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Christoph U. Keller
Professor of Experimental Astrophysics

Leiden Observatory
Leiden University, The Netherlands

Focal-plane wavefront sensing with high-order adaptive optics systems

1 Sep 2014

by Korkiakoski, Visa, Keller, Christoph U., Doelman, Niek, Kenworthy, Matthew, Otten, Gilles, Verhaegen, Michel, is now available here.

Abstract: We investigate methods to calibrate the non-common path aberrations at an adaptive optics system having a wavefront-correcting device working at an extremely high resolution (larger than 150x150). We use focal-plane images collected successively, the corresponding phase-diversity information and numerically efficient algorithms to calculate the required wavefront updates. The wavefront correction is applied iteratively until the algorithms converge. Different approaches are studied. In addition of the standard Gerchberg-Saxton algorithm, we test the extension of the Fast & Furious algorithm that uses three images and creates an estimate of the pupil amplitudes. We also test recently proposed phase-retrieval methods based on convex optimisation. The results indicate that in the framework we consider, the calibration task is easiest with algorithms similar to the Fast & Furious.

Fast & Furious focal-plane wavefront sensing

1 Sep 2014

by Korkiakoski, Visa, Keller, Christoph U., Doelman, Niek, Kenworthy, Matthew, Otten, Gilles, Verhaegen, Michel, is now available here.

Abstract: We present two complementary algorithms suitable for using focal-plane measurements to control a wavefront corrector with an extremely high spatial resolution. The algorithms use linear approximations to iteratively minimize the aberrations seen by the focal-plane camera. The first algorithm, Fast & Furious (FF), uses a weak-aberration assumption and pupil symmetries to achieve fast wavefront reconstruction. The second algorithm, an extension to FF, can deal with an arbitrary pupil shape; it uses a Gerchberg-Saxton style error reduction to determine the pupil amplitudes. Simulations and experimental results are shown for a spatial light modulator controlling the wavefront with a resolution of 170 x 170 pixels. The algorithms increase the Strehl ratio from ~0.75 to 0.98-0.99, and the intensity of the scattered light is reduced throughout the whole recorded image of 320 x 320 pixels. The remaining wavefront rms error is estimated to be ~0.15 rad with FF and ~0.10 rad with FF-GS.

Spectral line polarimetry with a channeled polarimeter

1 Sep 2014

by van Harten, Gerard, Snik, Frans, Rietjens, Jeroen H. H., Martijn Smit, J., Keller, Christoph U., is now available here.

Abstract: Not Available

Optimization-based wavefront sensorless adaptive optics for multiphoton microscopy

1 Sep 2014

by Antonello, Jacopo, van Werkhoven, Tim, Verhaegen, Michel, Truong, Hoa H., Keller, Christoph U., Gerritsen, Hans C., is now available here.

Abstract: Not Available

Atmospheric aerosol characterization with a ground-based SPEX spectropolarimetric instrument

1 Sep 2014

by van Harten, G., de Boer, J., Rietjens, J. H. H., Di Noia, A., Snik, F., Volten, H., Smit, J. M., Hasekamp, O. P., Henzing, J. S., Keller, C. U., is now available here.

Abstract: Characterization of atmospheric aerosols is important for understanding their impact on health and climate. A wealth of aerosol parameters can be retrieved from multi-angle, multi-wavelength radiance and polarization measurements of the clear sky. We developed a ground-based SPEX instrument (groundSPEX) for accurate spectropolarimetry, based on the passive, robust, athermal and snapshot spectral polarization modulation technique, and hence ideal for field deployment. It samples the scattering phase function in the principal plane in an automated fashion, using a motorized pan/tilt unit and automatic exposure time detection. Extensive radiometric and polarimetric calibrations were performed, yielding values for both random noise and systematic uncertainties. The absolute polarimetric accuracy at low degrees of polarization is established to be ~ 5 × 10-3. About 70 measurement sequences have been performed throughout four clear-sky days at Cabauw, the Netherlands. Several aerosol parameters were retrieved: aerosol optical thickness, effective radius, and complex refractive index for fine and coarse mode. The results are in good agreement with the co-located AERONET products, with a correlation coefficient of ρ = 0.932 for the total aerosol optical thickness at 550 nm.

Five-dimensional optical instrumentation: combining polarimetry with time-resolved integral-field spectroscopy

1 Sep 2014

by Rodenhuis, M., Snik, F., van Harten, G., Hoeijmakers, J., Keller, C. U., is now available here.

