Modern lightweight UV spectrographs for in situ studies of the exosphere and photo-ion population about Mercury

Important resonance transitions of most neutral species, either already known to be present in the exosphere of Mercury or likely to be found there, fall in the UV portion of the spectrum as do the principal resonance transitions of the ions of these same elements. The design and fabrication is presented of a new lightweight UV spectrograph, under development at Southwest Research Institute, which can easily detect these known or expected species. The instrument is an athermal design constructed for the most part of aluminum. It weighs less than 3 kg, and draws less than 3 W of power. The spectral coverage is 1800–3300 Å, which is sufficient to reach resonance lines of all the important species identified in this spectral passband. The predicted spectral resolution for a point source at infinity is ∼9 Å at 1800 Å, ∼13 Å at 2500 Å, and ∼15 Å at 3000 Å (λΔλ∼200). The spectrograph employs a 2-D microchannel plate wedge-and-strip detector with either a bi-alkali or cesium telluride (solar-blind) semi-transparent photocathode. For most of the important transitions expected, this instrument should be able to reach detection limits (signal-to-noise ratio ∼5) near 0.1 R in a 102 s exposure with a cesium telluride photocathode, or 0.08 R with a bi-alkali photocathode.