera for an ELT: characteristics, limitations and applications

Gilles ORBAN DE XIVRY — 2009-02-28T23:00:00Z

Submitted by Gilles ORBAN DE XIVRY

Authors

G. Orban de Xivry(1), S. Ihle(2), J. Ziegleder(1), L. Barl(1), R. Hartmann(2), S. Rabien(1), H. Soltau(2), L. Strueder(3)

Affiliations

(1) MPE, Giessenbachstrasse, 85748 Garching, Germany (2) PNSensor GmbH, Roemerstrasse 28, 80803 Muenchen, Germany (3) MPI Halbleiterlabor, Muenchen, Germany

Abstract

From low-order to high-order AO, future wave front sensors on ELTs require large, fast, and low-noise detectors with high quantum efficiency and low dark current. While a detector for a high-order Shack-Hartmann WFS does not exist yet, the current CCD technology pushed to its limits already provides several solutions for the ELT AO detector requirements. One of these devices is the new WFS pnCCD camera of ARGOS, the Ground-Layer Adaptive Optics system (GLAO) for LUCIFER at LBT. Indeed, with its 264x264 pixels, 48 mu m pixel size and 1kHz frame rate, this camera provides a technological solution to different needs of the AO systems for ELTs, such as low-order but as well possibly higher order correction using pyramid wavefront sensing. In this contribution, we present the newly developped WFS pnCCD camera of ARGOS and how it fulfills future detector needs of AO on ELTs.

Sky coverages on ELTs with a reference area much larger than the compensated one

Valentina VIOTTO — 2009-02-28T23:00:00Z

Submitted by Valentina VIOTTO

Authors

Viotto, V. (a,b), Ragazzoni, R. (a), Arcidiacono, C. (c), Dima, M. (a), Magrin, M. (a), Farinato, J. (a)

Affiliations

a - INAF, Astronomical Observatory of Padova b - University of Padova - Astronomy Department c - INAF, Astronomical Observatory of Arcetri

Abstract

Sky coverage for MCAO systems has been the subject of a large number of studies in the past decade or so. The resulting figures, further to have the chance of being refined by actual measurements carried out at the largest 8m class telescopes, have to be updated and validated for the most recent approaches in ELT adaptive optics corrections. In particular, techniques employing the use of several virtual deformable mirrors can take advantage of a field of view, inside which references can be found, that is much larger than the area where the correction is actually made. While a parallel with similar studies for Multi Objects AO can be traced down, we revise here the various proposed approaches and we show the results of similar computations using the novel concepts and gauging the outcome with the more recent on-sky available data. The comparison with similar computations that can be made for artificially generated beacon is briefly discussed.

Pyramids, layers and no laser guide stars!

Roberto RAGAZZONI — 2009-02-28T23:00:00Z

Submitted by Roberto RAGAZZONI

Authors

Roberto Ragazzoni, Marco Dima, Jacopo Farinato, Demetrio Magrin, Valentina Viotto

Affiliations

INAF - Astronomical Observatory of Padova

Abstract

A decade after the first achievement into the improved capabilities in sensing by the pyramid wavefront sensor and from the outlining of novel classes of Multi Conjugated Adaptive Optics experimental verification of such approaches has been vastly proved by results from MAD onboard VLT and from FLAO onboard LBT. Refinement and extensions of these techniques promises to achieve similar goals within references scattered in a Field of View much larger than the one being compensated, an approach only marginally exploited in the so-called Multiple Field of View approach while the adoption of virtual DMs would allow a much deeper exploitation of such possibilities. As in the meantime the diameter of the largest project shortened somehow it is time to gather all these concept, to assemble -possibly- in an efficient way in order to continue to pursue the goal of achieving diffraction limited imagery at a level concorrential with what is being promised by artificial references, by the usage of solely natural guide stars. The resulting approach is a robust one (as it is not incompatible with Laser generated beacons) and can be implemented into existing optical design of the current extremely large telescopes under development

Pyramid based locally closed loop wavefront sensor: an optomechanical study.

Demetrio MAGRIN — 2009-02-28T23:00:00Z

Submitted by Demetrio MAGRIN

Authors

Magrin D. (a), Dima M. (a), Farinato J. (a), Ragazzoni R. (a), Viotto, V. (a)(b)

Affiliations

a) INAF - Osservatorio Astronomico di Padova b) Universita' di Padova - Dipartimento di Astronomia

Abstract

Pyramid wavefront sensors have shown their ability to achieve ultimate performances in terms of usage of starlight photons. Several of these advantages, however, does relay on the fact that the starlight is focused onto the pyramid pin after being properly corrected. While conventional adaptive optics systems (including multi conjugated ones) automatically imply such a feature this is not true for Multi Objects or non conventional Multi Conjugated AO systems, or more in general for the ones where the compensated area is somehow different from the area searched for references. We develop the concept of locally closed loop systems in a compact way along with various approaches to metrologically measure the wavefront with enough absolute accuracy, in contrast with the accuracy to vanish the residual in the conventional use of wavefront sensors. We show an optomechanical study including a preliminary error budget and a choice of existing components in order to achieve the goal of detailed wavefront sensing with high dynamic range on the faintest references where a pyramid sensor can achieve significant results.