During the past decade, state-of-the-art planet-finder instruments like SPHERE@VLT, coupling coronagraphic devices and extreme adaptive optics systems, unveiled, thanks to large surveys, around 20 planetary mass companions at semi-major axis greater than 10 astronomical units. Direct imaging being the only detection technique to be able to probe this outer region of planetary systems, the SPHERE infrared survey for exoplanets (SHINE) was designed and conducted from 2015 to 2021 to study the demographics of such young gas giant planets around 400 young nearby solar-type stars. In this paper, we present the observing strategy, the data quality, and the point sources analysis of the full SHINE statistical sample as well as snapSHINE. Both surveys used the SPHERE@VLT instrument with the IRDIS dual band imager in conjunction with the integral field spectrograph IFS and the angular differential imaging observing technique. All SHINE data (650 datasets), corresponding to 400 stars, including the targets of the F150 survey, are processed in a uniform manner with an advanced post-processing algorithm called PACO ASDI. An emphasis is put on the classification and identification of the most promising candidate companions. Compared to the previous early analysis SHINE F150, the use of advanced post-processing techniques significantly improved by one or 2 magnitudes (x3-x6) the contrast detection limits, which will allow us to put even tighter constraints on the radial distribution of young gas giants. This increased sensitivity directly places SHINE as the largest and deepest direct imaging survey ever conducted. We detected and classified more than 3500 physical sources. One additional substellar companion has been confirmed during the second phase of the survey (HIP 74865 B), and several new promising candidate companions are awaiting second epoch confirmations.

We present new optical and near-IR images of debris disk around the F-type star HD 114082. We obtained direct imaging observations and analysed the TESS photometric time series data of this target with a goal to search for planetary companions and to characterise the morphology of the debris disk and the scattering properties of dust particles. HD 114082 was observed with the VLT/SPHERE instrument: the IRDIS camera in the K band together with the IFS in the Y, J and H band using the ADI technique as well as IRDIS in the H band and ZIMPOL in the I_PRIME band using the PDI technique. The scattered light images were fitted with a 3D model for single scattering in an optically thin dust disk. We performed aperture photometry in order to derive the scattering and polarized phase functions, polarization fraction and spectral scattering albedo for the dust particles in the disk. This method was also used to obtain the reflectance spectrum of the disk to retrieve the disk color and study the dust reflectivity in comparison to the debris disk HD 117214. We also performed the modeling of the HD 114082 light curve measured by TESS using the models for planet transit and stellar activity to put constraints on radius of the detected planet and its orbit. The debris disk appears as an axisymmetric debris belt with a radius of ~0.37$"$ (35 au), inclination of ~83$^\circ$ and a wide inner cavity. Dust particles in HD 114082 have a maximum polarization fraction of ~17% and a high reflectivity which results in a spectral scattering albedo of 0.65. The analysis of TESS photometric data reveals a transiting planetary companion to HD 114082 with a radius of $\sim$1~$\rm R_{J}$ on an orbit with a semi-major axis of $0.7 \pm 0.4$ au. Combining different data, we reach deep sensitivity limits in terms of companion masses down to ~5$M_{\rm Jup}$ at 50 au, and ~10 $M_{\rm Jup}$ at 30 au from the central star.

Spectroscopic techniques can extract information about the chiral structure of molecules. However, because these methods typically rely on chiroptical effects that are very weak, even small errors in our control of optical polarization can induce experimental artifacts. Distinguishing these artifacts from true signals remains an important practical problem. Here, we present a comprehensive study of chiroptical artifacts using Raman optical activity (ROA) as a challenging example. ROA measures the difference in Raman scattering between right- and left-circularly polarized light. While ROA spectra can yield valuable information about the vibrational modes and handedness of a chiral molecule, ROA is particularly prone to artifacts as signals are 10(3)-10(4) times smaller than in standard Raman spectroscopy. We develop a Mueller-matrix model to examine the origins of artifacts in ROA. We then combine our model with experimental examples from multiple ROA instruments to understand real-world artifacts. For example, we disprove a commonly held belief that ROA spectra that exhibit mirror symmetry about the intensity axis for an enantiomeric pair confirm a true signal. Based on our findings, we describe characteristics of common artifacts and propose a list of standard controls that researchers should perform to increase the likelihood that their data represent true signal. This work is intended as a resource for those working in chiroptical spectroscopy, providing methods to understand, identify, and avoid experimental artifacts.

