Sterrekundig Instituut Utrecht

Utrecht, Netherlands

Sterrekundig Instituut Utrecht

Utrecht, Netherlands

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Teriaca L.,Max Planck Institute for Solar System Research | Andretta V.,National institute for astrophysics | Auchere F.,University Paris - Sud | Brown C.M.,U.S. Navy | And 38 more authors.
Experimental Astronomy | Year: 2012

The solar outer atmosphere is an extremely dynamic environment characterized by the continuous interplay between the plasma and the magnetic field that generates and permeates it. Such interactions play a fundamental role in hugely diverse astrophysical systems, but occur at scales that cannot be studied outside the solar system. Understanding this complex system requires concerted, simultaneous solar observations from the visible to the vacuum ultraviolet (VUV) and soft X-rays, at high spatial resolution (between 0. 1′′ and 0. 3′′), at high temporal resolution (on the order of 10 s, i. e., the time scale of chromospheric dynamics), with a wide temperature coverage (0. 01 MK to 20 MK, from the chromosphere to the flaring corona), and the capability of measuring magnetic fields through spectropolarimetry at visible and near-infrared wavelengths. Simultaneous spectroscopic measurements sampling the entire temperature range are particularly important. These requirements are fulfilled by the Japanese Solar-C mission (Plan B), composed of a spacecraft in a geosynchronous orbit with a payload providing a significant improvement of imaging and spectropolarimetric capabilities in the UV, visible, and near-infrared with respect to what is available today and foreseen in the near future. The Large European Module for solar Ultraviolet Research (LEMUR), described in this paper, is a large VUV telescope feeding a scientific payload of high-resolution imaging spectrographs and cameras. LEMUR consists of two major components: a VUV solar telescope with a 30 cm diameter mirror and a focal length of 3. 6 m, and a focal-plane package composed of VUV spectrometers covering six carefully chosen wavelength ranges between 170 Å and 1270 Å. The LEMUR slit covers 280′′ on the Sun with 0. 14′′ per pixel sampling. In addition, LEMUR is capable of measuring mass flows velocities (line shifts) down to 2 km s -1 or better. LEMUR has been proposed to ESA as the European contribution to the Solar C mission. © 2011 Springer Science+Business Media B.V.


Roelfsema R.,NOVA ASTRON | Gisler D.,ETH Zurich | Pragt J.,NOVA ASTRON | Schmid H.M.,ETH Zurich | And 23 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2011

SPHERE (Spectro-Polarimetric High Contrast Exoplanet Research) is one of the first instruments which aim for the direct detection from extra-solar planets. The instrument will search for direct light from old planets with orbital periods of several months to several years as we know them from our solar system. These are planets which are in or close to the habitable zone. 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 from extrasolar planets. The search for reflected light from extra-solar planets is very demanding because the signal decreases rapidly with the orbital separation. For a Jupiter-sized object and a separation of 1 AU the planet/star contrast to be achieved is on the order of 10-8 for a successful detection. This is much more demanding than the direct imaging of young self-luminous planets. ZIMPOL is located behind an extreme AO system (SAXO) and a stellar coronagraph. SPHERE is foreseen to have first light at the VLT at the end of 2012. ZIMPOL is currently in the subsystem testing phase. We describe the results of verification and performance testing done at the NOVA-ASTRON lab. We will give an overview of the system noise performance, the polarimetric accuracy and the high contrast testing. For the high contrast testing we will describe the impact of crucial system parameters on the contrast performance. SPHERE is an instrument designed and built by a consortium consisting of IPAG, MPIA, LAM, LESIA, Fizeau, INAF, Observatoire de Genève, ETH, NOVA, ONERA and ASTRON in collaboration with ESO. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).