Abstract: We present implementations of optical instrumentation that records five dimensions of light: polarization state as a function of wavelength, two spatial dimensions, and time. We focus on the optimal integration of polarimetry within microlens-based integral-field spectroscopy. The polarimetric analyzer (or beam-splitter) and dispersing element could be implemented separately, but also amalgamated in the form of a polarization grating. We present optimizations for stacking the polarization-split spectra on a 2D detector. The polarimetric modulation can be performed in the temporal, the spatial or the spectral domain. Temporal modulation could be set up with achromatic optics conform the Stokes definition scheme, but a wide wavelength range generally demands a "polychromatic" modulation approach for which the modulation efficiency for all or some of the Stokes parameters is optimized at every wavelength. Spectral modulation (full-Stokes or optimized for linear polarization) yields instruments without any moving parts, for which all polarization information is obtained in one shot. We present first results from two polarimetric IFU instruments; the ExPo pIFU and LOUPE. The first is based on a rapid polychromatic modulator consisting of two FLCs and two fixed retarders, while the latter is based on spectral modulation for linear polarization. In addition to applications within astronomy and planetary science, we discuss remote-sensing applications for such instruments.

Snapshot coherence-gated direct wavefront sensing for multi-photon microscopy

1 Sep 2014

by van Werkhoven, T. I. M., Antonello, J., Truong, H. H., Verhaegen, M., Gerritsen, H. C., Keller, C. U., is now available here.

Abstract: Not Available

Instrumental polarisation at the Nasmyth focus of the E-ELT

23 Apr 2014

by de Juan Ovelar, M., Snik, F., Keller, C. U., Venema, L., is now available here.

Abstract: The ~39-m European Extremely Large Telescope (E-ELT) will be the largest telescope ever built. This makes it particularly suitable for sensitive polarimetric observations, as polarimetry is a photon-starved technique. However, the telescope mirrors may severely limit the polarimetric accuracy of instruments on the Nasmyth platforms by creating instrumental polarisation and/or modifying the polarisation signal of the object. In this paper we characterise the polarisation effects of the two currently considered designs for the E-ELT Nasmyth ports as well as the effect of ageing of the mirrors. By means of the Mueller matrix formalism, we compute the response matrices of each mirror arrangement for a range of zenith angles and wavelengths. We then present two techniques to correct for these effects that require the addition of a modulating device at the "polarisation-free" intermediate focus that acts either as a switch or as a part of a two-stage modulator. We find that the values of instrumental polarisation, Stokes transmission reduction and cross-talk vary significantly with wavelength, and with pointing, for the lateral Nasmyth case, often exceeding the accuracy requirements for proposed polarimetric instruments. Realistic ageing effects of the mirrors after perfect calibration of these effects may cause polarimetric errors beyond the requirements. We show that the modulation approach with a polarimetric element located in the intermediate focus reduces the instrumental polarisation effects down to tolerable values, or even removes them altogether. The E-ELT will be suitable for sensitive and accurate polarimetry, provided frequent calibrations are carried out, or a dedicated polarimetric element is installed at the intermediate focus.

Imaging the circumstellar environment of the young T Tauri star SU Aurigae

23 Apr 2014

by Jeffers, S. V., Min, M., Canovas, H., Rodenhuis, M., Keller, C. U., is now available here.

Abstract: The circumstellar environments of classical T Tauri stars are challenging to directly image because of their high star-to-disk contrast ratio. One method to overcome this is by using imaging polarimetry where scattered and consequently polarised starlight from the star's circumstellar disk can be separated from the unpolarised light of the central star. We present images of the circumstellar environment of SU Aur, a classical T Tauri star at the transition of T Tauri to Herbig stars. The images directly show that the disk extends out to 500 AU with an inclination angle of ~50°. Using interpretive models, we derived very small grains in the surface layers of its disk, with a very steep size- and surface-density distribution. Additionally, we resolved a large and extended nebulosity in our images that is most likely a remnant of the prenatal molecular cloud. The position angle of the disk, determined directly from our images, rules out a polar outflow or jet as the cause of this large-scale nebulosity.

Analysis and interpretation of 15 quarters of Kepler data of the disintegrating planet KIC 12557548 b

23 Apr 2014

by van Werkhoven, T. I. M., Brogi, M., Snellen, I. A. G., Keller, C. U., is now available here.

