One of the most appealing science objectives in astrophysics is the direct imaging of rocky exoplanets and the characterization of their atmosphere. The goal of detecting Biosignatures is theoretically within reach of the future Planetary Camera Spectrograph (PCS) for the European Extremely Large Telescope (ELT).  However, the image quality must be extremely good and requires an eXtreme Adaptive Optics (XAO) system:  a deformable mirror with tens of thousands of actuators is required to correct the Wave-Front (WF) of the starlight perturbed by Earth’s atmosphere.  To this end, the WF must be measured with nanometric precision with a WF Sensor (WFS) operating at very high speed (several kilohertz) and very high spatial resolution. This PhD thesis will take place in the context of Fourier Filtering WFS (FF-WFS), a new field which goal is to approach the fundamental limits of sensitivity.

We propose a new FF-WFS, the Bi-Orthogonal Foucault knife edge (in short Bi-O-edge) that is specifically designed for increased sensitivity in the framework of XAO (see https://arxiv.org/abs/2309.07485). Two variants are proposed:

1. The first one called ‘modulated Sharp Bi-O-edge’ can be seen as a mild evolution of the well-known Pyramid Wavefront Sensor (PWS), where the pyramid-like phase mask is replaced by two roof-top prisms operating in parallel which boosts the overall sensitivity. Like the PWS, the Sharp Bi-O-edge requires a modulation mirror running at the control frame rate to ensure robustness of the system.
2. In the second variant called ‘Grey Bi-O-edge’, the phase masks are replaced by amplitude masks which permits to increase even further the sensitivity while suppressing the need of the modulation mirror, such that the control frame rate is no longer limited by mechanical properties of moving devices.

Both variants have the capability to operate in a super-resolution mode to measure WF structures twice as fine as the usual limit set by the detector pixel size.

This thesis is proposed in collaboration with Laboratoire d’Astrophysique de Marseille (LAM) and the French Aerospace Lab (ONERA). The PhD studentship shall be shared between ESO and LAM/ONERA with a split ratio to de determined.

The thesis objectives are to consolidate the theory of the Bi-O-edge and to design, manufacture and evaluate prototypes of the masks and develop compact opto-mechanical solutions. The PhD candidate will have access to advanced simulation codes (Python) and to several experimental facilities: The GHOST lab bench at ESO, the LOOPS lab bench at LAM/ONERA and the PAPYRUS on-sky bench at Observatoire de Haute Provence (France). The PhD student will also contribute to the PCS project at ESO and to the LAM/ONERA project HARMONI.

The detailed objectives are the following:

1. Characterization of the sensitivity and dynamic range the Bi-O-edge WFS variants and compare to other FF-WFSs.
2. Tuning of the Bi-O-edge design to maximize the performance in presence of atmospheric turbulence and instrumental error. The super-resolution mode will be evaluated to deal with non-linear effects like optical gain, pupil fragmentation and low wind effect.
3. Design and manufacturing of prototypes of Grey Bi-O-edge masks and associated opto-mechanics.
4. Validation and characterization of the prototype(s) on the lab benches: GHOST and/or LOOPS.
5. On-shy demonstration of the Bi-O-edge using the PAPYRUS platform.

Contacts:

ESO: Christophe Vérinaud, Markus Kasper

LAM/ONERA: Cédric Taïssir Héritier