The polarimetric zero-level accuracy of spectropolarimetric measurements with ground-based solar telescopes usually suffers from systematic telescopic and instrumental effects which are difficult to model, and therefore, cannot be easily removed during post-measurement data reduction. Here, a novel measurement method to enhance the zero-level accuracy to an unprecedented level of such compromised measurements is presented. The method is comprised of adding a slow polarization modulation (< 1 Hz) before any polarizing component of the telescope to a high-sensitivity polarimeter with fast modulation (> 1 kHz). This additional slow modulation significantly mitigates systematic instrumental polarization signals induced by the telescope and post-focus instruments such as polarimetric offsets or cross-talk between polarization states. We present the results and limitations learned from implementing the method at the 45 cm Gregory-Coudé telescope at IRSOL, Locarno. The slow modulation is performed with a low-cost zero-order retarder film mounted in front of the telescope and is combined with the fast modulating Z¨urich IMaging POLarimeter (ZIMPOL). We find that the ground zero of polarization normalized to the intensity is determined within a few 10−5 . This level is consistently achieved over a wide wavelength range in the visible. An improvement of up to a few orders of magnitude for cases where the polarization offset induced by the telescope is as high as 10−2 is achieved. This measurement technique allows for enhancing the zero-level accuracy of solar polarimetry, which is crucial for scattering polarization measurements and their theoretical interpretations.

Many chiroptical spectroscopic techniques have been developed to detect chirality in molecular species and probe its role in biological processes. Raman optical activity (ROA) should be one of the most powerful methods, as ROA yields vibrational and chirality information simultaneously and can measure analytes in aqueous and biologically relevant solvents. However, despite its promise, the use of ROA has been limited, largely due to challenges in instrumentation. Here, we report a new approach to ROA that exploits high-frequency polarization modulation. High-frequency polarization modulation, usually implemented with a photoelastic modulator (PEM), has long been the standard technique in other chiroptical spectroscopies. Unfortunately, the need for simultaneous spectral and polarization resolution has precluded the use of PEMs in ROA instruments. We combine a specialized camera system (the Zurich imaging polarimeter, or ZIMPOL) with PEM modulation to perform ROA measurements. We demonstrate performance similar to the current standard in ROA instrumentation while reducing complexity and polarization artifacts. This development should aid researchers in exploiting the full potential of ROA for chemical and biological analysis.

Context. Large surveys with new-generation high-contrast imaging instruments are needed to derive the frequency and properties of exoplanet populations with separations from ~5 to 300 au. A careful assessment of the stellar properties is crucial for a proper understanding of when, where, and how frequently planets form, and how they evolve. The sensitivity of detection limits to stellar age makes this a key parameter for direct imaging surveys. Aims. We describe the SpHere INfrared survey for Exoplanets (SHINE), the largest direct imaging planet-search campaign initiated at the VLT in 2015 in the context of the SPHERE Guaranteed Time Observations of the SPHERE consortium. In this first paper we present the selection and the properties of the complete sample of stars surveyed with SHINE, focusing on the targets observed during the first phase of the survey (from February 2015 to February 2017). This early sample composed of 150 stars is used to perform a preliminary statistical analysis of the SHINE data, deferred to two companion papers presenting the survey performance, main discoveries, and the preliminary statistical constraints set by SHINE. Methods. Based on a large database collecting the stellar properties of all young nearby stars in the solar vicinity (including kinematics, membership to moving groups, isochrones, lithium abundance, rotation, and activity), we selected the original sample of 800 stars that were ranked in order of priority according to their sensitivity for planet detection in direct imaging with SPHERE. The properties of the stars that are part of the early statistical sample wererevisited, including for instance measurements from the Gaia Data Release 2. Rotation periods were derived for the vast majority of the late-type objects exploiting TESS light curves and dedicated photometric observations. Results. The properties of individual targets and of the sample as a whole are presented.

The SpHere INfrared Exoplanet (SHINE) project is a 500-star survey performed with SPHERE on the Very Large Telescope for the purpose of directly detecting new substellar companions and understanding their formation and early evolution. Here we present an initial statistical analysis for a subsample of 150 stars spanning spectral types from B to M that are representative of the full SHINE sample. Our goal is to constrain the frequency of substellar companions with masses between 1 and 75 M-Jup and semimajor axes between 5 and 300 au. For this purpose, we adopt detection limits as a function of angular separation from the survey data for all stars converted into mass and projected orbital separation using the BEX-COND-hot evolutionary tracks and known distance to each system. Based on the results obtained for each star and on the 13 detections in the sample, we use a Markov chain Monte Carlo tool to compare our observations to two different types of models. The first is a parametric model based on observational constraints, and the second type are numerical models that combine advanced core accretion and gravitational instability planet population synthesis. Using the parametric model, we show that the frequencies of systems with at least one substellar companion are 23.0(-9.7)(+13.5) 23.0 -9.7 +13.5 , 5.8(-2.8)(+4.7) 5.8 -2.8 +4.7 , and 12.6(-7.1)(+12.9)% 12.6 -7.1 +12.9 % for BA, FGK, and M stars, respectively. We also demonstrate that a planet-like formation pathway probably dominates the mass range from 1-75 M-Jup for companions around BA stars, while for M dwarfs, brown dwarf binaries dominate detections. In contrast, a combination of binary star-like and planet-like formation is required to best fit the observations for FGK stars. Using our population model and restricting our sample to FGK stars, we derive a frequency of 5.7(-2.8)(+3.8)% 5.7 -2.8 +3.8 , consistent with predictions from the parametric model. More generally, the frequency values that we derive are in excellent agreement with values obtained in previous studies.

