Observations of Late-Type Stars with the Infrared Spatial Interferometer

The Infrared Spatial Interferometer (IST) has been conducting mid-IR measurements of red supergiant and AGB stars for about 20 years. This paper reviews the ISI system and results with an emphasis on measurements of changes in stellar sizes and shapes, and those of the surrounding dust shells, over time scales of weeks to decades. The long-term measurement record provides important new observables for stellar theory. A new spectrometer-correlator system is discussed where this system will measure visibilities on-and-off individual molecular spectral lines.