Keck Observatory as an HWO Testbed: validating wavefront sensing and control schemes on a large segmented aperture in parallel with high-contrast science

TL;DR

The paper presents Keck Observatory as an HWO testbed to validate wavefront sensing and control on a large segmented aperture while enabling concurrent high-contrast science. It identifies critical HWO gaps in segment phasing and mid-spatial error handling, and outlines a multi-faceted approach combining Zernike-based wavefront sensing, capacitive edge sensors, and dual-mirror control architectures, validated on Keck and the SEAL testbed. Through on-sky demonstrations and targeted experiments, it aims to quantify control authority, actuator offload, and loop stability, and to produce an evidence-based HWO error budget and architecture recommendations. The work provides a framework for nested, multi-timescale control loops, enabling end-to-end evaluation of sensing modalities and informing future design choices for Habitable Worlds Observatory.

Abstract

Exoplanet direct imaging allows us to directly probe and characterize an exoplanet's atmosphere, searching for signs of life in its atmospheric signatures. Directly imaging an Earth-like planet around a Sun-like star requires reaching 10$^{-10}$ contrast levels and will be the goal of the Habitable Worlds Observatory (HWO). A key technical barrier to reaching such deep contrasts is maintaining wavefront stability on the order of tens of picometers, in particular in the presence of a segmented primary mirror. Keck Observatory is the only facility with all of the hardware components necessary for validating HWO segment phasing strategies: a large segmented primary mirror, capacitive edge sensors, deformable mirror, Zernike wavefront sensor (ZWFS), and high contrast science instruments. Taking advantage of these parallels, we are using Keck as a testbed for developing and validating HWO wavefront sensing and control loop strategies, as well as demonstrating the full system-level segment control architecture for HWO, using existing infrastructure. Recently, we set the stage for this work by using the ZWFS installed on the Keck II telescope to sense and correct the primary mirror segment pistons in closed-loop in parallel with science observations. This resulted in improved Strehl ratios on the NIRC2 science camera (Salama et al. 2024a). We now aim to directly address concerns related to control authority, actuator offload, and loop stability - tasks which require Keck's existing infrastructure, but which do not require picometer wavefront stability. Moreover, successful comparisons of observed and predicted performances will validate, on a real operating observatory, the HWO error budget methodology and in particular its approach to nested loops operating at multiple timescales.

Figures (2)

• Figure 2: Demonstration of using the ZWFS installed on the Keck II telescope to sense the primary mirror segment pistons and control them with the segment actuators in closed-loop, improving the Strehl ratios on the NIRC2 science camera. Left: ZWFS segment piston measurements before (top) and after (bottom) the ZWFS closed-loop. Right: NIRC2 PSF images before (top) and after (bottom) the ZWFS closed-loop. These measurements correspond to the 2025-06-17 Run 1 reported in Salama25. Many such closed-loop runs have now been conducted Salama24Salama24bSalama25.
• Figure 3: Top: HWO control architecture diagram (modified from Feinberg23). Bottom: Keck control architecture (modified from Chin22). Highlighting the parallel architecture and subsystems between HWO and Keck. Although operating at different speeds and in a different wavefront error regime, Keck can be used to develop and validate nested wavefront sensing and control loop strategies. With the exception of the laser truss system, we can take advantage of existing Keck infrastructure to demonstrate the full system-level segment control architecture for HWO.