Context. Young stars with debris disks are the most promising targets for an exoplanet search because debris indicate a successful formation of planetary bodies. Debris disks can be shaped by planets into ring structures that give valuable indications on the presence and location of planets in the disk.Aims. We performed observations of the Sco-Cen F star HD 117214 to search for planetary companions and to characterize the debris disk structure.Methods. HD 117214 was observed with the SPHERE subsystems IRDIS, IFS, and ZIMPOL at optical and near-IR wavelengths using angular and polarimetric differential imaging techniques. This provided the first images of scattered light from the debris disk with the highest spatial resolution of 25 mas and an inner working angle <0.1 ''. With the observations with IRDIS and IFS we derived detection limits for substellar companions. The geometrical parameters of the detected disk were constrained by fitting 3D models for the scattering of an optically thin dust disk. Investigating the possible origin of the disk gap, we introduced putative planets therein and modeled the planet-disk and planet-planet dynamical interactions. The obtained planetary architectures were compared with the detection limit curves.Results. The debris disk has an axisymmetric ring structure with a radius of 0.42(0.01)'' or similar to 45 au and an inclination of 71(+/- 2.5)degrees and exhibits a 0.4 '' (similar to 40 au) wide inner cavity. From the polarimetric data, we derive a polarized flux contrast for the disk of (F-pol)(disk)/F=(3.1 +/- 1.2)x10(-4) in the RI band.Conclusions. The fractional scattered polarized flux of the disk is eight times lower than the fractional IR flux excess. This ratio is similar to the one obtained for the debris disk HIP 79977, indicating that dust radiation properties are similar for these two disks. Inside the disk cavity we achieve high-sensitivity limits on planetary companions with a mass down to similar to 4 M-J at projected radial separations between 0.2 '' and 0.4 ''. We can exclude stellar companions at a radial separation larger than 75 mas from the star.

RefPlanets is a guaranteed time observation (GTO) programme that uses the Zurich IMaging POLarimeter (ZIMPOL) of SPHERE/VLT for a blind search for exoplanets in wavelengths from 600-900 nm. The goals of this study are the characterization of the unprecedented high polarimetic contrast and polarimetric precision capabilities of ZIMPOL for bright targets, the search for polarized reflected light around some of the closest bright stars to the Sun and potentially the direct detection of an evolved cold exoplanet for the first time. For our observations of Alpha Cen A and B, Sirius A, Altair, Eps Eri and Tau Ceti we used the polarimetric differential imaging (PDI) mode of ZIMPOL which removes the speckle noise down to the photon noise limit for angular separations >0.6". We describe some of the instrumental effects that dominate the noise for smaller separations and explain how to remove these additional noise effects in post-processing. We then combine PDI with angular differential imaging (ADI) as a final layer of post-processing to further improve the contrast limits of our data at these separations. For good observing conditions we achieve polarimetric contrast limits of 15.0-16.3 mag at the effective inner working angle of about 0.13", 16.3-18.3 mag at 0.5" and 18.8-20.4 mag at 1.5". The contrast limits closer in (<0.6") depend significantly on the observing conditions, while in the photon noise dominated regime (>0.6"), the limits mainly depend on the brightness of the star and the total integration time. We compare our results with contrast limits from other surveys and review the exoplanet detection limits obtained with different detection methods. For all our targets we achieve unprecedented contrast limits. Despite the high polarimetric contrasts we are not able to find any additional companions or extended polarized light sources in the data that has been taken so far.

