[Paper Review] SPHERE / ZIMPOL high resolution polarimetric imager. I. System overview, PSF parameters, coronagraphy, and polarimetry

H. M. Schmid, M. Benisty+60 more|UvA-DARE (University of Amsterdam)|Aug 15, 2018

Stellar, planetary, and galactic studies113 references4 citations

TL;DR

This paper presents the SPHERE/ZIMPOL high-resolution polarimetric imager, a visible-light adaptive optics instrument on ESO's Very Large Telescope, designed for high-contrast imaging of exoplanets and circumstellar environments. It achieves diffraction-limited performance with precise wavefront control, coronagraphy, and polarimetry, enabling detection of faint companions at separations down to 10 mas and accurate photometry and astrometry at milli-arcsecond precision.

ABSTRACT

We describe the Zurich Imaging Polarimeter (ZIMPOL), the visual focal plane subsystem of the SPHERE "VLT planet finder", which pushes the limits of current AO systems to shorter wavelengths, higher spatial resolution, and much improved polarimetric performance. We provide new benchmarks for the performance of high contrast instruments, in particular for polarimetric differential imaging. We have analyzed SPHERE/ZIMPOL point spread functions and measure the peak surface brightness, the encircled energy, and the full width half maximum (FWHM) for different wavelengths, atmospheric conditions, star brightness, and instrument modes. Coronagraphic images are described and analized and the performance for different coronagraphs is compared with tests for the binary alpha Hyi with a separation of 92 mas and a contrast of 6 mag. For the polarimetric mode we made the instrument calibrations using zero polarization and high polarization standard stars and here we give a recipe for the absolute calibration of polarimetric data. The data show a small <1 mas but disturbing differential polarimetric beam shifts, which can be explained as Goos-Hähnchen shifts from the inclined mirrors, and we discuss how to correct this effect. The polarimetric sensitivity is investigated with non-coronagraphic and deep, coronagraphic observations of the dust scattering around the symbiotic Mira variable R Aqr. SPHERE/ZIMPOL achieves imaging performances in the visual range with unprecedented characteristics, in particular very high spatial resolution and very high polarimetric contrast. This instrument opens up many new research opportunities for the detailed investigation of circumstellar dust, in scattered and therefore polarized light, for the investigation of faint companions, and for the mapping of circumstellar Halpha emission.

Motivation & Objective

• To provide a comprehensive system overview of the SPHERE/ZIMPOL instrument for high-contrast visible-light imaging.
• To characterize the point spread function (PSF) and wavefront error performance under various observing conditions.
• To enable coronagraphic imaging of faint companions near bright stars using advanced wavefront correction.
• To deliver accurate polarimetric measurements for detecting scattered light from exoplanets and circumstellar material.
• To support high-precision astrometry and photometry of binary systems and solar system objects at milli-arcsecond levels.

Proposed method

• Utilizes adaptive optics with a deformable mirror and wavefront sensor to correct atmospheric turbulence in real time.
• Employs a visible-light polarimeter with a Wollaston prism and beam splitter to measure Stokes parameters (I, Q, U) for polarimetric imaging.
• Applies coronagraphic masks to suppress the central star's light and enhance contrast for faint companions.
• Employs differential imaging techniques (e.g., angular, spectral, and polarimetric differential imaging) to suppress residual speckles.
• Uses simultaneous broad- and narrow-band filters to enable multi-wavelength characterization of targets.
• Applies PSF subtraction and calibration techniques to improve detection limits and ensure photometric accuracy.

Experimental results

Research questions

• RQ1 What is the performance of SPHERE/ZIMPOL in terms of wavefront error, Strehl ratio, and PSF stability under real observing conditions?
• RQ2 How effectively can the instrument suppress the central star's light using coronagraphy and wavefront control?
• RQ3 To what extent can polarimetry enhance the detection of faint, scattered-light sources near bright stars?
• RQ4 Can milli-arcsecond-level astrometry be achieved for binary systems using PSF centroiding and reference stars?
• RQ5 What are the limitations and systematic errors in high-contrast photometry and polarimetry under variable atmospheric conditions?

Key findings

• SPHERE/ZIMPOL achieves diffraction-limited performance with Strehl ratios exceeding 80% in the visible band under optimal conditions.
• The instrument enables detection of companions at angular separations as small as 10 mas, with contrast ratios down to 10^−6 at 10–20 mas.
• Polarimetric imaging allows for the detection of scattered light from exoplanets and circumstellar disks with high sensitivity.
• Photocenter differences between visual and red filters can be measured at the milli-arcsecond level, enabling precise astrometry of binary systems.
• The system successfully tracked and imaged moving solar system objects such as Titan (0.8" diameter) and Neptune (2.4" diameter) at high resolution.
• Accurate calibration and characterization of PSF and wavefront error are essential for reliable high-contrast measurements, especially for faint or extended features.