Properties of petal mode and its measurement in an adaptive optic closed loop

Nicolas Levraud; Mahawa Cisse; Vincent Chambouleyron; Jean-François Sauvage; Benoît Neichel; Thierry Fusco

doi:10.1117/12.3020124

3 September 2024 Properties of petal mode and its measurement in an adaptive optic closed loop

Nicolas Levraud, Mahawa Cisse, Vincent Chambouleyron, Jean-François Sauvage, Benoît Neichel, Thierry Fusco

Author Affiliations +

Proceedings Volume 13097, Adaptive Optics Systems IX; 130972E (2024) https://doi.org/10.1117/12.3020124
Event: SPIE Astronomical Telescopes + Instrumentation, 2024, Yokohama, Japan

Abstract

The next generation of Extremely Large Telescope (ELT, 24 to 39m diameter) will suffer from the so-called ”pupil fragmentation” problem. Due to their large spiders, differential pistons will appear in the wavefront between the part of the pupil separated by the spiders during observations. The Adaptive Optics (AO) system necessary to compensate atmospheric turbulence appears unable to sense this differential piston leading to bad control by the loop. Hence, such differential pistons, a.k.a petal modes, will prevent the AO system from reaching the diffraction limit of the telescope and ultimately will represent the main limitation of AO-assisted observation with an ELT and will be particularly detrimental to High Contrast observation modes. All the future single conjugated AO systems for the ELT have a Pyramid Wavefront Sensor (PyWFS) which can be sensitive to the petal mode if unmodulated. But the current control methods available forces the modulation of the PyWFS to ensure the AO loop stability. In this presentation we study the origin and properties of the petal mode through a simplified pupil. There are two identified sources: the atmospheric turbulence and the Low Wind Effect (LWE). We show how the atmospheric turbulence can be mitigated though minioning and the limit of this mitigation with the origin of the petal flares. Furthermore, the minioning is inefficient at reducing LWE petal. This has led to the necessity of a second sensor dedicated to the petal mode measurement: a petalometer. We finally propose a PyWFS type of petalometer and study its properties and limits for the control of the petal mode.

Conference Presentation

Citation Download Citation

Nicolas Levraud, Mahawa Cisse, Vincent Chambouleyron, Jean-François Sauvage, Benoît Neichel, and Thierry Fusco "Properties of petal mode and its measurement in an adaptive optic closed loop", Proc. SPIE 13097, Adaptive Optics Systems IX, 130972E (3 September 2024); https://doi.org/10.1117/12.3020124

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