Thermal infrared stellar interferometry using single-mode guided optics: first results with the TISIS experiment on IOTA

We report the first long baseline interferometric observations obtained in the thermal infrared with a single-mode fiber coupler. Stellar fringes have been found on alpha Bootis (Arcturus) and alpha Herculis, with a 21 m base line of the Infrared and Optical Telescope Array (IOTA), located at the Smi thsonian's Fred Lawrence Whipple Observatory (Mt Hopkins, Arizona). Beams f rom the telescopes are coherently combined through an X shaped fluoride gla ss single-mode fiber coupler. Although this device was not originally optim ized for operation in the L band (effective wavelength of 3.75 mu m), we fi nd good interferometric properties in that wavelength domain, with a rather high and steady instrumental transfer function, and very low dispersive ef fects. Observations are still limited by the poor sensitivity of the detect ors and not by the thermal background, which was minimized. Visibilities de rived from the interferograms are consistent with the published limb darken ed diameters of alpha Herculis. It is, to our knowledge, the first time int erferometric observations of Arcturus and alpha Her are reported in the L b and.