Abstract: Context. The Kepler object KIC 12557548 shows irregular eclipsing behaviour with a constant 15.685 h period, but strongly varying transit depth. The object responsible for this is believed to be a disintegrating planet forming a trailing dust cloud transiting the star. A 1D model of an exponentially decaying dust tail was found to reproduce the average eclipse in intricate detail. Based on radiative hydrodynamic modelling, the upper limit for the planet mass was found to be twice the mass of the Moon. Aims: In this paper we fit individual eclipses, in addition to fitting binned light curves, to learn more about the process underlying the eclipse depth variation. Additionally, we put forward observational constraints that any model of this planet-star system will have to match. Methods: We manually de-correlated and de-trended 15 quarters of Kepler data, three of which were observed in short cadence mode. We determined the transit depth, egress depth, and stellar intensity for each orbit and search for dependencies between these parameters. We investigated the full orbit by comparing the flux distribution of a moving phase window of interest versus the out-of-eclipse flux distribution. We fit short cadence data on a per-orbit basis using a two-parameter tail model, allowing us to investigate potential dust tail property variations. Results: We find two quiescent spells of ~30 orbital periods each where the transit depth is <0.1%, followed by relatively deep transits. Additionally, we find periods of on-off behaviour where >0.5% deep transits are followed by apparently no transit at all. Apart from these isolated events we find neither significant correlation between consecutive transit depths nor a correlation between transit depth and stellar intensity. We find a three-sigma upper limit for the secondary eclipse of 4.9 × 10-5, consistent with a planet candidate with a radius of less than 4600 km. Using the short cadence data we find that a 1D exponential dust tail model is insufficient to explain the data. We improved our model to a 2D, two-component dust model with an opaque core and an exponential tail. Using this model we fit individual eclipses observed in short cadence mode. We find an improved fit of the data, quantifying earlier suggestions by Budaj (2013, A&A, 557, A72) of the necessity of at least two components. We find that deep transits have most absorption in the tail, and not in a disk-shaped, opaque coma, but the transit depth and the total absorption show no correlation with the tail length.

Crowdsourced aerosol measurements using smartphone spectropolarimeters

23 Apr 2014

by Rietjens, J., Snik, F., Keller, C. U., Heinsbroek, R., van Harten, G., Heikamp, S., de Boer, J., Zeegers, E., Einarsen, L., Hasekamp, O., Smit, M., di Noia, A., Apituley, A., Mijling, B., Hendriks, E., Stammes, P., Volten, H., Vonk, J., Berkhout, S., Haaima, M., van der Hoff, R., Stam, D., Navarro, R., Bettonvil, F., is now available here.

Abstract: We present the development, organisation and results of a large citizen science project with the goal to measure and characterise atmospheric aerosols using a network of smartphone spectropolarimeters. The project, called ';iSPEX', was conceived and carried out in the Netherlands, and organised the first National iSPEX measurement day on July 8th 2013. During this day, more than 3000 people performed over 6000 measurements with their own smartphones using a special add-on and a dedicated app. These measurements were sent to a central database, processed and analysed using a vector-radiative transfer based inversion code in order to extract aerosol properties. The add-on that transforms the camera of the smartphone into a spectropolarimeter and thereby the smartphone into a scientific instrument, employs the method of spectral modulation [1]. The add-on is comprised of polymer parts and was mass-produced and distributed to almost 10000 people. A single measurement involves scanning the blue sky, thereby yielding the angular behaviour of the degree of linear polarisation as a function of wavelength. Although a single iSPEX measurement is not accurate enough, combining many measurements of a crowdsourced experiment with thousands of people should yield sufficiently accurate results that may be interpreted in terms of aerosol optical thickness and aerosol particle properties. By analysing not only the measured results, but also the motivation of the general public to participate, we learn about the possibilities to create a new kind of air quality measurement network. At the conference, we will demonstrate iSPEX and present the results of the first measurement day. We hope to convince you that iSPEX is not only a great outreach tool to engage the public in issues pertaining to atmospheric aerosols, but that it may also contribute to the solution of several urgent societal and scientific problems. [1] Snik, F., Karalidi, T., Keller, C.U.. Spectral modulation for full linear polarimetry. Appl. Opt. 48,1337 (2009) Fig. 1: An iSPEX participant measuring the scattered sun light. Fig. 2: Many iSPEX participants submitting their measured data.

Remote sensing of atmospheric aerosols with the SPEX spectropolarimeter

23 Apr 2014

by van Harten, G., Rietjens, J., Smit, M., Snik, F., Keller, C. U., di Noia, A., Hasekamp, O., Vonk, J., Volten, H., is now available here.

