# Cascade adaptive optics with a second stage based on a Zernike wavefront sensor for exoplanet observations

**Authors:** M. N'Diaye, A. Vigan, B. Engler, M. Kasper, K. Dohlen, S. Leveratto, J. Floriot, M. Marcos, C. Bailet, P. Bristow  
**arXiv:** [2411.11946](https://arxiv.org/abs/2411.11946)  
**Date:** 2024-11-18  
**Summary page:** https://www.summarizepaper.com/en/paper/2411.11946v1/

## Key points

- Ground-based instruments have improved high-contrast observations of disks and gas giant planets around nearby stars using extreme adaptive optics (XAO)
- Cascade adaptive optics systems with a fast second stage based on a Pyramid wavefront sensor or Zernike wavefront sensor (ZWFS) are emerging to mitigate atmospheric wavefront errors
- The innovative cascade AO configuration with a ZWFS-based control loop has been tested on the ESO's GHOST testbed, showing significant reduction in atmospheric residuals and improved wavefront error stability
- The ZWFS-based second-stage AO loop yields significant contrast gains, particularly evident in images captured with a Lyot coronagraph at short separations from the source
- Adjustments to control loop features further enhance contrast improvements even in more challenging observational conditions
- This study validates the effectiveness of employing a ZWFS-based second-stage AO loop as an efficient solution for addressing residual aberrations left by single-stage XAO systems during coronagraphic observations

## Summary

In the past decade, ground-based instruments have greatly improved high-contrast observations of disks and gas giant planets around nearby stars through the use of extreme adaptive optics (XAO). These systems have been successful in producing high-quality images with a Strehl ratio exceeding 90% in H-band under typical observing conditions and high-flux scenarios. However, they still leave behind residual aberrations that hinder the study of fainter or less massive exoplanets. To address this challenge, cascade adaptive optics systems with a fast second stage based on a Pyramid wavefront sensor have emerged as a promising solution to mitigate atmospheric wavefront errors. Recent advancements have introduced an alternative second-stage approach utilizing a Zernike wavefront sensor (ZWFS) to correct for these remaining AO residuals. This innovative cascade AO configuration with a ZWFS-based control loop has been implemented on the ESO's GHOST testbed to validate its efficacy in monochromatic light. Results from testing in median wind speed and seeing conditions demonstrate that the second-stage AO system incorporating a ZWFS and basic integrator can reduce atmospheric residuals by a factor of 6 while improving wavefront error stability by a gain of 2 from open to closed loop operation. Moreover, when faced with non-common path aberrations, the ZWFS-based second-stage AO loop also yields significant contrast gains, particularly evident in images captured with a Lyot coronagraph at short separations from the source. This success underscores the capability of this novel scheme to complement XAO loops and enhance imaging capabilities for detecting fainter or lighter close-in companions. Additionally, adjustments to control loop features further enhance contrast improvements even in more challenging observational conditions. Overall, this study validates the effectiveness of employing a ZWFS-based second-stage AO loop as an efficient solution for addressing residual aberrations left by single-stage XAO systems during coronagraphic observations of circumstellar environments. The collaborative efforts of researchers including M. N'Diaye, A. Vigan, B. Engler, M. Kasper, K. Dohlen, S. Leveratto, J. Floriot, M. Marcos, C. Bailet, and P. Bristow have contributed valuable insights towards advancing exoplanet observation techniques using advanced adaptive optics technologies.