Adaptive optics (AO) has become an indispensable tool at ground-based solar telescopes. AO enables the ground-based observer to overcome the adverse effects of atmospheric seeing and obtain diffraction limited observations. Over the last decade adaptive optics systems have been deployed at major ground-based solar telescopes and revitalized ground-based solar astronomy. The relatively small aperture of solar telescopes and the bright source make solar AO possible for visible wavelengths where the majority of solar observations are still performed. Solar AO systems enable diffraction limited observations of the Sun for a significant fraction of the available observing time at ground-based solar telescopes, which often have a larger aperture than equivalent space based observatories, such as HINODE. New ground breaking scientific results have been achieved with solar adaptive optics and this trend continues. New large aperture telescopes are currently being deployed or are under construction. With the aid of solar AO these telescopes will obtain observations of the highly structured and dynamic solar atmosphere with unprecedented resolution. This paper reviews solar adaptive optics techniques and summarizes the recent progress in the field of solar adaptive optics. An outlook to future solar AO developments, including a discussion of Multi-Conjugate AO (MCAO) and Ground-Layer AO (GLAO) will be given.

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Solar Adaptive Optics

The National Solar Observatory at Sacramento Peak is developing adaptive optics (AO) for solar astronomy. We are currently implementing a low order adaptive optics system that will correct approximately 20 Zernike modes. The system design permits future expansion future expansion to about 80 Zernike modes. We are using a correlating Shack-Hartmann wavefront sensor. Since a point source is generally not available on the sun, wavefront tilts have to be derived by applying correlation techniques to images of extended objects, such as granulation, probes and sunspots.

Solar adaptive optics at the National Solar Observatory

The phase diversity technique is studied as a wave-front sensor to be implemented with widely extended sources. The wave-front phase expanded on the Zernike polynomials is estimated from a pair of images (in focus and out of focus) by use of a maximum-likelihood approach. The propagation of the photon noise in the images on the estimated phase is derived from a theoretical analysis. The covariance matrix of the phase estimator is calculated, and the optimal distance between the observation planes that minimizes the noise propagation is determined. The phase error is inversely proportional to the number of photons in the images. The noise variance on the Zernike polynomials increases with the order of the polynomial. These results are confirmed with both numerical and experimental validations. The influence of the spectral bandwidth on the phase estimator is also studied with simulations.

Noise propagation in wave-front sensing with phase diversity

We present a progress report of the solar adaptive optics (AO) development program at the National Solar Observatory (NSO) and the Big Bear Solar Observatory (BBSO). Examples of diffraction-limited observations obtained with the NSO low-order solar adaptive optics system at the Dunn Solar Telescope (DST) are presented. The design of the high order adaptive optics systems that will be deployed at the DST and the BBSO is discussed. The high order systems will provide diffraction-limited observations of the Sun in median seeing conditions at both sites.

Solar adaptive optics: a progress report

La turbulence atmospherique perturbe la propagation de la lumiere provenant d'objets astronomiques. Au foyer du telescope, on observe sur l'image obtenue une perte de resolution angulaire. L'optique adaptative permet de retrouver la resolution theorique du telescope. L'Optique Adaptative Multiconjuguee devrait de plus permettre d'elagir le champ isoplanetique en sortie du systeme. Dans le contexte des futurs systemes d'Optique Adaptative (a haute dynamique ou a grand champ), la loi de commande du systeme doit etre choisie de facon a optimiser les performances. Elle doit notamment permettre de minimiser l'erreur temporelle ou l'anisoplanetisme residuel. Nous proposons une commande optimale au sens de la variance de phase residuelle minimale pour l'Optique Adaptative Classique et l'Optique Adaptative Multiconjuguee. L'approche que nous presentons est basee sur une estimation de la phase turbulente par filtrage de Kalman suivie d'une commande par retour d'etat, developpes dans un formalisme d'etat. Cette approche prend en compte les connaissances a priori disponibles sur le bruit de mesure, le profil de turbulence en altitude, la statistique de Kolmogorov ou encore la vitesse d'evolution de la turbulence. Nous quantifions le gain apporte par cette loi de commande vis-a-vis d'une approche traditionnelle par des simulations numeriques en OA classique et multiconjuguee et nous discutons la robustesse des performances approche optimale. Nous presentons egalement une mise en place d'une validation experimentale sur le banc BOA de l'Onera.

https://tel.archives-ouvertes.fr/tel-00004690/document

J. Fontanella

Papers

Wavefront Sensing from Extended Sources