Astronomers proposed System Parameters Recurrent INvasive Tracking, a fast and least-cost online calibration strategy for adaptive optics
A large fraction of ground-based astronomical observations relies on the performance of Adaptive Optics (AO) systems that allow to compensate in real time for the wave-front aberrations induced by the atmospheric turbulence. The principle of an AO system consists in measuring signals related to the phase using a Wave-Front-Sensor (WFS) that are converted by a Real-Time Computer (RTC) into commands to apply on a Deformable Mirror (DM) that cancels out the optical aberrations. This loop is usually operated in a feedback-loop running at least ten times faster than the typical evolution rate of the atmospheric turbulence.
Within a few years, the new generation of telescopes with diameters up to 39 meters, the Extremely Large Telescopes (ELT), will start their scientific operations. These giants will address fundamental astrophysical science cases such as direct imaging and characterization of rocky exoplanets located close to their orbiting star or the study of bulk and evolution of the first galaxies. The scientific potential of these new telescopes relies on challenging new AO systems’ features such as DM integrated inside the telescope itself, turning the telescopes into adaptive telescopes. The colossal size of these large adaptive telescopes and the complexity of their scientific instruments compel us to a complete rethinking, in order to improve the overall performance, but more specifically the sensitivity and the robustness of the AO systems to maximize the astrophysical return of AO-assisted instruments.
In particular, the ESO-ELT, will provide a challenging environment for the AO systems. First of all, the calibration of a large number of degrees of freedom (around 5000 actuators) with no external calibration source will be required. Furthermore, the use of a pre-focal DM far from the AO instruments with moving elements in the optical path may lead to regular evolution of the DM/WFS registration during the observations (rotation, shifts or higher order of pupil distortion of the DM actuators grid with respect to the WFS subapertures). These so-called mis-registrations have to be monitored and compensated as they will highly affect the AO performance or could create loop instabilities that will jeopardise the scientific observations
To overcome this challenge, it will be necessary to regularly monitor and compensate for these DM/WFS mis-registrations either by physically re-aligning some optical components or by updating the control matrix of the system. Now, Heritier and colleagues presented a new strategy to track mis-registrations using a pseudo-synthetic model of the AO system. The method is based on an invasive approach where signals are acquired on-sky, before or during the scientific operations, and fed to the model to extract the mis-registration parameters. They demonstrated that the method is applicable for both SH-WHS and PWFS as the algorithm includes a compensation for the PWFS optical gains.
“We introduced a method to compute the most sensitive modes to these mis-registrations that allows to reduce the number of degrees of freedom required by the algorithm and minimize the impact on the scientific performance.”
Their research is oriented to minimize the number of modes required by the algorithm to estimate accurately the mis-registration parameters. This is by identifying the most sensitive modes to the mis-registrations using a Principal Component Analysis of the sensitivity interaction matrices.
Using on-sky push-pull measurements, they investigated the accuracy achieved with only 3 PCA modes exploring different observing conditions. They found that, even in very low flux conditions, it is always possible to tune the push/pull amplitude and the measurement time to reach a very good estimation accuracy. In addition, it has been shown that the impact of these on-sky disturbance on the quality of the science PSF is fully negligible.
At last, they demonstrated that this procedure is performing extremely well for various mis-registrations evolving dynamically at the same time. By using only 3 PCA modes with an amplitude of 20 nm RMS, they could provide a tracking of the mis-registration parameters with an accuracy better than 1% of a subaperture.
“We demonstrated that, using only a few of these well selected signals, the method provides a very good accuracy on the parameters estimation, well under the targeted accuracy, and has a negligible impact on the scientific path. In addition, the method appears to be very robust to varying operating conditions of noise and atmospheric turbulence and performs equally for both Pyramid and Shack-Hartmann WFS.”— Heritier, lead author of the study
They concluded that the future step will require an experimental validation of the method, implementing it on an existing facility equipped with a secondary adaptive mirror. In addition, it will be relevant to investigate if this calibration strategy allows to retrieve other parameters such as optical gains for the PWFS, for instance to compensate properly Non Common Path Aberrations.
In addition, it will be required to study how this novel method performs taking into account more complex closed-loop effects specific to large adaptive telescopes equipped with a large adaptive secondary mirror. The couplings with pupil fragmentation effects due to the presence of thick spiders, deformable mirror saturation and the segmentation of the primary mirror will have to be investigated. In particular, in their recent paper, they put light on the coupling between the optical gains of PWFS and the presence of mis-registrations. In the context of the ELT, PWFS optical gains are expected to exhibit large variations which will require aggressive compensation strategies. An accurate tracking and compensation of the mis-registration will be required to prevent any bias in the optical gains estimation that would lead to loop instabilities or over/under compensation of Non-Common Path Aberrations.
Featured image: Principle of the Pseudo-Synthetic calibration. Experimental inputs representative of the real mis-registration state 𝜶 are injected into a mis-registration identification algorithm that provides an estimation 𝜶∗ © Heritier et al.
Reference: C. T. Heritier, T. Fusco, S. Oberti, B. Neichel, S. Esposito, P.-Y. Madec, “SPRINT: System Parameters Recurrent INvasive Tracking, a fast and least-cost online calibration strategy for adaptive optics”, pp. 1-17, 2021. https://arxiv.org/abs/2104.11261
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