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 alread y known to be present in the exosphere of Mercury or likely to be foun d there, fall in the UV portion of the spectrum as do the principal re sonance 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 t hese known or expected species. The instrument is an athermal design c onstructed for the most part of aluminum. It weighs less than 3 kg, an d draws less than 3 W of power. The spectral coverage is 1800-3300 Ang strom, which is sufficient to reach resonance lines of all the importa nt species identified in this spectral passband. The predicted spectra l resolution for a point source at infinity is similar to 9 Angstrom a t 1800 Angstrom, similar to 13 Angstrom at 2500 Angstrom, and similar to 15 Angstrom at 3000 Angstrom (lambda/Delta lambda similar to 200). The spectrograph employs a 2-D microchannel plate wedge-and-strip dete ctor with either a bi-alkali or cesium telluride (solar-blind) semi-tr ansparent photocathode. For most of the important transitions expected , this instrument should be able to reach detection limits (signal-to- noise ratio similar to 5) near 0.1 R in a 10(2) s exposure with a cesi um telluride photocathode, or 0.08 R with a bi-alkali photocathode. (C ) 1997 Elsevier Science Ltd.