Abstract: Characterizing atmospheric aerosols is key to understanding their influence on climate through their direct and indirect radiative forcing. This requires long-term global coverage, at high spatial (~km) and temporal (~days) resolution, which can only be provided by satellite remote sensing. Aerosol load and properties such as particle size, shape and chemical composition can be derived from multi-wavelength radiance and polarization measurements of sunlight that is scattered by the Earth's atmosphere at different angles. The required polarimetric accuracy of ~10^(-3) is very challenging, particularly since the instrument is located on a rapidly moving platform. Our Spectropolarimeter for Planetary EXploration (SPEX) is based on a novel, snapshot spectral modulator, with the intrinsic ability to measure polarization at high accuracy. It exhibits minimal instrumental polarization and is completely solid-state and passive. An athermal set of birefringent crystals in front of an analyzer encodes the incoming linear polarization into a sinusoidal modulation in the intensity spectrum. Moreover, a dual beam implementation yields redundancy that allows for a mutual correction in both the spectrally and spatially modulated data to increase the measurement accuracy. A partially polarized calibration stimulus has been developed, consisting of a carefully depolarized source followed by tilted glass plates to induce polarization in a controlled way. Preliminary calibration measurements show an accuracy of SPEX of well below 10^(-3), with a sensitivity limit of 2*10^(-4). We demonstrate the potential of the SPEX concept by presenting retrievals of aerosol properties based on clear sky measurements using a prototype satellite instrument and a dedicated ground-based SPEX. The retrieval algorithm, originally designed for POLDER data, performs iterative fitting of aerosol properties and surface albedo, where the initial guess is provided by a look-up table. The retrieved aerosol properties, including aerosol optical thickness, single scattering albedo, size distribution and complex refractive index, will be compared with the on-site AERONET sun-photometer, lidar, particle counter and sizer, and PM10 and PM2.5 monitoring instruments. Retrievals of the aerosol layer height based on polarization measurements in the O2A absorption band will be compared with lidar profiles. Furthermore, the possibility of enhancing the retrieval accuracy by replacing the look-up table with a neural network based initial guess will be discussed, using retrievals from simulated ground-based data.

Calibrating a high-resolution wavefront corrector with a static focal-plane camera

23 Apr 2014

by Korkiakoski, Visa, Doelman, Niek, Codona, Johanan, Kenworthy, Matthew, Otten, Gilles, Keller, Christoph U., is now available here.

Abstract: We present a method to calibrate a high-resolution wavefront-correcting device with a single, static camera, located in the focal plane; no moving of any component is needed. The method is based on a localized diversity and differential optical transfer functions (dOTF) to compute both the phase and amplitude in the pupil plane located upstream of the last imaging optics. An experiment with a spatial light modulator shows that the calibration is sufficient to robustly operate a focal-plane wavefront sensing algorithm controlling a wavefront corrector with ~40 000 degrees of freedom. We estimate that the locations of identical wavefront corrector elements are determined with a spatial resolution of 0.3% compared to the pupil diameter.

Iterative linear focal-plane wavefront correction

23 Apr 2014

by Smith, C. S., Marinică, R., den Dekker, A. J., Verhaegen, M., Korkiakoski, V., Keller, C. U., Doelman, N., is now available here.

Abstract: Not Available

SPHERE-ZIMPOL system testing: status report on polarimetric high contrast results

23 Apr 2014

by Roelfsema, Ronald, Gisler, Daniel, Pragt, Johan, Schmid, Hans Martin, Bazzon, Andreas, Dominik, Carsten, Baruffolo, Andrea, Boccaletti, Anthonoy, Beuzit, Jean-Luc, Costille, Anne, Dohlen, Kjetil, Downing, Mark, Elswijk, Eddy, de Haan, Menno, Hubin, Norbert, Kasper, Markus, Keller, Christoph, Kenworthy, Matthew, Lizon, Jean-Louis, Martinez, Patrice, Mouillet, David, Pavlov, Alexey, Puget, Pascal, Salasnich, Bernardo, Sauvage, Jean-Francois, Thalmann, Christian, Wildi, Francois, is now available here.

Abstract: SPHERE (Spectro-Polarimetric High Contrast Exoplanet Research) is one of the first instruments which aim for the direct detection from extra-solar planets. SPHERE commissioning is foreseen in 2013 on the VLT. ZIMPOL (Zurich Imaging Polarimeter) is the high contrast imaging polarimeter subsystem of the ESO SPHERE instrument. ZIMPOL is dedicated to detect the very faint reflected and hence polarized visible light (600-900 nm) from extrasolar planets. It is located behind an extreme AO system (SAXO) and a stellar coronagraph. We present the first high contrast polarimetric results obtained for the fully integrated SPHERE-ZIMPOL system. We have measured the polarimetric high contrast performance of several coronagraphs: a Classical Lyot on substrate, a suspended Classical Lyot and two 4 Quadrant Phase Mask coronagraphs. We describe the impact of crucial system parameters - Adaptive Optics, Coronagraphy and Polarimetry - on the contrast performance.

