The pyramid wavefront sensor (P-WFS) has replaced the Shack-Hartmann (SH-) WFS as sensor of choice for high performance adaptive optics (AO) systems in astronomy because of its flexibility in pupil sampling, its dynamic range, and its improved sensitivity in closed-loop application. Usually, a P-WFS requires modulation and high precision optics that lead to high complexity and costs of the sensor. These factors limit the competitiveness of the P-WFS with respect to other WFS devices for AO correction in visual science. Here, we present a cost effective realization of AO correction with a non-modulated P-WFS and apply this technique to human retinal in vivo imaging using optical coherence tomography (OCT). P-WFS based high quality AO imaging was, to the best of our knowledge for the first time, successfully performed in 5 healthy subjects and benchmarked against the performance of conventional SH-WFS based AO. Smallest retinal cells such as central foveal cone photoreceptors are visualized and we observed a better quality of the images recorded with the P-WFS. The robustness and versatility of the sensor is demonstrated in the model eye under various conditions and in vivo by high-resolution imaging of other structures in the retina using standard and extended fields of view.
Advanced Adaptive Optics (AO) instruments have applications in ophthalmic imaging, Free-Space Optical Communications (FSOC) and the future generation of Extremely Large Telescopes (ELTs). These AO systems are designed to perform real-time corrections of dynamic wavefront aberrations. The corrections can be performed by converting wavefront measurements into Deformable Mirror (DM) actuator commands. The role of the DM is to mitigate aberrations by restoring a planar wavefront. Optimal DM actuator commands therefore require precise phase measurements across the entire wavefront. Reconstructing a wavefront from Wavefront Sensor (WFS) data is an inverse problem that depends on the type of WFS implemented. Nonlinear Fourier-type WFSs are included in the design of many current and upcoming AO systems. Conventionally, these sensors perform AO control based on simplifications and linearisations of the underlying models. However, in nonlinear regimes, approximation errors critically degrade image quality. This study looks at overcoming nonlinear wavefront sensing regimes by introducing a nonlinear, iterative algorithm for Fourier-type wavefront reconstruction. The algorithm used is well-known in the field of inverse problems. The underlying mathematical theory for modeling Fourier-type WFSs is provided, along with how these models can be used to perform nonlinear wavefront reconstruction. A significant advantage of the analysis presented is its generalised applicability to any Fourier-type sensor. The only input required is the mathematical expression for the optical element transfer function. The generalised and full mathematical model of Fourier-type WFSs is introduced in a Sobolev space setting. Necessary inputs are derived for the nonlinear iterative algorithms, such as Fréchet derivatives and adjoints. The generalised theory is then expanded to solve the inverse problem of wavefront reconstruction for all Fourier-type WFSs. Moreover, the study concentrates on the Pyramid WFS (PWFS) - one of the most well-known Fourier-type WFSs - and shows a Hilbert transform representation of the amplitude of the incoming light on its detector. The developed theory is demonstrated using a simulated PWFS to measure an example wavefront.
METIS is the European Extremely Large Telescope (ELT) 1st-generation Mid-Infrared ELT Imager and Spectrograph. It will offer spectroscopic, imaging and coronagraphic capabilities from 3 up to 13 microns with Adaptive-Optics correction.With its Final Design Review due late 2022 we report on the wavefront control strategy devised to meet the METIS science and technological requirements. Such strategy addresses challenging aspects as i) the appearance of differential petal piston modes in the presence of secondary mirror support struts caused either by numerical processing or the actual, physical low-wind effect, ii) the numerical pupil derotation and mis-reg compensation, iii) the adaptation to transient disturbance signals such as telescope-to-instrument handover control and iv) the compliance with constrained modal control of the pre-focal beam corrector mirrors (M4/M5).The overall METIS wavefront control strategy consists in a split approach cemented in a sequence of steps: 1) Tikhonov-regularised spatial wavefront estimation/reconstruction on a zonal Cartesian coordinate system tied to the pyramid (P-WFS) sampling pixel grid, 2) the regularised projection onto a global modal control space including correction of mis-registrations and rotation between the P-WFS coordinate grid and the ELTs M4/M5, and 3) the time-filtering through the application of proportional-integral control before converting to actuator commands readied for the ELTs collaborative TT off-loading scheme whilst avoiding hitting the mirrors constraints in amplitude, speed and force.We present physical-optics simulation results of the whole AO system obtained with prototyped instances of the real-time and soft-real-time computers including sensitivity analysis with respect to observational, atmospheric, non-atmospheric (telescope-intrinsic such as wind-induced low-order modes comprising tip-tilt) and instrument-specific conditions and disturbances.An error budget is put together that meets the METIS science requirements in terms of wavefront error with reassuring margins thus endorsing the strategy devised.
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