Ralf Flicker

Gemini multiconjugate adaptive optics system review - I. Design, trade-offs and integration

The Gemini multiconjugate adaptive optics system (GeMS) at the Gemini South telescope in Cerro Pachon is the first sodium-based multilaser guide star (LGS) adaptive optics system. It uses five LGSs and two deformable mirrors to measure and compensate for atmospheric distortions. The GeMS project started in 1999, and saw first light in 2011. It is now in regular operation, producing images close to the diffraction limit in the near-infrared, with uniform quality over a field of view of two square arcminutes. This paper is the first one in a two-paper review of GeMS. It describes the system, explains why and how it was built, discusses the design choices and trade-offs, and presents the main issues encountered during the course of the project. Finally, we briefly present the results of the system first light.

Principles, limitations, and performance of multiconjugate adaptive optics

Multi-Conjugate Adaptive Optics (MCAO) holds the promise of moderate to large adaptively compensated field of view with uniform image quality. This paper is a first effort to analyze the fundamental limitations of such systems, and that are mainly related to the finite number of deformable mirrors and guide stars. We demonstrate that the ultimate limitation is due to the vertical discretization of the correction. This effect becomes more severe quite rapidly with increasing compensated field of view or decreasing wavelength, but does not depend at first order on the telescope aperture. We also discuss limitations associated with the use of laser guide stars and ELT related issues.

Comparison of multiconjugate adaptive optics configurations and control algorithms for the Gemini-South 8-m telescope

This paper focuses on two main categories of the multiconjugate adaptive optics (MCAO) parameter space for performance optimization: the geometrical configuration of guide stars and deformable mirrors (DMs), and the wavefront reconstructors. From the first category it is shown how, for a fixed reconstructor and imaging wavelength, the performance metrics with a few important exceptions improve with an increasing number of (1) DMs, (2) actuators per DM and (3) guide stars. The metrics are seen to degrade with (1) an increasing field of compensation and (2) DM conjugation altitude mismatch with the significant turbulent atmospheric layers. In the second category, this study also compares the performance with a fixed MCAO configuration using the least- square estimator (LSE) and the maximum a posteriori estimator (MAP) for wavefront reconstruction. The MAP is shown to perform significantly better than the LSE at low or intermediate signal-to-noise ratios (SNRs), and somewhat better even in the absence of noise due to its a priori knowledge of the phase statistics.

Sequence of phase correction in multiconjugate adaptive optics.

The way in which the sequence of phase correction impinges on performance in multiconjugate adaptive optics systems is described. When multiple phase modulators with different conjugate ranges are used, the conjugate images of the phase modulators in the atmosphere must be reimaged in the reverse order post focus, and adaptive phase correction applied in this sequence for perfect amplitude and phase cancellation. Performing the correction without relay optics results in residual amplitude and phase aberrations. It is shown, by Monte Carlo simulations of Fresnel propagation, that the effects of wave optical propagation become nonnegligible at visible wavelengths and for large air masses.

Anisoplanatic deconvolution of adaptive optics images.

A modified method for maximum-likelihood deconvolution of astronomical adaptive optics images is presented. By parametrizing the anisoplanatic character of the point-spread function (PSF), a simultaneous optimization of the spatially variant PSF and the deconvolved image can be performed. In the ideal case of perfect information, it is shown that the algorithm is able to perfectly cancel the adverse effects of anisoplanatism down to the level of numerical precision. Exploring two different modes of deconvolution (using object bases of pixel values or stellar field parameters), we then quantify the performance of the algorithm in the presence of Poissonian noise for crowded and noncrowded stellar fields.

Tilt anisoplanatism in laser-guide-star-based multiconjugate adaptive optics - Reconstruction of the long exposure point spread function from control loop data

A method is presented for estimating the long exposure point spread function (PSF) degradation due to tilt aniso- planatism in a laser-guide-star-based multiconjugate adaptive optics systems from control loop data. The algorithm is tested in numerical Monte Carlo simulations of the separately driven low-order null-mode system, and is shown to be robust and accurate with less than 10% relative error in both H and K bands down to a natural guide star (NGS) magnitude of mR= 21, for a symmetric asterism with three NGS on a 30 arcsec radius. The H band limiting magnitude of the null-mode system due to NGS signal-to-noise ratio and servo-lag was estimated previously to mR = 19. At this magnitude, the relative errors in the reconstructed PSF and Strehl are here found to be less than 5% and 1%, suggesting that the PSF retrieval algorithm will be applicable and reliable for the full range of operating conditions of the null-mode system.

Efficient first-order performance estimation for high-order adaptive optics systems

It is shown how first-order performance estimation of high-orderadaptive optics (AO) systems may be efficiently implemented in a hybridnumerical simulation by the use of 1) sparse matrix techniques forwavefront reconstruction, 2) undersampled pupil-plane turbulence-inducedaberrations, and 3) analytical models that compensate - in the limit ofinfinite exposure time - for the errors introduced by undersampling. Asparse preconditioned conjugate gradient (PCG) method is applied forwavefront reconstruction, and it is seen that acceptable AO performancemay be achieved at a relative error tolerance of 0.01, at which thecomputational cost of the sparse PCG scales approximately asO(n1.2), where n is the number of actuators in the system.Estimations of adaptive optics performance for extremely high-ordersystems are presented, including multi-conjugate andlaser-guide-star-based systems. The scaling laws for AO performance withtelescope diameter D and turbulence outer scale L0 coupled with the useof laser guide stars are also investigated. It is shown that a single ora small number of laser guide stars (LGS) may still provide a usefullevel of compensation to telescopes with diameters in the range 30-100m, if L0 is on the order of or smaller than D. The deviations fromKolmogorov theory are also investigated for LGS AO. To the best of theauthors knowledge, results presented for a n=65 282 case represent thelargest multi-conjugate adaptive optics system simulated in full todate. (Less)

Efficient simulation of adaptive optics technologies for the Euro50 telescope

An efficient adaptive optics (AO) simulation code was developed, which enables first-order simulations of extremely high-order systems. The Monte-Carlo-type code employs a sparse conjugate gradient algorithm for wavefront reconstruction, and a separation of spatial frequencies into two domains to economize on the number crunching. High-order multi-conjugate adaptive optics (MCAO) systems are thereby readily simulated on a single standard PC. The code is presently being applied to MCAO design studies for the Euro50 extremely large telescope (ELT), addressing a number of issues not previously subjected to realistic simulation due to the excessive computational load. We report in this paper on the latest results obtained from simulating two specific aspects of the Euro50 AO system: turbulence prediction and laser guide star (LGS) beacon sythesis. The two studies presented here are represetative examples of a number of technology studies being enabled by the new fast simulation codes.

Some aspects of science with a 50-m AO telescope

Among the science challenges of the Extremely Large Telescopes (ELTs), four object types are studied for performance with a 50 m ELT with adaptive optics (AO), Euro50. Emphasis is on planetary systems and very distant objects. For planetary systems and their evolution, we examine high resolution imaging of the nuclei of comets and high-resolution imaging, photometry and low and intermediate resolution spectroscopy of Kuiper-Belt objects. Imaging of Earth-like planets is discussed. The very high contrast imaging necessary for these purposes is discussed together with the relevant error sources. Finally, photometry and classification of supernovae is discussed and examined. The performance of a 50 m AO ELT is compared to corresponding data obtainable with current VLTs equipped with AO.