Calibrating a high-resolution wavefront corrector with a static focal-plane camera

7 Oct 2013

by Korkiakoski, Visa, Doelman, Niek, Codona, Johanan, Kenworthy, Matthew, Otten, Gilles, Keller, Christoph U., is now available here.

Abstract: We present a method to calibrate a high-resolution wavefront-correcting device with a single, static camera, located in the focal plane; no moving of any component is needed. The method is based on a localized diversity and differential optical transfer functions (dOTF) to compute both the phase and amplitude in the pupil plane located upstream of the last imaging optics. An experiment with a spatial light modulator shows that the calibration is sufficient to robustly operate a focal-plane wavefront sensing algorithm controlling a wavefront corrector with ~40 000 degrees of freedom. We estimate that the locations of identical wavefront corrector elements are determined with a spatial resolution of 0.3% compared to the pupil diameter.

Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 . I. Spectropolarimetric observations in all four Stokes parameters

7 Oct 2013

by Rusomarov, N., Kochukhov, O., Piskunov, N., Jeffers, S. V., Johns-Krull, C. M., Keller, C. U., Makaganiuk, V., Rodenhuis, M., Snik, F., Stempels, H. C., Valenti, J. A., is now available here.

Abstract: Context. High-resolution spectropolarimetric observations provide simultaneous information about stellar magnetic field topologies and three-dimensional distributions of chemical elements. High-quality spectra in the Stokes IQUV parameters are currently available for very few early-type magnetic chemically peculiar stars. Here we present analysis of a unique full Stokes vector spectropolarimetric data set, acquired for the cool magnetic Ap star HD 24712 with a recently commissioned spectropolarimeter. Aims: The goal of our work is to examine the circular and linear polarization signatures inside spectral lines and to study variation of the stellar spectrum and magnetic observables as a function of rotational phase. Methods: HD 24712 was observed with the HARPSpol instrument at the 3.6-m ESO telescope over a period of 2010-2011. We achieved full rotational phase coverage with 43 individual Stokes parameter observations. The resulting spectra have a signal-to-noise ratio of 300-600 and resolving power exceeding 105. The multiline technique of least-squares deconvolution (LSD) was applied to combine information from the spectral lines of Fe-peak and rare earth elements. Results: We used the HARPSPol spectra of HD 24712 to study the morphology of the Stokes profile shapes in individual spectral lines and in LSD Stokes profiles corresponding to different line masks. From the LSD Stokes V profiles we measured the longitudinal component of the magnetic field, ⟨Bz⟩, with an accuracy of 5-10 G. We also determined the net linear polarization from the LSD Stokes Q and U profiles. Combining previous ⟨Bz⟩ measurements with our data allowed us to determine an improved rotational period of the star, Prot = 12.45812 ± 0.00019 d. We also measured the longitudinal magnetic field from the cores of Hα and Hβ lines. The analysis of ⟨Bz⟩ measurements showed no evidence for a significant radial magnetic field gradient in the atmosphere of HD 24712. We used our ⟨Bz⟩ and net linear polarization measurements to determine parameters of the dipolar magnetic field topology. We found that magnetic observables can be reasonably well reproduced by the dipolar model, although significant discrepancies remain at certain rotational phases. We discovered rotational modulation of the Hα core and related it to a non-uniform surface distribution of rare earth elements. Based on observations collected at the European Southern Observatory, Chile (ESO programs 084.D-0338, 085.D-0296, 086.D-0240).Figure 3 and Appendix A are available in electronic form at http://www.aanda.org

Are there tangled magnetic fields on HgMn stars?

7 Oct 2013

by Kochukhov, O., Makaganiuk, V., Piskunov, N., Jeffers, S. V., Johns-Krull, C. M., Keller, C. U., Rodenhuis, M., Snik, F., Stempels, H. C., Valenti, J. A., is now available here.

