Closed-loop atmospheric dispersion compensation: On-sky demonstration of sub-milliarcsecond residual dispersion across H-band

On ground-based telescopes employing adaptive optics (AO) systems, atmospheric dispersion compensation is essential to deliver high quality imaging, and critical for coronagraphy and precision astrometry. In AO systems delivering high-Strehl, residual dispersion if often a dominant source of error, and is especially challenging to correct on large aperture telescopes. Imperfect compensation by atmospheric dispersion compensator (ADC) can be due to errors in the atmospheric dispersion estimation (usually derived from local temperature and pressure measurements), or calibration errors in the instrument and ADC optics. These limitations can be addressed with a high-precision on-sky measurement of residual dispersion in a closed-loop control scheme. In this work, we present a focal plane based technique, which utilizes the chromatic scaling of speckles to measure residual dispersion (atmospheric and optical) in the final science image. By using an adaptive speckle grid generated using a deformable mirror (DM) with a sufficiently large number of actuators, the residual dispersion is accurately measured and subsequently corrected in a closed-loop control. We demonstrate on-sky residual dispersion of 0.75 mas across H-band, reduced from 8.4 mas before closing the loop. This work will be useful for upcoming extremely large telescopes (ELTs) to achieve sub-milliarcsecond precision on astrometry and residual dispersion.