Roelfsema R.,NOVA ASTRON | Schmid H.M.,ETH Zurich | Pragt J.,NOVA ASTRON | Gisler D.,ETH Zurich | And 30 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

ZIMPOL 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 from extrasolar planets. ZIMPOL is located behind an extreme AO system (SAXO) and a stellar coronagraph. SPHERE is foreseen to have first light at the VLT at the end of 2011. ZIMPOL is currently in the manufacturing, integration and testing phase. We describe the optical, polarimetric, mechanical, thermal and electronic design as well as the design trade offs. Specifically emphasized is the optical quality of the key performance component: the Ferro-electric Liquid Crystal polarization modulator (FLC). Furthermore, we describe the ZIMPOL test setup and the first test results on the achieved polarimetric sensitivity and accuracy. These results will give first indications for the expected overall high contrast system performance. SPHERE is an instrument designed and built by a consortium consisting of LAOG, MPIA, LAM, LESIA, Fizeau, INAF, Observatoire de Genève, ETH, NOVA, ONERA and ASTRON in collaboration with ESO. © 2010 Copyright SPIE - The International Society for Optical Engineering.


Snik F.,Sterrekundig Instituut Utrecht | Rietjens J.H.H.,SRON Netherlands Institute for Space Research | Van Harten G.,Sterrekundig Instituut Utrecht | Stam D.M.,SRON Netherlands Institute for Space Research | And 9 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2010

SPEX (Spectropolarimeter for Planetary EXploration) is an innovative, compact instrument for spectropolarimetry, and in particular for detecting and characterizing aerosols in planetary atmospheres. With its ∼1-liter volume it is capable of full linear spectropolarimetry, without moving parts. The degree and angle of linear polarization of the incoming light is encoded in a sinusoidal modulation of the intensity spectrum by an achromatic quarter-wave retarder, an athermal multiple-order retarder and a polarizing beam-splitter in the entrance pupil. A single intensity spectrum thus provides the spectral dependence of the degree and angle of linear polarization. Polarimetry has proven to be an excellent tool to study microphysical properties (size, shape, composition) of atmospheric particles. Such information is essential to better understand the weather and climate of a planet. The current design of SPEX is tailored to study Martian dust and ice clouds from an orbiting platform: a compact module with 9 entrance pupils to simultaneously measure intensity spectra from 400 to 800 nm, in different directions along the flight direction (including two limb viewing directions). This way, both the intensity and polarization scattering phase functions of dust and cloud particles within a ground pixel are sampled while flying over it. We describe the optical and mechanical design of SPEX, and present performance simulations and initial breadboard measurements. Several flight opportunities exist for SPEX throughout the solar system: in orbit around Mars, Jupiter and its moons, Saturn and Titan, and the Earth. © 2010 SPIE.


Van Harten G.,Sterrekundig Instituut Utrecht | Snik F.,Sterrekundig Instituut Utrecht | Rietjens J.H.H.,SRON Netherlands Institute for Space Research | Smit J.M.,SRON Netherlands Institute for Space Research | And 14 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2011

We present the Spectropolarimeter for Planetary EXploration (SPEX), a high-accuracy linear spectropolarimeter measuring from 400 to 800 nm (with 2 nm intensity resolution), that is compact (∼ 1 liter), robust and lightweight. This is achieved by employing the unconventional spectral polarization modulation technique, optimized for linear polarimetry. The polarization modulator consists of an achromatic quarter-wave retarder and a multiple-order retarder, followed by a polarizing beamsplitter, such that the incoming polarization state is encoded as a sinusoidal modulation in the intensity spectrum, where the amplitude scales with the degree of linear polarization, and the phase is determined by the angle of linear polarization. An optimized combination of birefringent crystals creates an athermal multiple-order retarder, with a uniform retardance across the field of view. Based on these specifications, SPEX is an ideal, passive remote sensing instrument for characterizing planetary atmospheres from an orbiting, air-borne or ground-based platform. By measuring the intensity and polarization spectra of sunlight that is scattered in the planetary atmosphere as a function of the single scattering angle, aerosol microphysical properties (size, shape, composition), vertical distribution and optical thickness can be derived. Such information is essential to fully understand the climate of a planet. A functional SPEX prototype has been developed and calibrated, showing excellent agreement with end-to-end performance simulations. Calibration tests show that the precision of the polarization measurements is at least 2 • 10-4. We performed multi-angle spectropolarimetric measurements of the Earth's atmosphere from the ground in conjunction with one of AERONET's sun photometers. Several applications exist for SPEX throughout the solar system, a.o. in orbit around Mars, Jupiter and the Earth, and SPEX can also be part of a ground-based aerosol monitoring network. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).


