PhD defense: Mahawa Cissé

PhD defense entitled by Mahawa Cissé : “Optimising Fourier Filtering Wavefront Sensing for High-Contrast Imaging: Applications to Extremely Large Telescopes“. The defense will occur on Tuesday, December 17, at 4:00 PM (French time) in the LAM amphitheatre.

The presentation will be held in English, and you can find the abstract below.

Abstract:

Since 1995, thousands of exoplanets have been identified using various techniques including high-contrast imaging (or direct imaging) which contributes to the detection and spectral characterisation of these planets. To function optimally, this type of instrument must receive a wavefront corrected for all aberrations. They therefore integrate adaptive optics (AO) systems, composed of a deformable mirror, a wavefront sensor (WFS) and a real-time computer (RTC), to measure and compensate in real time all phase perturbations induced by atmospheric turbulence, the telescope and the instrument itself.

The advent of giant telescopes (from 24 to 39min diameter) presents new challenges for AO systems in terms of wavefront correction allowing the detection of Exo-Earth (an exoplanet similar to Earth). The primary mirrors of the next generation of giant telescopes will be segmented to reach the desired size. The problems related to the segmentation of the primary mirror (in particular, related to the shadow cast by the secondary mirror supports) require the implementation of innovative measurement and phase correction strategies to manage both pupil fragmentation and atmospheric turbulence. Classical approaches for wavefront measurements (Shack-Hartmann, Modulated Pyramid) are optimised to measure atmospheric turbulence but are insensitive to the differential piston generated by fragmented pupils. Conversely, some other sensors such as unmodulated pyramids or Zernike masks are more sensitive to pupil fragmentation but less effective in measuring atmospheric turbulence due to their very reduced linearity domain.

This PhD thesis focuses on new generation of WFSs that will be an integral part of the AO systems of giant segmented telescopes. These sensors belong to the Fourier filtering WFS family of which the Zernike mask and the unmodulated pyramid are examples. The main objective of my PhD was to develop a thorough understanding of the nonlinear response of these sensors and its impact on wavefront estimation. I then proposed two solutions to reduce the nonlinear effects: a modification of the WFS and a modification of the reconstructor used for wavefront estimation