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Register a new userLaser Guide Stars for Extremely Large Telescopes: Efficient Shack-Hartmann Wavefront Sensor Design using Weighted center-of-gravity algorithm
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Over the last few years increasing consideration has been given to the study of Laser Guide Stars (LGS) for the measurement of the disturbance introduced by the atmosphere in optical and near-infrared astronomical observations from the ground. A possible method for the generation of a LGS is the excitation of the Sodium layer in the upper atmosphere at approximately 90 km of altitude. Since the Sodium layer is approximately 10 km thick, the artificial reference source looks elongated, especially when observed from the edge of a large aperture. The spot elongation strongly limits the performance of the most common wavefront sensors. The centroiding accuracy in a Shack-Hartmann wavefront sensor, for instance, decreases proportionally to the elongation (in a photon noise dominated regime). To compensate for this effect a straightforward solution is to increase the laser power, i.e. to increase the number of detected photons per subaperture. The scope of the work presented in this paper is twofold: an analysis of the performance of the Weighted Center of Gravity algorithm for centroiding with elongated spots and the determination of the required number of photons to achieve a certain average wavefront error over the telescope aperture.
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Sodium Laser Guide Stars (LGSs) are elongated sources due to the thickness and the finite distance of the sodium layer. The fluctuations of the sodium layer altitude and atom density profile induce errors on centroid measurements of elongated spots, and generate spurious optical aberrations in closed--loop adaptive optics (AO) systems. According to an analytical model and experimental results obtained with the University of Victoria LGS bench demonstrator, one of the main origins of these aberrations, referred to as LGS aberrations, is not the Centre-of-Gravity (CoG) algorithm itself, but the thresholding applied on the pixels of the image prior to computing the spot centroids. A new thresholding method, termed ``radial thresholding, is presented here, cancelling out most of the LGS aberrations without altering the centroid measurement accuracy.
The new class of large telescopes, as the future ELT, are designed to work with Laser Guide Star (LGS) tuned to a resonance of atmosphere sodium atoms. This wavefront sensing technique presents complex issues for an application to big telescopes due to many reasons mainly linked to the finite distance of the LGS, the launching angle, Tip-tilt indetermination and focus anisoplanatism. The implementation of a laboratory Prototype for LGS wavefront sensor (WFS) at the beginning of the phase study of MAORY, the Multi-conjugate Adaptive Optics RelaY for the ELT first light, has been indispensable to investigate specific mitigation strategies to the LGS WFS issues. This paper shows the test results of LGS WFS Prototype under different working conditions. The accuracy within which the LGS images are generated on the Shack-Hartmann (SH) WFS has been cross-checked with the MAORY simulation code. The experiments show the effect of noise on the centroiding precision, the impact of LGS image truncation on the wavefront sensing accuracy as well as the temporal evolution of sodium density profile and LGS image under-sampling.
In typical adaptive optics applications, the atmospheric residual turbulence affects the wavefront sensor response decreasing its sensitivity. On the other hand, wavefront sensors are generally calibrated in diffraction limited condition, and, so, the interaction matrix sensitivity differs from the closed loop one. The ratio between the two sensitivities, that we will call the sensitivity loss factor, has to be estimated to retrieve a well-calibrated measurement. The spots size measurement could give a good estimation, but it is limited to systems with spots well sampled and uniform across the pupil. We present an algorithm to estimate sensitivity loss factor from closed loop data, based on the known parameters of the closed loop transfer functions. Here we preferred for simplicity the Shack-Hartmann WFS, but the algorithm we propose can be extended to other WFSs.
In recent years, detectors with sub-electron readout noise have been used very effectively in astronomical adaptive optics systems. Here, we compare readout noise models for the two key faint flux level detector technologies that are commonly used: EMCCD and scientific CMOS (sCMOS) detectors. We find that in almost all situations, EMCCD technology is advantageous, and that the commonly used simplified model for EMCCD readout is appropriate. We also find that the commonly used simple models for sCMOS readout noise are optimistic, and recommend that a proper treatment of the sCMOS rms readout noise probability distribution should be considered during instrument performance modelling and development.
We investigate the improvements in Shack-Hartmann wavefront sensor image processing that can be realised using total variation minimisation techniques to remove noise from these images. We perform Monte-Carlo simulation to demonstrate that at certain signal-to-noise levels, sensitivity improvements of up to one astronomical magnitude can be realised. We also present on-sky measurements taken with the CANARY adaptive optics system that demonstrate an improvement in performance when this technique is employed, and show that this algorithm can be implemented in a real-time control system. We conclude that total variation minimisation can lead to improvements in sensitivity of up to one astronomical magnitude when used with adaptive optics systems.
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