SPHERE/ZIMPOL high resolution polarimetric imager

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 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 imaging performances ...

The widest Hα survey of accreting protoplanets around nearby transition disks

“…with a 41 × 41 actuators wavefront control, pupil stabilization, and differential tip-tilt control (Petit et al 2014). The instrument has three science subsystems: the infrared dual-band imager and spectrograph (IRDIS; Dohlen et al 2008), an integral field spectrograph (IFS; Claudi et al 2008), and the Zimpol rapid-switching imaging polarimeter (ZIMPOL; Thalmann et al 2008;Schmid et al 2018). The latter, which is the only subsystem in visible light, has been used for the survey.…”

“…The observing strategy was to take simultaneous images in the Hα narrow band filter (N_Ha; λ c = 656.9 nm, ∆λ = 1 nm) and in Hα continuum (Cont_Ha; λ c = 644.9 nm, ∆λ = 4.1 nm). We refer the reader to Schmid et al (2018) for more details about the filters. All the observations were taken in pupil stabilized mode in order to take advantage of the speckles suppression technique of the angular differential imaging (ADI; Marois et al 2006).…”

Polarimetry of Water Ice Particles Providing Insights on Grain Size and Degree of Sintering on Icy Planetary Surfaces

“…Spatially resolved polarimetric observations of the icy satellites could help to identify areas affected by these processes. Such observations can be achieved using large ground-based telescopes, such as the VLT equipped with SPHERE and its high-angular resolution polarimetric imager ZIMPOL (Schmid et al, 2018). Much higher-spatial resolution could be provided by polarimetric instruments onboard spacecrafts.…”

A new white dwarf companion around the Δμ star GJ 3346

“…Several types of coronagraphic devices for stellar diffraction suppression are provided, including apodized pupil Lyot coronagraphs (Soummer 2005) and achromatic four-quadrants phase masks (Boccaletti et al 2008). The instrument has three science subsystems: the infrared dual-band imager and spectrograph (IRDIS, Dohlen et al 2008), an integral field spectrograph (IFS; Claudi et al 2008) and the Zimpol rapid-switching imaging polarimeter (ZIMPOL; Schmid et al 2018).…”