Snik F.,Sterrekundig Instituut Utrecht | De Wijn A.G.,High Altitude Observatory | Ichimoto K.,Kyoto University | Fischer C.E.,Sterrekundig Instituut Utrecht | And 2 more authors.
Astronomy and Astrophysics | Year: 2010

Context. The weak, turbulent magnetic fields that supposedly permeate most of the solar photosphere are difficult to observe, because the Zeeman effect is virtually blind to them. The Hanle effect, acting on the scattering polarization in suitable lines, can in principle be used as a diagnostic for these fields. However, the prediction that the majority of the weak, turbulent field resides in intergranular lanes also poses significant challenges to scattering polarization observations because high spatial resolution is usually difficult to attain. Aims. We aim to measure the difference in scattering polarization between granules and intergranules. We present the respective center-to-limb variations, which may serve as input for future models. Methods. We perform full Stokes filter polarimetry at different solar limb positions with the CN band filter of the Hinode-SOT Broadband Filter Imager, which represents the first scattering polarization observations with sufficient spatial resolution to discern the granulation. Hinode-SOT offers unprecedented spatial resolution in combination with high polarimetric sensitivity. The CN band is known to have a significant scattering polarization signal, and is sensitive to the Hanle effect. We extend the instrumental polarization calibration routine to the observing wavelength, and correct for various systematic effects. Results. The scattering polarization for granules (i.e., regions brighter than the median intensity of non-magnetic pixels) is significantly larger than for intergranules.We derive that the intergranules (i.e., the remaining non-magnetic pixels) exhibit (9.8 ± 3.0)% lessscattering polarization for 0.2 < μ ≤ 0.3, although systematic effects cannot be completely excluded. Conclusions. These observations constrain MHD models in combination with (polarized) radiative transfer in terms of CN band line formation, radiation anisotropy, and magnetic fields. © 2010 ESO.


Boccaletti A.,Paris Observatory | Schneider J.,Paris Observatory | Traub W.,NASA | Lagage P.-O.,SAP | And 26 more authors.
Experimental Astronomy | Year: 2012

SPICES (Spectro-Polarimetric Imaging and Characterization of Exoplanetary Systems) is a five-year M-class mission proposed to ESA Cosmic Vision. Its purpose is to image and characterize long-period extrasolar planets and circumstellar disks in the visible (450-900 nm) at a spectral resolution of about 40 using both spectroscopy and polarimetry. By 2020/2022, present and near-term instruments will have found several tens of planets that SPICES will be able to observe and study in detail. Equipped with a 1. 5 m telescope, SPICES can preferentially access exoplanets located at several AUs (0. 5-10 AU) from nearby stars (<25 pc) with masses ranging from a few Jupiter masses to Super Earths (~2 Earth radii, ~10 M ⊕) as well as circumstellar disks as faint as a few times the zodiacal light in the Solar System. © 2012 Springer Science+Business Media B.V.


Jonker P.G.,SRON Netherlands Institute for Space Research | Jonker P.G.,Harvard - Smithsonian Center for Astrophysics | Jonker P.G.,Radboud University Nijmegen | Torres M.A.P.,Harvard - Smithsonian Center for Astrophysics | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2010

In this paper we report the discovery of CXO J122518.6+144545, a peculiar X-ray source with a position 3.6 ± 0.2 arcsec off-nuclear from a Sloan Digital Sky Survey Data Release 7 (SDSS DR7) z= 0.0447 galaxy. The 3.6 arcsec offset corresponds to 3.2 kpc at the distance of the galaxy. The 0.3-8 keV X-ray flux of this source is 5 × 10-14 erg cm-2 s-1 and its 0.3-8 keV luminosity is 2.2 × 1041 erg s-1 (2.7 × 1041 erg s-1; 0.5-10 keV) assuming that the source belongs to the associated galaxy. We find a candidate optical counterpart in archival Hubble Space Telescope/Advanced Camera for Surveys g′-band observations of the field containing the galaxy obtained on 2003 June 16. The observed magnitude of g′= 26.4 ± 0.1 corresponds to an absolute magnitude of -10.1. We discuss the possible nature of the X-ray source and its associated candidate optical counterpart and conclude that the source is either a very blue Type IIn supernova, an ultraluminous X-ray source with a very bright optical counterpart or a recoiling supermassive black hole. © 2010 The Authors. Journal compilation © 2010 RAS.

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