Detecting faint companions in close proximity to stars is one of the major goals of current/planned ground- and space-based high-contrast imaging instruments. High-performance coronagraphs can ...suppress the diffraction features and gain access to companions at small angular separation. However, the uncontrolled pointing errors degrade the coronagraphic performance by leaking starlight around the coronagraphic focal-plane mask, preventing the detection of companions at small separations. A Lyot-stop low-order wavefront sensor (LLOWFS) was therefore introduced to calibrate and measure these aberrations for focal-plane phase mask coronagraphs. This sensor quantifies the variations in wavefront error decomposed into a few Zernike modes by reimaging the diffracted starlight rejected by a reflective Lyot stop. The technique was tested with several coronagraphs on the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system at the Subaru Telescope. The wavefront was decomposed into 15 and 35 Zernike modes with an occulting and focal-plane phase mask coronagraph, respectively, which were used to drive a closed-loop correction in the laboratory. Using a 2000-actuator deformable mirror, a closed-loop pointing stability between 10−3-10−4 λ/D was achieved in the laboratory in H-band, with sub nanometer residuals for the other Zernike modes (Noll index > 4 ). On-sky, the low-order control of 10+ Zernike modes for the phase-induced amplitude apodization and the vector vortex coronagraphs was demonstrated, with a closed-loop pointing stability of 10 − 4 λ D under good seeing and 10 − 3 λ D under moderate seeing conditions readily achievable.
ABSTRACT We propose a new algorithm dramatically enhancing the efficiency of the lucky imaging technique for AO-corrected images in the visible range. It is achieved by a selection based on the ...relative strength of signal for each spatial frequency in the Fourier domain, making a more efficient use of information contained in each frame. Realistic simulations show that our algorithm allows us to reach the diffraction limit in the visible range on an AO-equipped 8 m telescope and enhances the Strehl ratio of an AO long exposure by a factor of up to 4. It outperforms the lucky imaging technique at an equivalent selection ratio. The fraction of selected data in simulation is also boosted from two to eight times for a given Strehl-ratio performance.
Éteindre une étoile, voilà une drôle d’idée ! C’est pourtant que ce que l’on propose aux étudiants avec cette expérience. L’objectif est de comprendre le principe du coronographe appliqué à ...l’observation de planètes extrasolaires. Avec un montage simple, on démontre comment atténuer optiquement la lumière d’une étoile sans modifier l’image d’une planète extrasolaire qui orbite autour de cette dernière.
Avec 4000 planètes découvertes en 20 ans dont plus de la moitié dans les trois dernières années, l’étude des planètes extrasolaires est devenue un thème majeur de l’astronomie moderne. La découverte ...de planètes différentes de celles observées dans notre système solaire interroge les modèles de formation et d’évolution des systèmes planétaires. Pour bien comprendre la nature de ces nouveaux objets, les données actuelles sont trop limitées et des grands projets notamment spatiaux seront nécessaires dans les années qui viennent.
Context. Small inner working angle coronagraphs, such as the vortex phase mask, are essential to exploit the full potential of ground-based telescopes in the context of exoplanet detection and ...characterization. However, the drawback of this attractive feature is a high sensitivity to pointing errors, which degrades the performance of the coronagraph. Aims. We propose a tip-tilt retrieval technique based on the analysis of the final coronagraphic image, hereafter called Quadrant Analysis of Coronagraphic Images for Tip-tilt Sensing (QACITS). Methods. Under the assumption of small phase aberrations, we show that the behavior of the vortex phase mask can be simply described from the entrance pupil to the Lyot stop plane with Zernike polynomials. This convenient formalism is used to establish the theoretical basis of the QACITS technique. We performed simulations to demonstrate the validity and limits of the technique, including the case of a centrally obstructed pupil. Results. The QACITS technique principle is validated with experimental results in the case of an unobstructed circular aperture, as well as simulations in presence of a central obstruction. The typical configuration of the Keck telescope (24% central obstruction) has been simulated with additional high order aberrations. In these conditions, our simulations show that the QACITS technique is still adapted to centrally obstructed pupils and performs tip-tilt retrieval with a precision of 5 × 10-2λ/D when wavefront errors amount to λ/ 14 rms and 10-2λ/D for λ/ 70 rms errors (with λ the wavelength and D the pupil diameter). Conclusions. We have developed and demonstrated a tip-tilt sensing technique for vortex coronagraphs. The implementation of the QACITS technique is based on the analysis of the scientific image and does not require any modification of the original setup. Current facilities equipped with a vortex phase mask can thus directly benefit from this technique to improve the contrast performance close to the axis.
Residual wavefront errors in optical elements limit the performance of coronagraphs. To improve their efficiency, different types of devices have been proposed to correct or calibrate these errors. ...In this article, we study one of these techniques proposed by Baudoz et al. (2006), and called Self-Coherent Camera (SCC). The principle of this instrument is based on the lack of coherence between the stellar light and the planet that is searched for. After recalling the principle of the SCC, we simulate its performance under realistic conditions and compare it with the performance of differential imaging.
To cite this article: R. Galicher, P. Baudoz, C. R. Physique 8 (2007).
La qualité de surface des optiques limite les performances des coronographes. La correction ou l'étalonnage de ces défauts optiques permet d'en améliorer l'efficacité. Nous étudions dans cet article une technique, proposée par Baudoz et al. (2006), qui permet d'étalonner les tavelures créées au plan focal par les défauts de surface d'onde. Le principe de cet instrument, appelé Self-Coherent Camera (SCC), est basé sur l'absence de cohérence entre l'étoile et la planète. Après un rappel du principe de la SCC, nous présentons une comparaison de ses performances avec celles de l'imagerie différentielle.
Pour citer cet article : R. Galicher, P. Baudoz, C. R. Physique 8 (2007).
High performance coronagraphic imaging of faint structures around bright stars at small angular separations requires fine control of tip, tilt, and other low order aberrations. When such errors occur ...upstream of a coronagraph they result in starlight leakage, which reduces the dynamic range of the instrument. This issue has been previously addressed for occulting coronagraphs by sensing the starlight before or at the coronagraphic focal plane. One such solution, the coronagraphic low order wave-front sensor (CLOWFS), uses a partially reflective focal plane mask to measure pointing errors for Lyot-type coronagraphs. To deal with pointing errors in low inner working angle phase mask coronagraphs which do not have a reflective focal plane mask, we have adapted the CLOWFS technique. This new concept relies on starlight diffracted by the focal plane phase mask being reflected by the Lyot stop towards a sensor which reliably measures low order aberrations such as tip and tilt. This reflective Lyot-based wavefront sensor is a linear reconstructor which provides high sensitivity tip-tilt error measurements with phase mask coronagraphs. Simulations show that the measurement accuracy of pointing errors with realistic post adaptive optics residuals are ≈10-2λ/D per mode at λ = 1.6 μm for a four quadrant phase mask. In addition, we demonstrate the open loop measurement pointing accuracy of 10-2λ/D at 638 nm for a four quadrant phase mask in the laboratory.
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