First on-sky fringes in K-band of an ULI-manufactured integrated optics beam combiner at the CHARA observatory

Abstract

Long-baseline interferometric observations in the Near Infrared reach angular resolutions on the order of milliarcseconds. With the long baselines of the CHARA array, science cases such as exozodiacal dust of early-type main sequence stars can be probed to study early-stage planet formation with percentage-precision visibility. Photonic-based instruments have been a successful technical solution to combine the light of telescopes of longbaseline interferometers, such as GRAVITY. The integrated-optics technology brings a compact foot print which reduces the instrument sensitivity to its environment, and wavefront filtering when it is coupled with singlemode optical fibres. Following the decommissioning of the JouFLU instrument, the CHara ARray Integrated Optics Testbench (CHARIOT) aims to implement new technologies of integrated-optics beam combiners in the K-band for long-baseline interferometry. The long-term philosophy and goals are to offer a complete fibred plug-and-play platform to easily test and characterise on sky innovative photonic components at the CHARA array. Currently, CHARIOT integrates a proof-of-concept two-telescope fibre-connectorised integrated-optics beam combiner manufactured using the Ultrafast Laser-Inscription fabrication technique which presents low-OH contamination and high transparency over the whole K-band. This talk gives an update on the CHARIOT development, and presents its first on-sky results. Sub-1% precision squared visibilities could be measured on stars of K magnitude ≲ 3 and fringes were detected on stars of magnitude up to to K-mag = 5 (HD 210824). Those results are comparable to the JouFLU instrument. With additional improvements such as decreasing the fibre length, using the MIRC-X instrument as a fringe tracker to correct for the atmospheric turbulence, and cryo-cooling the beam combiner system, the limiting magnitude and precision of the visibility estimation for fainter stars would be increased even further.