Abstract: Context. Several recent spectrophotometric studies failed to detect significant global magnetic fields in late-B HgMn chemically peculiar stars, but some investigations have suggested the presence of strong unstructured or tangled fields in these objects. Aims: We used detailed spectrum synthesis analysis to search for evidence of tangled magnetic fields in high-quality observed spectra of eight slowly rotating HgMn stars and one normal late-B star. We also evaluated recent sporadic detections of weak longitudinal magnetic fields in HgMn stars based on the moment technique. Methods: Our spectrum synthesis code calculated the Zeeman broadening of metal lines in HARPS spectra, assuming an unstructured, turbulent magnetic field. A simple line formation model with a homogeneous radial field distribution was applied to assess compatibility between previous longitudinal field measurements and the observed mean circular polarization signatures. Results: Our analysis of the Zeeman broadening of magnetically sensitive spectral lines reveals no evidence of tangled magnetic fields in any of the studied HgMn or normal stars. We infer upper limits of 200-700 G for the mean magnetic field modulus - much smaller than the field strengths implied by studies based on differential magnetic line intensification and quadratic field diagnostics. The new HARPSpol longitudinal field measurements for the extreme HgMn star HD 65949 and the normal late-B star 21 Peg are consistent with zero at a precision of 3-6 G. Re-analysis of our Stokes V spectra of the spotted HgMn star HD 11753 shows that the recent moment technique measurements retrieved from the same data are incompatible with the lack of circular polarization signatures in the spectrum of this star. Conclusions: We conclude that there is no evidence for substantial tangled magnetic fields on the surfaces of studied HgMn stars. We cannot independently confirm the presence of very strong quadratic or marginal longitudinal fields for these stars, so results from the moment technique are likely to be spurious. Based on observations collected at the European Southern Observatory, Chile (ESO programmes 084.D-0338, 085.D-0296, 086.D-0240).

The color dependent morphology of the post-AGB star HD 161796

7 Oct 2013

by Min, M., Jeffers, S. V., Canovas, H., Rodenhuis, M., Keller, C. U., Waters, L. B. F. M., is now available here.

Abstract: Context. Many protoplanetary nebulae show strong asymmetries in their surrounding shells, pointing to asymmetries during the mass loss phase. Questions concerning the origin and the onset of deviations from spherical symmetry are important for our understanding of the evolution of these objects. Here we focus on the circumstellar shell of the post-AGB star HD 161796. Aims: We aim to detect signatures of an aspherical outflow, and to derive its properties. Methods: We used the imaging polarimeter the Extreme Polarimeter (ExPo), a visitor instrument at the William Herschel Telescope, to accurately image the dust shell surrounding HD 161796 in various wavelength filters. Imaging polarimetry allows us to separate the faint, polarized, light that comes from circumstellar material from the bright, unpolarized, light from the central star. Results: The shell around HD 161796 is highly aspherical. A clear signature of an equatorial density enhancement can be seen. This structure is optically thick at short wavelengths and changes its appearance to optically thin at longer wavelengths. In the classification of the two different appearances of planetary nebulae from HST images it changes from being classified as DUst-Prominent Longitudinally-EXtended (DUPLEX) at short wavelengths to star-obvious low-level-elongated (SOLE) at longer wavelengths. This strengthens the interpretation that these two appearances are manifestations of the same physical structure. Furthermore, we find that the central star is hotter than often assumed and the relatively high observed reddening is a consequence of circumstellar rather than interstellar extinction. Based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofsicaŋsica de Canarias.

Observing Circumstellar Neighbourhoods with the Extreme Polarimeter

7 Oct 2013

by Rodenhuis, M., Canovas, H., Jeffers, S. V., Min, M., Keller, C. U., is now available here.

Abstract: The study of circumstellar environments at ever higher contrasts has generated considerable interest in recent years. One method to increase the contrast is to observe the linearly polarized light scattered by the circumstellar material while suppressing the unpolarized stellar flux. This paper presents some sample imaging polarimetry results obtained with the Extreme Polarimeter (ExPo). ExPo operates in the visible part of the spectrum, and currently achieves a polarimetric sensitivity of 10-4. Despite the demise of the Utrecht Astronomical Institute, where this instrument was developed, the instrument is still being used and upgraded. It has now moved to the Leiden Observatory.

Sterrekundig Instituut Utrecht: The Last Years

7 Oct 2013

by Keller, C. U., is now available here.

Abstract: I describe the last years of the 370-year long life of the Sterrekundig Instituut Utrecht, which was the second-oldest university observatory in the world and was closed in early 2012 after the Faculty of Science and the Board of Utrecht University decided, without providing qualitative or quantitative arguments, to remove astrophysics from its research and education portfolio.

Astronomical Polarimetry: Polarized Views of Stars and Planets

7 Oct 2013

by Snik, Frans, Keller, Christoph U., is now available here.

