PhD defense of Nicolas Levraud

The defense will take place on the 11th of December in the morning at 10am, in the main amphitheater of the Laboratoire d’Astrophysique de Marseille, 38 rue Frédéric Joliot Curie, 13013 Marseille. The presentation will also be broadcasted via zoom : https://univ-amu-fr.zoom.us/j/87564991610?pwd=RnljdkwzOFJDaENlY0RCM3RqN2R4dz09

Title : Toward the ultimate Wave-Front-Sensor for High Contrast Imaging : Application to Large Binocular and European Extremely Large Telescopes”

Abstract:
One of Astronomy’s most important subjects is the existence of extraterrestrial life. Science started working and theorizing seriously on the subject as far as the 16th century. But so far, the universe remains empty and silent.
The discovery of the first exoplanet in 1995, 51Peg b, started the modern search for extraterrestrial life. A decade later the first direct imaging of exoplanet allowed the study of the presence of biomarkers. This achievement was allowed by the ‘Very Large’ telescope generation (mirror from 6 to 10m) and their instruments.
Adaptive optics (AO) in particular was the game-changer.  A wavefront sensor allows the measurement of atmospheric turbulence, and a deformable mirror allows the correction of this turbulence. This leads to the concentration of the starlight making its suppression to study its close environment possible (this is called coronography).

The new ‘extremely large’ telescope generation is under construction (ELT, GMT, and TMT, with mirrors from 29 to 39m). Their size should make more exoplanets directly observable if AO functions correctly. But during the current design phase of the instrument, a new kind of defect has appeared called the petal mode. This mode appears due to the complexity of the new telescope’s pupil. These pupils appear to the AO separated into multiple fragments. The AO then treats each fragment as independent areas, which end up with dephasing between each other. The consequence is that their resolutions are limited by the size of a fragment. This makes the ELT as efficient as a 15m telescope instead of a 39m.
The aim of this thesis is to study the petal mode. In particular, we studied the origin of this mode and proposed solutions. At first, we studied in detail the designated wavefront sensor of the new giant telescopes: the pyramid sensor. We showed the conditions making this sensor unable to measure the petal mode.We showed that with the modulation the PyWFS can’t measure the petal mode.
We propose to use 2 sensors to allow the AO to control the petal mode. One will be dedicated to the atmospheric turbulence and one to the petal mode. From our study of the pyramid we studied in which conditions (in particular the absence of modulation) the pyramid could be used as the petal mode sensor. This led to different modifications of the pyramid to ease the measurement of petal: the addition of a spatial filter to reduce the effect of residual turbulence (extendable to other wavefront sensors), and an asymetrisation of the pyramid to increase its response to petal mode.
The different proposed sensors have been tested in simulation and show satisfying results for median conditions of the ELT. The proposed solutions should then correct the petal mode for the ELT and restore the expected resolution of a 39m telescope.