Context. With an orbital distance comparable to that of Saturn in the solar system, beta Pictoris b is the closest (semi-major axis similar or equal to 9 au) exoplanet that has been imaged to orbit a star. Thus it offers unique opportunities for detailed studies of its orbital, physical, and atmospheric properties, and of disk-planet interactions. With the exception of the discovery observations in 2003 with NaCo at the Very Large Telescope (VLT), all following astrometric measurements relative to beta Pictoris have been obtained in the southwestern part of the orbit, which severely limits the determination of the planet's orbital parameters. Aims. We aimed at further constraining beta Pictoris b orbital properties using more data, and, in particular, data taken in the northeastern part of the orbit. Methods. We used SPHERE at the VLT to precisely monitor the orbital motion of beta beta Pictoris b since first light of the instrument in 2014. Results. We were able to monitor the planet until November 2016, when its angular separation became too small (125 mas, i.e., 1.6 au) and prevented further detection. We redetected beta Pictoris b on the northeast side of the disk at a separation of 139 mas and a PA of 30 degrees in September 2018. The planetary orbit is now well constrained. With a semi-major axis (sma) of a = 9.0 +/- 0.5 au (1 sigma), it definitely excludes previously reported possible long orbital periods, and excludes beta Pictoris b as the origin of photometric variations that took place in 1981. We also refine the eccentricity and inclination of the planet. From an instrumental point of view, these data demonstrate that it is possible to detect, if they exist, young massive Jupiters that orbit at less than 2 au from a star that is 20 pc away.

Observations of circumstellar environments that look for the direct signal of exoplanets and the scattered light from disks have 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 brought about a new generation of high-contrast imagers and spectrographs on large ground-based telescopes with much better performance. One of the most productive imagers is the Spectro-Polarimetic High contrast imager for Exoplanets REsearch (SPHERE), which was 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), were designed to efficiently cover the near-infrared range in a single observation for an efficient search of young planets. The third instrument, ZIMPOL, was designed for visible polarimetric observation to look for the reflected light of exoplanets and the light scattered by debris disks. These three scientific instruments enable the study of circumstellar environments at unprecedented angular resolution, both in the visible and the near-infrared. In this work, we thoroughly present SPHERE and its on-sky performance after four years of operations at the VLT.

Sr I 4607 Å spectral line shows one of the strongest scattering polarization signals in the visible solar spectrum. The amplitudes of these signals are expected to vary at granular spatial scales. This variation can be due to changes in the magnetic field intensity and orientation (Hanle effect) as well as due to spatial and temporal variations in the plasma properties. Measuring the spatial variation of such polarization signal would allow us to study the properties of the magnetic fields at subgranular region. But, the observations are challenging since both high spatial resolution and high spectropolarimetric sensitivity are required at the same time. To the aim of measuring these spatial variations at granular scale, we carried out a spectropolarimetric measurement with the Zurich IMaging POLarimeter (ZIMPOL), at the GREGOR solar telescope at different limb distances on solar disk. Our results show a spatial variation of scattering linear polarization signals in Sr I 4607 Å line at the granular scale at every μ, starting from 0.2 to 0.8. The correlation between the polarization signal amplitude and the continuum intensity imply statistically that the scattering polarization is higher at the granular regions than in the intergranular lanes.

Context. The Sr I 4607 angstrom spectral line shows one of the strongest scattering polarization signals in the visible solar spectrum. The amplitude of this polarization signal is expected to vary at granular spatial scales, due to the combined action of the Hanle effect and the local anisotropy of the radiation field. Observing these variations would be of great interest because it would provide precious information on the small-scale activity of the solar photosphere. At present, few detections of such spatial variations have been reported. This is due to the difficulty of these measurements, which require combining high spatial (similar to 0.1 ''), spectral (<= 20 m angstrom), and temporal resolution (<1 min) with increased polarimetric sensitivity (similar to 10(-4)). Aims. We aim to detect spatial variations at granular scales of the scattering polarization peak of the Sr I 4607 angstrom line at different limb distances, and to study the correlation with the continuum intensity. Methods. Using the Zurich IMaging POLarimeter (ZIMPOL) system mounted at the GREGOR telescope and spectrograph in Tenerife, Spain, we carried out spectro-polarimetric measurements to obtain the four Stokes parameters in the Sr I line at different limb distances, from mu = 0.2 to mu = 0.8, on the solar disk. Results. Spatial variations of the scattering polarization signal in the Sr I 4607 angstrom line, with a spatial resolution of about 0.66 '', are clearly observed at every mu. The spatial scale of these variations is comparable to the granular size. A statistical analysis reveals that the linear scattering polarization amplitude in this Sr I spectral line is positively correlated with the intensity in the continuum, corresponding to the granules, at every mu.

