Design optimization of far UV spectro-imagers for ionosphere characterization from space

Abstract

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.