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

Abstract

Context. The SPHERE "planet finder" is an extreme adaptive optics (AO) instrument for high resolution and high contrast observations at the Very Large Telescope (VLT). We describe the Zurich Imaging Polarimeter (ZIMPOL), the visual focal plane subsystem of SPHERE, which pushes the limits of current AO systems to shorter wavelengths, higher spatial resolution, and much improved polarimetric performance. Aims. We present a detailed characterization of SPHERE/ZIMPOL which should be useful for an optimal planning of observations and for improving the data reduction and calibration. We aim to provide new benchmarks for the performance of high contrast instruments, in particular for polarimetric differential imaging. Methods. We have analyzed SPHERE/ZIMPOL point spread functions (PSFs) and measure the normalized 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 the peak flux attenuation and the off-axis flux transmission are determined. Simultaneous images of the coronagraphic focal plane and the pupil plane are analyzed and the suppression of the diffraction rings by the pupil stop is investigated. We compared the performance at small separation for different coronagraphs with tests for the binary α Hyi with a separation of 92 mas and a contrast of ∆m ≈ 6 m . 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. Results. SPHERE/ZIMPOL reaches routinely an angular resolution (FWHM) of 22−28 mas, and a normalized peak surface brightness of SB 0 − m star ≈ −6.5 m arcsec −2 for the V-, R-and I-band. The AO performance is worse for mediocre ∼ > 1.0 seeing conditions, faint stars m R ∼ > 9 m , or in the presence of the "low wind" effect (telescope seeing). The coronagraphs are effective in attenuating the PSF peak by factors of > 100, and the suppression of the diffracted light improves the contrast performance by a factor of approximately two in the separation range 0.06 −0.20 . The polarimetric sensitivity is ∆p < 0.01 % and the polarization zero point can be calibrated to better than ∆p ≈ 0.1 %. The contrast limits for differential polarimetric imaging for the 400 s I-band data of R Aqr at a separation of ρ = 0.86 are for the surface brightness contrast SB pol (ρ) − m star ≈ 8 m arcsec −2 and for the point source contrast m pol (ρ) − m star ≈ 15 m and much lower limits are achievable with deeper observations. Conclusions. SPHERE/ZIMPOL achieves imagi...

A search for accreting young companions embedded in circumstellar disks

“…The data were all obtained with the ZIMPOL sub-instrument of the adaptive optics (AO) assisted high-contrast imager SPHERE (Beuzit et al 2008;Petit et al 2008;Fusco et al 2016), which is installed at the Very Large Telescope (VLT) of the European Southern Observatory (ESO) on Paranal in Chile. A detailed description of ZIMPOL can be found in Schmid et al (2018). Some of the data were collected within the context of the Guaranteed Time Observations (GTO) program of the SPHERE consortium; others were obtained in other programs and downloaded from the ESO data archive (program IDs are listed in Table 1).…”

“…of disk material (Biller et al 2012;Price et al 2018). Avenhaus et al (2017) obtained polarimetric differential imaging data with SPHERE/ZIMPOL in the very broad band (VBB, as defined in Schmid et al 2018) optical filter, revealing new substructures, and resolving the innermost regions of the disk (down to 0 . 025).…”

Differential speckle polarimetry at Cassegrain and Nasmyth foci

“…In Figure 9 we plotted for comparison the detection limit curve in terms of polarized flux for VLT/ZIMPOL (Schmid et al 2018). This coronagraph operates in the wavelength range from 600 nm to 900 nm.…”

“…These slopes are produced by differential polarization aberrations caused by oblique reflection (Breckinridge et al 2015). The effect was analysed in detail by Schmid et al (2018) for the case of SPHERE/ZIMPOL. They dubbed it "polarimetric differential beamshift".…”

“…The thin dashed line stands for typical detection threshold for the SOAR speckle interferometer(Tokovinin et al 2010) (total flux). The thin line with squares stands for typical detection threshold for the VLT/ZIMPOL(Schmid et al 2018) (polarized flux).…”