AO4ELT5 Proceedings

Sensing and control of segmented mirrors with a pyramid wavefront sensor in the presence of spiders

Schwartz, Noah (UKATC), Sauvage, Jean-François (ONERA), Correia, Carlos (LAM), Petit, Cyril (ONERA), Quiros-Pacheco, Fernando (GMTO), Fusco, Thierry (ONERA), Dohlen, Kjetil (LAM), El Hadi, Kacem (LAM), Thatte, Niranjan (Oxford), Clarke, Fraser (Oxford), Paufique, Jérome (ESO), Vernet, Joel (ESO)

The segmentation of the telescope pupil (due to the spiders and the segmented M4) will create areas of phase isolated by the width of the spiders on the wavefront sensor (WFS) detector, breaking the spatial continuity of the wavefront data. The poor sensitivity of the Pyramid WFS (PWFS) to differential piston will lead to badly seen and therefore uncontrollable differential pistons. In close loop operation, differential pistons between segments will settle around integer values of the average sensing wavelength lambda. The differential pistons typically range from one to tens of time the sensing wavelength and vary rapidly over time, leading to extremely poor performance. In addition, aberrations created by atmospheric turbulence will naturally contain some differential piston between the segments. It is typically a relatively large multiple of the sensing wavelength. Removing piston contribution over each of the DM segments leads to poor performance. In an attempt to reduce the impact of unwanted differential pistons that are injected by the AO correction, we compare three different approaches. We first limit ourselves to only use the information measured by the PWFS, in particular by reducing the modulation. We show that using this information sensibly is important but will not be sufficient. We discuss possible ways of improvement by using prior information. A second approach is based on phase closure of the DM commands and assumes the continuity of the correction wavefront over the entire unsegmented pupil. The last approach is based on the pair-wise slaving of edge actuators and shows the best results. We compare the performance of these methods using realistic end-to-end simulations. We find that pair-wise slaving leads to a small increase of the total wavefront error, only adding between 20-45 nm RMS in quadrature for seeing conditions between 0.45-0.85”. Finally, we discuss the possibility of combining the different proposed solutions to increase robustness.

DOI: 10.26698/AO4ELT5.0015- Proceeding PDF

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