Multireference adaptive optics and problem of widening field-of-view (original) (raw)
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Multiple field of view layer-oriented adaptive optics
Astronomy and Astrophysics, 2002
In layer-oriented adaptive optics, multiconjugation is performed in a much more efficient way than conventional wavefront sensing. This improved efficiency is impressive for high altitude layers and moderate for ground ones. On the other hand, high altitude layers can be covered with only a limited field of view (where one can search for natural guide stars) while for ground layers the usable field of view is limited essentially by practical reasons. We introduce the further concept of multiple field of view layer-oriented adaptive optics where a combination of sampling and covered field leads easily to sky coverages for 8 m class telescopes that nearly approach the whole sky with the usage solely of natural guide stars. The extension of the concept to much larger apertures is also discussed.
Optimal wave-front reconstruction strategies for multiconjugate adaptive optics
Journal of the Optical Society of America A, 2001
We propose an optimal approach for the phase reconstruction in a large field of view (FOV) for multiconjugate adaptive optics. This optimal approach is based on a minimum-mean-square-error estimator that minimizes the mean residual phase variance in the FOV of interest. It accounts for the C n 2 profile in order to optimally estimate the correction wave front to be applied to each deformable mirror (DM). This optimal approach also accounts for the fact that the number of DMs will always be smaller than the number of turbulent layers, since the C n 2 profile is a continuous function of the altitude h. Links between this optimal approach and a tomographic reconstruction of the turbulence volume are established. In particular, it is shown that the optimal approach consists of a full tomographic reconstruction of the turbulence volume followed by a projection onto the DMs accounting for the considered FOV of interest. The case where the turbulent layers are assumed to match the mirror positions [model-approximation (MA) approach], which might be a crude approximation, is also considered for comparison. This MA approach will rely on the notion of equivalent turbulent layers. A comparison between the optimal and MA approaches is proposed. It is shown that the optimal approach provides very good performance even with a small number of DMs (typically, one or two). For instance, good Strehl ratios (greater than 20%) are obtained for a 4-m telescope on a 150-arc sec ϫ 150-arc sec FOV by using only three guide stars and two DMs.
Efficient wavefront sensing for space-based adaptive optics
Journal of Astronomical Telescopes, Instruments, and Systems, 2020
Future large space telescopes will be equipped with adaptive optics (AO) to overcome wavefront aberrations and achieve high contrast for imaging faint astronomical objects, such as earth-like exoplanets and debris disks. In contrast to AO that is widely used in ground telescopes, space-based AO systems will use focal plane wavefront sensing to measure the wavefront aberrations. Focal plane wavefront sensing is a class of techniques that reconstruct the light field based on multiple focal plane images distorted by deformable mirror (DM) probing perturbations. In this paper, we report an efficient focal plane wavefront sensing approach for space-based AO which optimizes the DM probing perturbation and thus also the integration time for each image. Simulation of the AO system equipped with a vortex coronagraph has demonstrated that our new approach enables efficient information acquisition and significantly reduces the time needed for achieving high contrast in space.
Angular anisoplanatism in laser guide star adaptive optics
SPIE Proceedings, 2006
The image quality obtained using laser guide star adaptive optics (LGS AO) is degraded by the fact that the wavefront aberrations experienced by light from the LGS and from the science object differ. In this paper we derive an analytic expression for the variance of the difference between the two wavefronts as a function of angular distance between the LGS and the science object. This error is a combination of focal anisoplanatism and angular anisoplanatism. We show that the wavefront error introduced by observing a science object displaced from the guide star is smaller for LGS AO systems than for natural guide star AO systems.
Angular anisoplanatism in laser guide star adaptive optics
Advances in Adaptive Optics II, 2006
The image quality obtained using laser guide star adaptive optics (LGS AO) is degraded by the fact that the wavefront aberrations experienced by light from the LGS and from the science object differ. In this paper we derive an analytic expression for the variance of the difference between the two wavefronts as a function of angular distance between the LGS and the science object. This error is a combination of focal anisoplanatism and angular anisoplanatism. We show that the wavefront error introduced by observing a science object displaced from the guide star is smaller for LGS AO systems than for natural guide star AO systems.
