Thesis title
Development of spectropolarimeters for the near to far ultraviolet : from design to experimental validation.
Composition of the jury
- Antonella Barucci (Astronome, LIRA) : President of the jury
- Thierry Lépine (Professeur associé, Institut d’Optique) : referee
- Frans Snik (!Professeur associé, Leiden University) : referee
- Evelyne Alecian (chercheur CNRS, IPAG) : examiner
- Jean-Claude Bouret (Directeur de recherche CNRS, LAM) : examiner
- Juan Larruquert (Professeur, CSIC) : examiner
- Frank Brachet (CNES) : invited
- Coralie Neiner (CNRS Directrice de recherche , LIRA) : supervisor
- Jean-Michel Reess (Ingénieur de recherche, LIRA) : co-supervisor
Abstract
Spectropolarimetry (the measurement of the intensity and polarisation of light as a function of wavelength) enables unique measurements in astrophysics : among other things, it gives access to stellar magnetic fields through the Zeeman and Hanle effects, to the geometry of unresolved circumstellar media through scattering, and to exoplanetary atmospheres. The ultraviolet domain is particularly valuable in this respect, as it concentrates the resonance lines of hot, highly ionised plasmas as well as the Lyman-α line ; yet no broadband spectropolarimeter currently operates there. The polarised signatures of interest are weak (typically below 0.1 % of the intensity), which imposes a demanding polarimetric measurement accuracy requirement, of the order of 10⁻³.
This thesis presents the development of ultraviolet spectropolarimeters, from optical design to experimental validation, in the framework of three space missions : the CASSTOR demonstrator, the Polstar SMEX proposal, and the Pollux instrument concept for NASA’s future flagship mission, the Habitable Worlds Observatory (HWO). These missions share a common polarimetric principle but split into two different technological approaches dictated by wavelength.
In the near- and mid-ultraviolet (118-380 nm), where transmissive birefringent components remain usable, a Stokes-Mueller formalism was developed to model, optimise, and tolerance polarimeters based on rotating stacks of thin MgF₂ plates. I applied it to the CASSTOR, Polstar, and Pollux polarimeters. I designed, assembled, and aligned a dedicated vacuum test bench. Its first light and the first reconstruction of polarisation states are reported here.
Below 120 nm, where no transmissive birefringent material exists, a fully reflective architecture is required. I adapted the same Stokes-Mueller formalism to enable the development, design, and tolerancing of the Pollux FUV polarimeter (a rotating K-mirror modulator followed by an analyser mirror). I adapted the test bench to this wavelength range, and a first FUV analyser (MgF₂ on B₄C) was manufactured in collaboration with a Madrid-based team.
These developments raise the maturity of UV polarimeter technologies and feed directly into the design of the Pollux UV spectropolarimeters for the future HWO space mission.
