PhD Defense: Julien Fourez

Evolution and Latest Developments in Optical Surfaces to Meet the New Requirements of Astronomical Telescopes: Case Studies for the Dark Energy Survey, Virgo, and ELT Projects”

Julien Fourez (Thales SESO) will defend his PhD by VAE (Validation of Acquired Experience) at LAM on Monday, October 5, at 2:00 PM.

Jury Members:

• Guillaume Druart – ONERA – Reviewer
• Yvan Sortais – Institut d’Optique IOGS – Reviewer
• Myriam Zerrad – Institut Fresnel – Examiner
• Hélène Krol – CILAS – Examiner
• Philippe Amram – Jury President
• Emmanuel Hugot – PhD Supervisor
• Laurent Pinard – Laboratoire des Matériaux Avancés – Invited Guest

Thesis Abstract

Since 1609, when Galileo created the first astronomical instrument with optical surfaces, optical surfaces for astronomy have continually evolved. This thesis aims to characterize these developments in greater detail, their scientific significance, and the various associated techniques. We observe that optical surfaces have historically evolved toward larger sizes, while simultaneously achieving higher precision and incorporating more complex shapes.

This thesis is based on case studies from the development of certain telescopes and their instruments, such as Dark Energy Survey, Extremely Large Telescope (ELT), GREGOR Solar Telescope, PARAS Spectrograph, or Virgo Gravitational Wave Detector.

We note a ceiling in the size of monolithic mirrors at 8.4 meters in diameter. We analyze the evolution of flatness specifications toward a continuous topographic distribution of defects, such as power spectral density or structure function. We also define a difficulty criterion for the manufacturing and measurement of non-spherical optical surfaces, applicable to freeform surfaces.

The manufacturing challenges are explained through two extreme case studies: the production of a one-meter diameter lens with a large aperture (f/0.7) for wide-field astronomy and for the emerging field of gravitational waves, achieving a shape precision of less than 0.35 nm RMS over a useful area of 235 mm.

Along the way, we explore some of the relevant techniques for addressing these optical surface manufacturing challenges by analyzing the properties of Fourier transforms, their limitations, and the advantages of the symmetries of Zernike polynomials.