Split atmospheric tomography using laser and natural guide stars

Abstract

Laser guide star (LGS) atmospheric tomography is described in the literature as integrated minimum-variance tomographic wavefront reconstruction from a concatenated wavefront-sensor measurement vector consisting of many high-order, tip/tilt (TT)-removed LGS measurements, supplemented by a few low-order natural guide star (NGS) components essential to estimating the TT and tilt anisoplanatism (TA) modes undetectable by the TT-removed LGS wavefront sensors (WFSs). The practical integration of these NGS WFS measurements into the tomography problem is the main subject of this paper. A split control architecture implementing two separate control loops driven independently by closed-loop LGS and NGS measurements is proposed in this context. Its performance is evaluated in extensive wave optics Monte Carlo simulations for the Thirty Meter Telescope (TMT) LGS multiconjugate adaptive optics (MCAO) system, against the delivered performance of the integrated control architecture. Three iterative algorithms are analyzed for atmospheric tomography in both cases: a previously proposed Fourier domain preconditioned conjugate gradient (FDPCG) algorithm, a simple conjugate gradient (CG) algorithm without preconditioning, and a novel layer-oriented block Gauss-Seidel conjugate gradient algorithm (BGS-CG). Provided that enough iterations are performed, all three algorithms yield essentially identical closed-loop residual RMS wavefront errors for both control architectures, with the caveat that a somewhat smaller number of iterations are required by the CG and BGS-CG algorithms for the split approach. These results demonstrate that the split control approach benefits from (i) a simpler formulation of minimum-variance atmospheric tomography allowing for algorithms with reduced computational complexity and cost (processing requirements), (ii) a simpler, more flexible control of the NGS-controlled modes, and (iii) a reduced coupling between the LGS-and NGS-controlled modes. Computation and memory requirements for all three algorithms are also given for the split control approach for the TMT LGS AO system and appear feasible in relation to the performance specifications of current hardware technology.

Increased sky coverage with optimal correction of tilt and tilt-anisoplanatism modes in laser-guide-star multiconjugate adaptive optics

“…The MV controller proposed here fits on the framework of ad hoc split tomography (AHST), in which the LGS and NGS correction loops are driven independently-adopted as the baseline option for NFIRAOS [19]. Such a scheme is known to be suboptimal, which led to a joint estimate of the NGS LGS modes in the so called minimum-variance split tomography (MVST) scheme [20,21].…”

Reconstruction Strategies for GeMS

“…In a first approximation, we will assume that the LGS provide a true measurement of the atmospheric Tip-Tilt, and that no NGS are needed. In fact, the NGS low order part is supposed to be orthogonal to the LGS correction, and thus can be considered separately [10]. This assumption will then affect the absolute value of the performance given below, but it should not modify the relative trends.…”

Simulation model based approach for long exposure atmospheric point spread function reconstruction for laser guide star multiconjugate adaptive optics

“…The system implements separate high-order LGS and low-order NGS servo loops [26]. The LGS loop runs a tomographic minimum variance wavefront reconstruction algorithm at 800 Hz from measurements from six high-order LGS Shack-Hartmann WFSs (each of order 60 × 60).…”