SPHERE: the exoplanet imager for the Very Large Telescope

Abstract: Observations of circumstellar environments to look for the direct signal of exoplanets and the scattered light from disks has significant instrumental implications. In the past 15 years, major developments in adaptive optics, coronagraphy, optical manufacturing, wavefront sensing and data processing, together with a consistent global system analysis have enabled a new generation of high-contrast imagers and spectrographs on large ground-based telescopes with much better performance. One of the most productive is the Spectro-Polarimetic High contrast imager for Exoplanets REsearch (SPHERE) designed and built for the ESO Very Large Telescope (VLT) in Chile. SPHERE includes an extreme adaptive optics system, a highly stable common path interface, several types of coronagraphs and three science instruments. Two of them, the Integral Field Spectrograph (IFS) and the Infra-Red Dual-band Imager and Spectrograph (IRDIS), are designed to efficiently cover the near-infrared (NIR) range in a single observation for efficient young planet search. The third one, ZIMPOL, is designed for visible (VIR) polarimetric observation to look for the reflected light of exoplanets and the light scattered by debris disks. This suite of three science instruments enables to study circumstellar environments at unprecedented angular resolution both in the visible and the near-infrared. In this work, we present the complete instrument and its on-sky performance after 4 years of operations at the VLT.

• The paper presents SPHERE as an advanced exoplanet imager using extreme adaptive optics and coronagraphs to achieve contrasts exceeding 10⁻⁶.
• It details the use of IRDIS, IFS, and ZIMPOL for dual-band imaging, spectral, and polarimetric analysis of exoplanetary systems.
• The study reports a 90% H-band Strehl ratio and outlines future upgrades like improved NCPA correction to enhance direct imaging capabilities.

Overview of SPHERE: The Exoplanet Imager for the Very Large Telescope

The paper presents an exhaustive account of the development, design, and operational capabilities of the Spectro-Polarimetric High-contrast Exoplanet Research (SPHERE) instrument mounted on the Very Large Telescope (VLT) in Chile. SPHERE was crafted to facilitate advancements in the direct imaging of exoplanets and the analysis of circumstellar disks. This essay provides an overview of the key elements presented in the paper.

Instrument Design and Components

SPHERE integrates several state-of-the-art features and systems:

1. Extreme Adaptive Optics (XAO): The adaptive optics system, named SAXO, is a critical component providing high-quality wavefront correction. This system employs a fast (1380 Hz) tip-tilt mirror and a 41x41 actuator high-order deformable mirror, enabling correction of atmospheric turbulence to achieve high Strehl ratios in the near-infrared, thereby enhancing the contrast performance necessary for exoplanet imaging.
2. Coronagraphs: SPHERE utilizes advanced coronagraphic techniques, including the apodized-pupil Lyot coronagraph (APLC) and the four-quadrant phase-mask coronagraph (4QPM), to significantly attenuate starlight and improve the detection capabilities of faint companions around bright stars.
3. Science Instruments:
   • Infrared Dual-band Imager and Spectrograph (IRDIS): Facilitates dual-band imaging and polarimetric studies vital for understanding circumstellar environments and exoplanet atmospheres.
   • Integral Field Spectrograph (IFS): Provides a low-resolution spectral data cube over a field of view around the target star, instrumental in distinguishing exoplanetary signals using spectral differential imaging.
   • Zurich Imaging Polarimeter (ZIMPOL): Delivers high-resolution polarimetric imaging in the visible light spectrum, particularly beneficial for characterizing disks and searching for polarimetric signals of scattered light from exoplanets.

Key Achievements and Performance Metrics

This extensive paper reports that SPHERE has achieved significant milestones in exoplanet imaging and characterization:

• High Angular Resolution and Contrast: The XAO system consistently achieves an $H$-band Strehl ratio of around 90% enabling SPHERE to reach contrasts exceeding $10^{-6}$ at small angular separations from the host star.
• Spectral and Polarimetric Analysis: The combinatorial use of IRDIS, IFS, and ZIMPOL allows comprehensive analysis across different spectral and polarimetric domains, supporting various observation modes including dual-polarimetry and long-slit spectroscopy with IRDIS, and high-contrast differential polarimetric imaging with ZIMPOL.

Future Prospects

The paper outlines possible future enhancements to maintain and improve SPHERE’s scientific yield:

• NCPA Correction: Continuous improvement in the correction of non-common path aberrations holds potential for contrast enhancement.
• Coronagraph Development: Development of next-generation coronagraphs with smaller inner working angles could potentially increase SPHERE’s capabilities in the discovery and characterization of planets closer to their host stars.
• High-Resolution Imaging: Future integration with high-resolution spectrographs could facilitate detailed spectral characterization of exoplanetary atmospheres.

Conclusion

SPHERE stands as a testament to the collaborative effort in advancing direct imaging capabilities. It not only represents a leap in technology but also continuously challenges the boundaries of current astronomical instrumentation practices. The design choices, though fraught with complexity, have proven effective and continue to guide future upgrades aimed at unraveling the mysteries of planetary formation and diversity outside our solar system.