Journal of the Optical Society of America A, 2009
The performances of various estimators for wavefront sensing applications such as adaptive optics (AO) are compared. Analytical expressions for the bias and variance terms in the mean squared error (MSE) are derived for the minimum-norm maximum likelihood (MNML) and the maximum a posteriori (MAP) reconstructors. The MAP estimator is analytically demonstrated to yield an optimal trade-off that reduces the MSE, hence leading to a better Strehl ratio. The implications for AO applications are quantified thanks to simulations on 8-m-and 42-m-class telescopes. We show that the MAP estimator can achieve twice as low MSE as MNML methods do. Large AO systems can thus benefit from the high quality of MAP reconstruction in O͑n͒ operations, thanks to the fast fractal iterative method (FrIM) algorithm (Thiébaut and Tallon, submitted to J. Opt. Soc. Am. A).
Astronomy & Astrophysics, 2002
In layer-oriented adaptive optics, multiconjugation is performed in a much more efficient way than conventional wavefront sensing. This improved efficiency is impressive for high altitude layers and moderate for ground ones. On the other hand, high altitude layers can be covered with only a limited field of view (where one can search for natural guide stars) while for ground layers the usable field of view is limited essentially by practical reasons. We introduce the further concept of multiple field of view layer-oriented adaptive optics where a combination of sampling and covered field leads easily to sky coverages for 8 m class telescopes that nearly approach the whole sky with the usage solely of natural guide stars. The extension of the concept to much larger apertures is also discussed.
Exact wavefront correction in adaptive optics
Proceedings of the 16th IFAC World Congress, 2005, 2005
In this work we consider an adaptive optical system in which a deformable mirror is controlled to compensate for random wavefront disturbances. For most systems of this type, the shape of the mirror is taken as a linear function of the wavefront error, leading to satisfactory results in linear regimes. Here, the geometric shape of the mirror leading to a perfect correction of the wavefront is derived. Next, a control is designed to reach that geometric shape when the deformable mirror is a membrane mirror with electrostatic actuators. Numerical simulations illustrating the improvements supplied by the geometric approach are also reported here.
The Flexible Adaptive Optics Concept
Proceedings of the Adaptive Optics for Extremely Large Telescopes 5
The availability of small deformable mirrors with large number of actuators and stroke, on one hand, and versatile wavefront sensors (pyramid WFS), on the other, allows the development of AO systems whose equivalent pitch can be adapted freely to the guide stars and seeing conditions. On moderately large telescopes (4 m class) this flexibility allows a performance (Strehl and limiting magnitude) always better than what a classical, frozen Shack-Hartman design would allow. Moreover, adding several natural guide star wavefront sensors, we believe a single system could do anything from extreme AO to GLAO, i.e using the same hardware, and adapting the control software parameters to the observation mode. Taking advantage of such a system requires the use of a zoom optics in the imager in order to optimally match the plate scale with the PSF. We demonstrate that sub-optimal pixel size would result in a significant loss in term of science data reduction, in particular object detectability. The raison d'être of such a versatile system is to popularize AO in community of astronomers not familiar with it, by allowing, on the same telescope, high resolution as well as seeing improved observations. This is particularly important for countries where very few large telescopes are available. On the long term, we think that most moderate size telescopes will have this sort of flexible multipurpose AO systems as a default. The flexible AO concept will be implemented on the new 4 m Turkish telescope, DAG.
New challenges for adaptive optics: extremely large telescopes
Monthly Notices of the Royal Astronomical Society, 2000
The performance of an adaptive optics (AO) system on a 100-m diameter ground-based telescope working in the visible range of the spectrum is computed using an analytical approach. The target Strehl ratio of 60 per cent is achieved at 0.5 mm with a limiting magnitude of the AO guide source near R magnitude , 10Y at the cost of an extremely low sky coverage. To alleviate this problem, the concept of tomographic wavefront sensing in a wider field of view using either natural guide stars (NGS) or laser guide stars (LGS) is investigated. These methods use three or four reference sources and up to three deformable mirrors, which increase up to 8-fold the corrected field size (up to 60 arcsec at 0.5 mm). Operation with multiple NGS is limited to the infrared (in the J band this approach yields a sky coverage of 50 per cent with a Strehl ratio of 0.2). The option of open-loop wavefront correction in the visible using several bright NGS is discussed. The LGS approach involves the use of a faint R , 22 NGS for low-order correction, which results in a sky coverage of 40 per cent at the Galactic poles in the visible.