Fast and Furious Wavefront Sensing Algorithm

W. M. Keck Observatory (WMKO) is home to the world's most scientifically productive adaptive optics (AO) systems due to the continuously upgraded AO technology and expanding set of AO-fed science instruments. The AO system’s hardware linchpin – the deformable mirror (DM) – was installed in the early 2000’s and its small number of actuators now limits the overall performance of the Keck II AO system. A full AO system project called High order Advanced Keck Adaptive optics (HAKA) upgrades the existing 349-actuator DM with a state-of-the-art 2900-actuator DM. This change will give Keck a unique observing opportunity to directly imaging and spectroscopically characterizing populations of old, cold exoplanets that are not accessible to any direct imaging facility, ground or space, as well as enable Z-Band AO corrections to probe the atmospheres of our Solar System’s ice giants and map the surfaces of the Galilean moons. The HAKA science program will be carried out by existing and up-coming AO science instruments on the Keck II telescope:

  1. Near-infrared camera (NIRC2)

  2. Single-mode fiber-fed spectrograph (KPIC + NIRSPEC)

  3. Mid-infrared coronagraphic integral field spectrograph (SCALES)

  4. Fiber-fed high-resolution spectrograph (HISPEC).

HAKA builds on a recent series of successful upgrades to the Keck II AO system including a new real-time controller, high-speed wavefront sensor camera, and near-infrared pyramid wavefront sensor. With HAKA, these upgrades transform the Keck II AO system from general purpose AO to a state-of-the-art planet-characterization machine with far-reaching capabilities for all science cases even moving to optical wavelengths.

The current Keck 2 AO DM is a 20x20 actuator, 7 mm spaced, Xinetics DM. The HAKA DM is an upgrade with a 58x58 actuator, 2.5 mm spaced ALPAO DM; this oversized to fit an11.4 m diameter and 20 cm spacing at the Keck primary mirror. These two DMs can be directly swapped without any changes to the AO bench science optical path. This ALPAO DM is currently being developed and tested by multiple groups around the world. The ALPAO DM has larger stroke and lower hysteresis than the current Xinetics DM, including operation down to -30C (allowing for a future cold AO system operation option).

The HAKA DM will require an upgrade to the current Shack-Hartmann Wavefront Sensor (SHWFS). The Shack-Hartmann wavefront sensor pupil relay optics will be redesigned to produce a 2.8 times larger pupil image on the lenslet array. The existing lenslet arrays, and reducer optics in front of the OCAM2K camera, are large enough to utilize the larger pupil; the distance between the field stop and lenslet array will be increased. The main change will be using 56 subapertures and a total of 224 pixels instead of the current SHWFS, which only utilizes 20 subaperatures.

Selected Publications

Guthery et al., Optimizing Keck adaptive optics: correcting residual errors with focal-plane wavefront sensing, SPIE, 2024

Bottom et al., Sequential coronagraphic low-order wavefront control, AO4ELT7, 2023

Guthery et al., From Demonstration to Operation: High Contrast Imaging Tools at Keck Observatory, AO4ELT7, 2023

Bos et al., “Fast” and Furious focal-plane wavefront sensing at W. M. Keck Observatory, 2021

Team (Current)

Charlotte E Guthery, Co-Pi, W. M. Keck Observatory

Mike Bottom, Co-Pi, University of Hawaii - Institute of Astronomy

Jacques Delorme, AO Scientist, W. M. Keck Observatory

Samuel Walker, Graduate Student, university of Hawaii - Manoa

Sebastien Vievard, Postdoctoral Scholar, Subaru Telescope