It is proposed to augment the Infrared Spatial Interferometer which has been built and is operating on Mount Wilson so that it can do interferometry on spectral lines. Since the system uses heterodyne detection, it can provide very sensitive detection at narrow bandwidths and hence can be conveniently adapted to spectral work. Prominent molecular spectral lines of gases recently emitted by stars and in their surrounding dust shells have already been studied by heterodyne detection in the 10 micron region. It is proposed here to enhance the ISI so that it can carry out spatial interferometry at high angular resolutions on these and other spectral lines as well as on continuum radiation, with which it now works. The work involves installation of two sets of carefully matched IF filters for the two telescopes, IF switching, and IF chopping between filters. It will also involve field work in testing the system and its use, so that techniques can be checked and perfected, and initial results of high quality obtained. This work can open up a new aspect of spatial interferometry and provide new, highly specific information on molecular formation and excitation, as well as distribution of gas densities, temperatures, and accelerations.
