Large Binocular Telescope Interferometer Adaptive Optics: On-sky performance and lessons learned

The Large Binocular Telescope Interferometer is a high contrast imager and interferometer that sits at the combined bent Gregorian focus of the LBT’s dual 8.4 m apertures. The interferometric science drivers dictate 0.1 ” resolution with 103 − 104 contrast at 10 µm, while the 4 µm imaging science drivers require even greater contrasts, but at scales>0.2”. In imaging mode, LBTI’s Adaptive Optics system is already delivering 4 µm contrast of 104 − 105 at 0.3′ ′ − 0.75′ ′ in good conditions. Even in poor seeing, it can deliver up to 90 % Strehl Ratio at this wavelength. However, the performance could be further improved by mitigating Non-Common Path Aberrations. Any NCPA remedy must be feasible using only the current hardware: the science camera, the wavefront sensor, and the adaptive secondary mirror. In preliminary testing, we have implemented an “eye doctor ” grid search approach for astigmatism and trefoil, achieving 5 % improvement in Strehl Ratio at 4 µm, with future plans to test at shorter wavelengths and with more modes. We find evidence of NCPA variability on short timescales and discuss possible upgrades to ameliorate time-variable effects