We investigate the adoption of Machine Learning techniques for piston sensing in the context of segmented primary mirror telescopes by the means of numerical simulations. Considering a Natural Guide Star Wavefront Sensor, composed by one high order modes sensor plus a second sensor dedicated to the differential piston modes, we focus on the latter and tackle the problem of providing an accurate estimation for the piston modes coefficients from a defocused image of the system PSF.

We consider as a baseline algorithm a customized version of LIFT (which is based on a Maximum Likelihood Estimation) and compare its performance with a Deep Neural Network (DNN) regression. After considering several DNN architectures, we designed a simple one and performed some degree of hyperparameter optimization on it to obtain the final DNN version.

The infrared low order sensor (IRLOS) upgrade project was recently launched to increase the sky coverage of GALACSI narrow-field mode (NFM).1, 2 While the baseline is to perform low-order wavefront sensing with a 2x2 Shack-Hartmann wavefront sensor (SHWFS) operating in the J+H band, a full-pupil mode was proposed to address the faintest end of the magnitude range by concentrating the photons from the full aperture in a single point spread function (PSF). In this context, we have investigated the wavefront sensing approach called the linearized focal-plane technique (LIFT). It enables the retrieval of low-order modes such as tip/tilt, defocus, astigmatism (and possibly more) from a single focal-plane PSF of a very faint natural guide star (NGS) target. LIFT is a phase diversity technique based on introducing a known amount of astigmatism into the optical path. The morphological change induced by the astigmatic shift allows encoding information about the phase aberrations into the PSF morphology.

MORFEO/MAORY is the post-focal adaptive optics instrument of the ELT. It is designed to provide the 53×53 arcsec field of view of MICADO with MCAO correction based on split-tomography, where the Low-Order modes are sensed by three NGS-based WFS. To maximize the sky-coverage the LO-WFS are 2×2 subapertures Shack- Hartmann sensors working in the H band, making use of the FREDA detectors. MAORY also implements 3 dedicated NGS-based truth sensors to measure at slow rate the true higher order atmospheric aberrations and to de-trend the LGS WFS measurements. These WFS work with the visible light of the NGS to feed a 10 × 10 SH sensor that makes use of the ALICE detector. Each unit of LOR WFS is provided with a couple of orthogonal linear stages to allow for the NGS acquisition in a 80 arcsec radius. The 3 LOR WFS are arranged at 120° geometry on a common support structure that rigidly connects them to MICADO and its rotator.