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

TL;DR

This paper evaluates the on-sky performance of the Large Binocular Telescope Interferometer's adaptive optics system, highlighting current capabilities, challenges with non-common path aberrations, and potential improvements for high-contrast imaging.

Contribution

It presents on-sky performance data of LBTI's adaptive optics, introduces a grid search method for NCPA correction, and discusses lessons learned and future upgrade plans.

Findings

Achieved 10^4-10^5 contrast at 4 μm in good conditions.

Implemented a grid search for NCPA correction, improving Strehl ratio by 5%.

Identified variability in NCPA on short timescales.

Abstract

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 $10^3-10^4$ contrast at $10~\mu m$, while the $4~\mu m$ imaging science drivers require even greater contrasts, but at scales $>$0.2''. In imaging mode, LBTI's Adaptive Optics system is already delivering $4~\mu m$ contrast of $10^4-10^5$ 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 $4~\mu m$, 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