1999A&A...346..181M

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 α Bootis (Arcturus) and α Herculis, with a 21m baseline of the Infrared and Optical Telescope Array (IOTA), located at the Smithsonian's Fred Lawrence Whipple Observatory (Mt Hopkins, Arizona). Beams from the telescopes are coherently combined through an X shaped fluoride glass single-mode fiber coupler. Although this device was not originally optimized for operation in the L band (effective wavelength of 3.75µm), we find good interferometric properties in that wavelength domain, with a rather high and steady instrumental transfer function, and very low dispersive effects. Observations are still limited by the poor sensitivity of the detectors and not by the thermal background, which was minimized. Visibilities derived from the interferograms are consistent with the published limb darkened diameters of α Herculis. It is, to our knowledge, the first time interferometric observations of Arcturus and α Her are reported in the L band.