Abstract: Polarization is a fundamental property of light from astronomical objects, and measuring that polarization often yields crucial information, which is unobtainable otherwise.This chapter reviews the useful formalisms for describing polarization in the optical regime, the mechanisms for the creation of such polarization, and methods for measuring it. Particular emphasis is given on how to implement a polarimeter within an astronomical facility, and on how to deal with systematic effects that often limit the polarimetric performance.

News ArchiveFocal-plane wavefront sensing with high-order adaptive optics systemsFast & Furious focal-plane wavefront sensingSpectral line polarimetry with a channeled polarimeterOptimization-based wavefront sensorless adaptive optics for multiphoton microscopyAtmospheric aerosol characterization with a ground-based SPEX spectropolarimetric instrumentFive-dimensional optical instrumentation: combining polarimetry with time-resolved integral-field spectroscopySnapshot coherence-gated direct wavefront sensing for multi-photon microscopyInstrumental polarisation at the Nasmyth focus of the E-ELTImaging the circumstellar environment of the young T Tauri star SU AurigaeAnalysis and interpretation of 15 quarters of Kepler data of the disintegrating planet KIC 12557548 bCrowdsourced aerosol measurements using smartphone spectropolarimetersRemote sensing of atmospheric aerosols with the SPEX spectropolarimeterCalibrating a high-resolution wavefront corrector with a static focal-plane cameraIterative linear focal-plane wavefront correctionSPHERE-ZIMPOL system testing: status report on polarimetric high contrast resultsCalibrating a high-resolution wavefront corrector with a static focal-plane cameraThree-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 . I. Spectropolarimetric observations in all four Stokes parametersAre there tangled magnetic fields on HgMn stars?The color dependent morphology of the post-AGB star HD 161796Observing Circumstellar Neighbourhoods with the Extreme PolarimeterSterrekundig Instituut Utrecht: The Last YearsAstronomical Polarimetry: Polarized Views of Stars and PlanetsThe color dependent morphology of the post-AGB star HD161796Magnetically Controlled Accretion on the Classical T Tauri Stars GQ Lupi and TW HydraeHARPS Spectropolarimetry of the Classical T Tauri Stars GQ Lup and TW HyaAstronomical Polarimetry: Polarized Views of Stars and PlanetsObserving the Earth as an exoplanet with LOUPE, the lunar observatory for unresolved polarimetry of EarthSemidefinite programming for model-based sensorless adaptive opticsUnusual Stokes V profiles during flaring activity of a delta sunspotPotential of phase-diversity for metrology of active instrumentsModeling the instrumental polarization of the VLT and E-ELT telescopes with the M&m's codeA spectro-polarimetric integral field spectrograph for EPICS-EPOLThe extreme polarimeter: design, performance, first results and upgradesSearching for signs of habitability with LOUPE, the Lunar Observatory of Unresolved Polarimetry of EarthEvidence for the disintegration of KIC 12557548 bSterrekundig Instituut Utrecht: The Last YearsLinear analytical solution to the phase diversity problem for extended objects based on the Born approximationBilinear solution to the phase diversity problem for extended objects based on the Born approximationExperimental validation of optimization concepts for focal-plane image processing with adaptive opticsFOAM: the modular adaptive optics frameworkExtremely fast focal-plane wavefront sensing for extreme adaptive opticsEvidence for the disintegration of KIC 12557548 bExtremely fast focal-plane wavefront sensing for extreme adaptive opticsConstraining the circumbinary envelope of Z Canis Majoris via imaging polarimetryData driven identification and aberration correction for model-based sensorless adaptive opticsHARPS spectropolarimetry of classical T Tauri starsSPEX2Earth, a novel spectropolarimeter for remote sensing of aerosols and cloudsiSPEX: everybody can measure atmospheric aerosols with a smartphone spectropolarimeteriSPEX: the creation of an aerosol sensor network of smartphone spectropolarimetersObserving the Earth as an exoplanetMultiwavelength imaging polarimetry of Venus at various phase anglesMagnetism, chemical spots, and stratification in the HgMn star ϕ PhoenicisCoherence-gated wavefront sensing for microscopy using fringe analysisSignatures of Water Clouds on Exoplanets: Numerical Simulations.New Insights into Stellar Magnetism from the Spectropolarimetry in All Four Stokes ParametersInnovative Imaging of Young Stars: First Light ExPo ObservationsSimulating Polarized Light from ExoplanetsData Reduction Approach for the Extreme PolarimeterDesign and Prototype