Optical design of the FUV spectrographic imager for the IMAGE mission

This paper describes the original concept and the optical design of the IMAGE mission FUV spectrographic imager (SI). The instrument goal is to spectrally separate and image the electron and proton auroras. A 30 angstrom (3 nm) spectral resolution is required to isolate the electron auroras (1356 angstrom). The proton aurora imaging requires to efficiently mask the geocoronal Lyman-alpha line (1216 angstrom), in order to image the Doppler shifted Lyman-alpha light (1217 - 1223 angstrom). A classical SI combines a telescope with a spectrometer. Our SI is consisting of a reverse combination: (1) a multi-slits Wadsworth monochromator designed to spectrally isolate the two bandwidths (electrons and protons auroras), (2) a two mirror imager with a crossed delay line detector producing the final imaging on each spectral channel.