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In the context of a space mission aiming at characterizing the phenomenon of mag-
netic reconnection, a spectrographic imager is needed to gather data in the far ul-
traviolet domain. Its observation target is set to the polar aurora, in the ionosphere
of the Earth, as their emission spectra contain valuable information. Specificaly, the
instrument is expected to feature two channels, at wavelengths around the atomic
hydrogen Lyman-alpha emission line (at 121.567 nm) and around a specific atomic
oxygen emission line (at 135.6 nm).
The geocoronal Lyman-alpha emission of atomic hydrogen being much brighter
than the emission of the proton aurora, it may impair the instrument’s ability to
image the Doppler-shifted auroral Lyman-alpha. Therefore, it must be rejected to
prevent the detector from saturating and effectively rendering information around
that wavelength unusable. To solve this problem, the chosen method is a pair of slit
grilles, which is to be placed at the entrance and exit planes of the spectrographic
part of the instrument. Moreover, considering that the observation targets lie in the
far ultraviolet domain and are quite close one another, a Czerny-Turner design with
a diffraction grating is chosen for the aforementioned spectrographic element.
Thanks to specific softwares, an optical optimization of the setup can be per-
formed. For the channel pertaining to the Doppler-shifted Lyman-alpha emissions,
specific care is given to the computed leakage of the geocoronal emission and trans-
mission of the neighboring wavelengths.

far ultraviolet --- spectrographic imager --- wavelength rejection --- Ingénierie, informatique & technologie > Ingénierie aérospatiale

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This thesis deals with the analytical design of a static spectropolarimeter for space use, based on birefringent media. It is about an instrument able to provide access to all four Stokes parameters of the detected light, at different wavelengths. In the first part of the thesis, the most important concepts and methods of polarimetry and of their applications in astrophysics are presented. Further, the proposed instrument is described, and the modulation function is derived, using the Mueller calculus. Because the material of the spectropolarimeter is Magnesium Fluoride (MgF2), an important attention was offered to the variation of the birefringence with wavelength. Two models for the description of the chromaticity of MgF2 were proposed and compared. Then the uncertainty calculus corresponding to this type of instrument was developed. Adjacently, considering the presence of noise, an extraction algorithm for the Stokes parameters was build and the behavior of the instrument in the presence of noise was tested. In the last part of the research, the off-axis case was also studied. After a detailed mathematical determination of the ray tracing with the help of Huygens constructions, several scenarios for the off-axis incidence were analyzed and the main limitations of the system were discovered.

polarization --- spectropolarimeter --- birefringence --- Zeeman effect --- Hanle effect --- polarization exoplanets --- modulation --- Physique, chimie, mathématiques & sciences de la terre > Aérospatiale, astronomie & astrophysique