# Increasing the raw contrast of VLT/SPHERE with the dark-hole technique. II. On-sky wavefront correction and coherent differential imaging

**Authors:** Axel Potier, Johan Mazoyer, Zahed Wahhaj, Pierre Baudoz, Gael Chauvin, Raphael Galicher, Garreth Ruane  
**arXiv:** [2208.11244](https://arxiv.org/abs/2208.11244)  
**Date:** 2022-08-24  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2208.11244v1/

## Key points

- Cutting-edge technologies such as adaptive optics (AO) systems, coronagraphy, and advanced post-processing techniques are crucial in direct imaging of exoplanets.
- Coronagraphs reduce starlight interference to enhance the visibility of exoplanets in relation to their host stars.
- AO systems minimize optical aberrations to ensure high-quality images that complement the effectiveness of coronagraphs.
- Post-processing methods refine images by eliminating residual stellar speckles caused by noncommon path aberrations (NCPAs) and diffraction effects from telescope obstructions, improving contrast performance and image quality.
- An efficient approach using an iterative dark-hole (DH) algorithm has been developed to reduce speckle intensity attributed to NCPAs during observational sessions with the VLT/SPHERE instrument.
- This technique leverages a pair-wise probing estimator and a controller based on electric field conjugation, achieving significant improvements in contrast performance with just 1 minute of exposure time compared to traditional post-processing techniques.
- An alternative calibration method based on starlight coherence results in a threefold enhancement in post-processed contrast levels root mean square (rms).
- These advancements have the potential to optimize future design, development, and observational strategies for next-generation ground-based exoplanet imagers tailored for telescopes ranging from 10m to 40m.
- The on-sky demonstration represents a pivotal milestone in advancing ground-based exoplanet imaging technologies, enhancing precision and efficiency in capturing distant planetary systems within our universe.

## Summary

In the realm of direct imaging of exoplanets, cutting-edge technologies such as adaptive optics (AO) systems, coronagraphy, and advanced post-processing techniques play crucial roles. These advancements have greatly improved our ability to observe and study distant planetary systems within our universe. Coronagraphs are instrumental in reducing starlight interference to enhance the visibility of exoplanets in relation to their host stars. Meanwhile, AO systems ensure high-quality images by minimizing optical aberrations that could compromise the effectiveness of coronagraphs. This combination allows for more precise and accurate observations. Post-processing methods further refine these images by identifying and eliminating residual stellar speckles caused by noncommon path aberrations (NCPAs) and diffraction effects from telescope obstructions. These techniques have become increasingly important in improving contrast performance and enhancing the overall quality of images. The primary objective of this study is to showcase an efficient approach for reducing speckle intensity attributed to NCPAs during observational sessions conducted using the VLT/SPHERE instrument. To achieve this goal, researchers have implemented an iterative dark-hole (DH) algorithm designed to eliminate stellar speckles directly on-sky prior to commencing scientific observations. This innovative technique leverages a pair-wise probing estimator along with a controller based on electric field conjugation, marking a significant advancement in the field. The results of this study demonstrate a notable reduction in the standard deviation of normalized intensity within raw images, with improvements reaching up to fivefold in corrected regions compared to existing calibration strategies under typical VLT conditions. Remarkably, this enhanced contrast performance is achieved with just 1 minute of exposure time, rivaling median outcomes obtained through post-processing techniques that typically necessitate hour-long observation sequences on SPHERE. Additionally, researchers introduce an alternative calibration method that capitalizes on starlight coherence, resulting in a threefold enhancement in post-processed contrast levels root mean square (rms). These findings underscore the potential for optimizing future design, development, and observational strategies for next-generation ground-based exoplanet imagers tailored for telescopes ranging from 10m to 40m. Overall, this on-sky demonstration represents a pivotal milestone in advancing the capabilities and efficacy of ground-based exoplanet imaging technologies. It sets the stage for enhanced precision and efficiency in capturing distant planetary systems within our universe. With continued advancements and refinements, these techniques have the potential to greatly expand our understanding of exoplanets and their host stars.