Sr I 4607\AA spectral line shows one of the strongest scattering polarization signals in the visible solar spectrum. The amplitudes of these signals are expected to vary at granular spatial scales. This variation can be due to changes in the magnetic field intensity and orientation (Hanle effect) as well as due to spatial and temporal variations in the plasma properties. Measuring the spatial variation of such polarization signal would allow us to study the properties of the magnetic fields at subgranular region. But, the observations are challenging since both high spatial resolution and high spectropolarimetric sensitivity are required at the same time. To the aim of measuring these spatial variations at granular scale, we carried out a spectro-polarimetric measurement with the Zurich IMaging POLarimeter (ZIMPOL), at the GREGOR solar telescope at different limb distances on solar disk. Our results show a spatial variation of scattering linear polarization signals in Sr I 4607\AA line at the granular scale at every $\mu$, starting from 0.2 to 0.8. The correlation between the polarization signal amplitude and the continuum intensity imply statistically that the scattering polarization is higher at the granular regions than in the intergranular lanes.

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 Δ 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 Δ p < 0.01% and the polarization zero point can be calibrated to better than Δ 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 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 H α emission.

Context. The scattering polarization signal observed in the photospheric Sr I 4607 Å line is expected to vary at granular spatial scales. This variation can be due to changes in the magnetic field intensity and orientation (Hanle effect), but also to spatial and temporal variations in the plasma properties. Measuring the spatial variation of such polarization signal would allow us to study the properties of the magnetic fields at subgranular scales, but observations are challenging since both high spatial resolution and high spectropolarimetric sensitivity are required. Aims. We aim to provide observational evidence of the polarization peak spatial variations, and to analyze the correlation they might have with granulation. Methods. Observations conjugating high spatial resolution and high spectropolarimetric precision were performed with the Zurich IMaging POLarimeter, ZIMPOL, at the GREGOR solar telescope, taking advantage of the adaptive optics system and the newly installed image derotator. Results. Spatial variations of the scattering polarization in the Sr I 4607 Å line are clearly observed. The spatial scale of these variations is comparable with the granular size. Small correlations between the polarization signal amplitude and the continuum intensity indicate that the polarization is higher at the center of granules than in the intergranular lanes.

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.

Context. R Aqr is a symbiotic binary system consisting of a mira variable, a hot companion with a spectacular jet outflow, and an extended emission line nebula. Because of its proximity to the Sun, this object has been studied in much detail with many types of high resolution imaging and interferometric techniques. We have used R Aqr as test target for the visual camera subsystem ZIMPOL, which is part of the new extreme adaptive optics (AO) instrument SPHERE at the Very Large Telescope (VLT). Aims. We describe SPHERE/ZIMPOL test observations of the R Aqr system taken in Hα and other filters in order to demonstrate the exceptional performance of this high resolution instrument. We compare our observations with data from the Hubble Space Telescope (HST) and illustrate the complementarity of the two instruments. We use our data for a detailed characterization of the inner jet region of R Aqr. Methods. We analyze the high resolution ≈25 mas images from SPHERE/ZIMPOL and determine from the Hα emission the position, size, geometric structure, and line fluxes of the jet source and the clouds in the innermost region <2′′ (<400 AU) of R Aqr. The data are compared to simultaneous HST line filter observations. The Hα fluxes and the measured sizes of the clouds yield Hα emissivities for many clouds from which one can derive the mean density, mass, recombination time scale, and other cloud parameters. Results. Our Hα data resolve for the first time the R Aqr binary and we measure for the jet source a relative position 45 mas West (position angle −89.5◦) of the mira. The central jet source is the strongest Hα component with a flux of about 2.5 × 10−12 erg cm−2 s−1. North east and south west from the central source there are many clouds with very diverse structures. Within 0.5′′ (100 AU) we see in the SW a string of bright clouds arranged in a zig-zag pattern and, further out, at 1′′−2′′, fainter and more extended bubbles. In the N and NE we see a bright, very elongated filamentary structure between 0.2′′−0.7′′ and faint perpendicular “wisps” further out. Some jet clouds are also detected in the ZIMPOL [O I] and He I filters, as well as in the HST-WFC3 line filters for Hα, [O III], [N II], and [O I]. We determine jet cloud parameters and find a very well defined correlation Ne ∝ r−1.3 between cloud density and distance to the central binary. Densities are very high with typical values of Ne ≈ 3 × 105 cm−3 for the “outer” clouds around 300 AU, Ne ≈ 3 × 106 cm−3 for the “inner” clouds around 50 AU, and even higher for the central jet source. The high Ne of the clouds implies short recombination or variability timescales of a year or shorter. Conclusions. Hα high resolution data provide a lot of diagnostic information for the ionized jet gas in R Aqr. Future Hα observations will provide the orientation of the orbital plane of the binary and allow detailed hydrodynamical investigations of this jet outflow and its interaction with the wind of the red giant companion.