Sparse-matrix wavefront reconstruction: simulations and experiments

MacMartin, Douglas G.; Troy, Mitchell; Brack, Gary L.; Burruss, Rick; Dekany, Richard

doi:10.1117/12.457134

Cited by 9 publications

(13 citation statements)

References 9 publications

(15 reference statements)

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“…Simulations use the actuator/sensor configuration of the Palomar Observatory AO system; the local and hierarchic algorithms have also been experimentally validated at Palomar. 14 The approach is also applicable to a broader class of problems involving many sensors and actuators, including control of a segmented primary mirror.…”

Section: Introductionmentioning

confidence: 99%

Local, hierarchic, and iterative reconstructors for adaptive optics

MacMartin

2003

J. Opt. Soc. Am. A

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Adaptive optics systems for future large optical telescopes may require thousands of sensors and actuators. Optimal reconstruction of phase errors using relative measurements requires feedback from every sensor to each actuator, resulting in computational scaling for n actuators of n 2 . The optimum local reconstructor is investigated, wherein each actuator command depends only on sensor information in a neighboring region. The resulting performance degradation on ''global'' modes is quantified analytically, and two approaches are considered for recovering global performance. Combining local and global estimators in a two-layer hierarchic architecture yields computations scaling with n 4/3 ; extending this approach to multiple layers yields linear scaling. An alternative approach that maintains a local structure is to allow actuator commands to depend on both local sensors and prior local estimates. This iterative approach is equivalent to a temporal low-pass filter on global information and gives a scaling of n 3/2 . The algorithms are simulated by using data from the Palomar Observatory adaptive optics system. The analysis is general enough to also be applicable to active optics or other systems with many sensors and actuators.

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Section: Introductionmentioning

confidence: 99%

Local, hierarchic, and iterative reconstructors for adaptive optics

MacMartin

2003

J. Opt. Soc. Am. A

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“…While this does not allow us to benefit from a computational speedup, it allows rapid sequential testing of different algorithms. The same approach was used in [4,7]. The 3217 actuator PALM-3000 system currently in development [12] is being designed with sufficient computation to allow full VMM reconstructors; this will permit similar experiments on a much larger system to be conducted in the future.…”

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confidence: 99%

“…Many faster methods have been proposed and analyzed using computer simulations. Examples include the conjugate-gradient (CG) [1], Fourier-domain (FD) [2], blended FD-CG [3], and sparse methods [4]. Recently, it was shown through simulation that a single-iteration multigrid (SIMG) method is as effective as CG methods for both multiconjugate [5] and single-conjugate adaptive optics (MCAO and SCAO, respectively) [6].…”

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confidence: 99%

“…[6]. In this Letter, we detail the experimental validation of this work on a real SCAO system.Two computationally efficient methods that have previously been tested on-sky are the FD method [7] and a sparse method [4]. These methods are O͑n log n͒.…”

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confidence: 99%

“…Two computationally efficient methods that have previously been tested on-sky are the FD method [7] and a sparse method [4]. These methods are O͑n log n͒.…”

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Experimental validation of single-iteration multigrid wavefront reconstruction at the Palomar Observatory

et al. 2008

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Single-iteration multigrid (SIMG) wavefront reconstruction schemes were implemented and validated on the adaptive optics system at the Hale 5.1 m telescope at the Palomar Observatory. Results indicate that even the simplest such method produces a performance indistinguishable from that of the standard leastsquares reconstructor for both bright and dim guide stars. SIMG provides a dramatic reduction in computational cost when compared to vector-matrix multiplication and can be implemented in parallel, making it the obvious choice for reconstruction in future large-scale adaptive optics systems. [6]. In this Letter, we detail the experimental validation of this work on a real SCAO system.Two computationally efficient methods that have previously been tested on-sky are the FD method [7] and a sparse method [4]. These methods are O͑n log n͒. The SIMG is O͑n͒ and shows no performance degradation when compared to the leastsquares reconstructor.A good model for the wavefront sensor (WFS) is [6,8] where x is the wavefront phase, y are the measurements, v is white noise, and G is a sparse influence matrix. The least-squares reconstruction matrix is found by taking the pseudoinverse. In practice, we can compute it by evaluating K = ͑G T G + ⑀I͒ −1 G T for a small ⑀. This ensures that unobservable modes such as piston and waffle are zeroed out. The SIMG method [6] uses a single multigrid sweep with an initial guess of 0 to obtain an approximate solution to the equationIf the measurements are taken in open-loop, x is the wavefront phase. In this case, x 0 = 0 is a bad guess, so multiple iterations are required to achieve acceptable convergence [6]. However, when we operate in closedloop, x is the change in wavefront phase between successive time steps, and only one iteration is required. Our tests were performed on the Palomar Adaptive Optics (PALAO) system on the Hale 5.1 m telescope [9]. The PALAO system has a deformable mirror (DM) with 241 active actuators and a ShackHartmann WFS array with 256 subapertures, producing a total of 512 measurements. The DM and WFS are aligned in a Fried geometry.The AO system collects measurements y at up to 2 kHz. Tip and tilt are removed from the wavefront using a fast-steering mirror (FSM) and proportionalintegral (PI) controller. The rest of the wavefront offset x is reconstructed by VMM: x = Ky. This estimate is fed back through a second PI loop to the DM. The closed-loop corrected wavefront is split using a dichroic mirror. The near-infrared portion is sent to the Palomar High Angle Resolution Observer (PHARO) [10]. See Fig. 1.To implement the SIMG on the PALAO system, virtual actuators were added to fill the 17ϫ 17 grid containing the circular actuator arrangement, and the influence matrix G was augmented with zeros appropriately, as in [3]. The new system is y = Ḡ x, where Ḡ is 512ϫ 289 instead of 512ϫ 241. This new system is September 15, 2008 / Vol. 33, No. 18 / OPTICS LETTERS 2047 0146-9592/08/182047-3/$15.00

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PALM-3000: EXOPLANET ADAPTIVE OPTICS FOR THE 5 m HALE TELESCOPE

Dekany

Roberts

Burruss

et al. 2013

ApJ

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We describe and report first results from PALM-3000, the second-generation astronomical adaptive optics (AO) facility for the 5.1 m Hale telescope at Palomar Observatory. PALM-3000 has been engineered for high-contrast imaging and emission spectroscopy of brown dwarfs and large planetary mass bodies at near-infrared wavelengths around bright stars, but also supports general natural guide star use to V ≈ 17. Using its unique 66 × 66 actuator deformable mirror, PALM-3000 has thus far demonstrated residual wavefront errors of 141 nm rms under ∼1 seeing conditions. PALM-3000 can provide phase conjugation correction over a 6. 4 × 6. 4 working region at λ = 2.2 μm, or full electric field (amplitude and phase) correction over approximately one-half of this field. With optimized back-end instrumentation, PALM-3000 is designed to enable 10 −7 contrast at 1 angular separation, including post-observation speckle suppression processing. While continued optimization of the AO system is ongoing, we have already successfully commissioned five back-end instruments and begun a major exoplanet characterization survey, Project 1640.

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Sparse-matrix wavefront reconstruction: simulations and experiments

Cited by 9 publications

References 9 publications

Local, hierarchic, and iterative reconstructors for adaptive optics

Local, hierarchic, and iterative reconstructors for adaptive optics

Experimental validation of single-iteration multigrid wavefront reconstruction at the Palomar Observatory

PALM-3000: EXOPLANET ADAPTIVE OPTICS FOR THE 5 m HALE TELESCOPE

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