Studies of Evolved Stars with a Mid-Infrared Interferometer

element stellar interferometer which employs heterodyne detection to gather starlight in the mid-infrared (9–12µm) wavelength range. Each element of the ISI is a 1.65m tele-scope, equipped with appropriate optical and detection systems. Successful operation of the ISI requires that the star image in each telescope be precisely centered on a signal detector. In order to accomplish this, near-infrared (2µm) guiding and tip-tilt correc-tion systems have been built and incorporated into the ISI telescopes. Each system uses an InSb infrared camera to determine small errors in the position of the source image. Slower fluctuations in position, occurring over a period of a few seconds, are corrected by adjusting the telescope tracking. Fast fluctuations are caused by the atmosphere, and are removed using a controllable tilting mirror. A significant improvement in signal quality, provided by this fast tip-tilt correction, is demonstrated by data taken on the supergiant α Orionis. To be suitable for study by the ISI, a source must have substantial mid-infrared flux. Evolved stars, typically very luminous and surrounded by one or more shells of dust, are the most common type of target for the ISI. The new guiding system enables the ISI to track these objects, which, because of their dust shells, often have insufficient visible flux for a guiding system using a silicon detector. During the 1997 observin