Results of the ExPo Imaging PolarimeterPlanetary science: In search of biosignaturesObserving the Earth as an exoplanet with LOUPE, the Lunar Observatory for Unresolved Polarimetry of EarthDirect imaging of a massive dust cloud around R Coronae BorealisSignatures of Water Clouds on Exoplanets: Numerical Simulations.Joint optimization of phase diversity and adaptive optics: demonstration of potentialThe effects of disk and dust structure on observed polarimetric images of protoplanetary disksMagnetism, chemical spots, and stratification in the HgMn star phi PhoenicisSpectropolarimeter for planetary exploration (SPEX): performance measurements with a prototypeNo magnetic field in the spotted HgMn star μ LeporisPrototyping for the Spectropolarimeter for Planetary EXploration (SPEX): calibration and sky measurementsM&m's: an error budget and performance simulator code for polarimetric systemsThe ZIMPOL high contrast imaging polarimeter for SPHERE: sub-system test resultsFast horizontal flows in a quiet sun MHD simulation and their spectroscopic signaturesThe search for magnetic fields in mercury-manganese starsSpectral and polarimetric characterization of gazeous and telluric planets with SEE COASTData-reduction techniques for high-contrast imaging polarimetry. Applications to ExPoData Reduction Techniques for High Contrast Imaging Polarimetry. Applications to ExPoFirst Detection of Linear Polarization in the Line Profiles of Active Cool StarsChemical spots in the absence of magnetic field in the binary HgMn star 66 EridaniThe Polarization Optics for the European Solar TelescopeThe HARPS PolarimeterHARPSpol — The New Polarimetric Mode for HARPSEPOL: the exoplanet polarimeter for EPICS at the E-ELTImaging polarimetry of protoplanetary disks: feasibility and usabilityImaging polarimetry of circumstellar environments with the Extreme PolarimeterThe search for magnetic fields in mercury-manganese starsThe search for magnetic fields in mercury-manganese starsThe HARPS polarimeterObservations of solar scattering polarization at high spatial resolutionThe polarization optics for the European Solar Telescope (EST)EPOL: the exoplanet polarimeter for EPICS at the E-ELTThe ZIMPOL high-contrast imaging polarimeter for SPHERE: design, manufacturing, and testingEPICS: direct imaging of exoplanets with the E-ELTSPEX: the spectropolarimeter for planetary explorationObservations of solar scattering polarization at high spatial resolutionInversions of High-Cadence SOLIS-VSM Stokes ObservationsEPICS, the exoplanet imager for the E-ELTTwo Ways of Improving Stokes InversionsStatistics of Convective Collapse Events in the Photosphere and Chromosphere Observed with the HINODE SOTThe case for spectropolarimetry with SPEX on EJSMStatistics of convective collapse events in the photosphere and chromosphere observed with the Hinode SOTAn IFU for diffraction-limited 3D spectroscopic imaging: laboratory and on-site testsPolarimetric Measurements of Protoplanetary Disks with ExPo Tim van Werkhoven The Prototype of the Small Synoptic Second Solar Spectrum Telescope (S^5T)Polarimetry from the Ground UpVector Magnetic Field Inversions of High Cadence SOLIS-VSM DataSOLIS Vector Spectromagnetograph: Status and ScienceA Brief History of the Second Solar SpectrumStatistics of convective collapse events in the photosphere and chromosphere observed with the Hinode SOTCharacterization of Extra-solar Planets with Direct-Imaging TechniquesPolarization Properties of Real Aluminum Mirrors, I. Influence of the Aluminum Oxide LayerSuper earth explorer: a coronagraphic off-axis space telescopePolarization properties of real aluminum mirrors; I. Influence of the aluminum oxide layerSpectral modulation for full linear polarimetryThe Prototype of the Small Synoptic Second Solar Spectrum Telescope (S5T)An analytical model to demonstrate the reliability of reconstructed `active longitudes'.Polarimetry of Mars with SPEX, an Innovative SpectropolarimeterDiversity among other worlds: characterization of exoplanets by direct detectionA New Era in Solar Thermal-IR Astronomy: the NSO Array Camera (NAC) on the McMath-Pierce TelescopeSPEX: an in-orbit spectropolarimeter for planetary explorationDesign of a laboratory simulator to test exoplanet imaging polarimetryThe Extreme Polarimeter (ExPo): design of a sensitive imaging polarimeterSPHERE ZIMPOL: overview and performance simulationThe upgrade of HARPS to a full-Stokes high-resolution spectropolarimeterDesign of a laboratory simulator to test exoplanet imaging polarimetryThe Extreme Polarimeter (ExPo): design of a sensitive imaging polarimeterSPHERE ZIMPOL: overview and performance simulationThe upgrade of HARPS to a full-Stokes high-resolution spectropolarimeterPolarimetry from the Ground UpNew web page online