Achromatizing a liquid-crystal spectropolarimeter: Retardance vs Stokes-based calibration of HiVIS

Astronomical spectropolarimeters can be subject to many sources of systematic error which limit the precision and accuracy of the instrument. We present a calibration method for observing high-resolution polarized spectra using chromatic liquid-crystal variable retarders (LCVRs). These LCVRs allow for polarimetric modulation of the incident light without any moving optics at frequencies >10Hz. We demonstrate a calibration method using pure Stokes input states that enables an achromatization of the system. This Stokes-based deprojection method reproduces input polarization even though highly chromatic instrument effects exist. This process is first demonstrated in a laboratory spectropolarimeter where we characterize the LCVRs and show example deprojections. The process is then implemented the a newly upgraded HiVIS spectropolarimeter on the 3.67m AEOS telescope. The HiVIS spectropolarimeter has also been expanded to include broad-band full-Stokes spectropolarimetry using achromatic wave-plates in addition to the tunable full-Stokes polarimetric mode using LCVRs. These two new polarimetric modes in combination with a new polarimetric calibration unit provide a much more sensitive polarimetric package with greatly reduced systematic error.Comment: Accepted in PAS

SPHERE: the exoplanet imager for the Very Large Telescope

Observations of circumstellar environments to look for the direct signal of exoplanets and the scattered light from disks has 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 enabled a new generation of high-contrast imagers and spectrographs on large ground-based telescopes with much better performance. One of the most productive is the Spectro-Polarimetic High contrast imager for Exoplanets REsearch (SPHERE) 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), are designed to efficiently cover the near-infrared (NIR) range in a single observation for efficient young planet search. The third one, ZIMPOL, is designed for visible (VIR) polarimetric observation to look for the reflected light of exoplanets and the light scattered by debris disks. This suite of three science instruments enables to study circumstellar environments at unprecedented angular resolution both in the visible and the near-infrared. In this work, we present the complete instrument and its on-sky performance after 4 years of operations at the VLT.Comment: Final version accepted for publication in A&

Spatially coupled inversion of spectro-polarimetric image data I: Method and first results

When inverting solar spectra, image degradation effects that are present in the data are usually approximated or not considered. We develop a data reduction method that takes these issues into account and minimizes the resulting errors. By accounting for the diffraction PSF of the telescope during the inversions, we can produce a self-consistent solution that best fits the observed data, while simultaneously requiring fewer free parameters than conventional approaches. Simulations using realistic MHD data indicate that the method is stable for all resolutions, including those with pixel scales well beyond those that can be resolved with a 0.5m telescope, such as the Hinode SOT. Application of the presented method to reduce full Stokes data from the Hinode spectro-polarimeter results in dramatically increased image contrast and an increase in the resolution of the data to the diffraction limit of the telescope in almost all Stokes and fit parameters. The resulting data allow for detecting and interpreting solar features that have so far only been observed with 1m class ground-based telescopes. The new inversion method allows for accurate fitting of solar spectro-polarimetric imaging data over a large field of view, while simultaneously improving the noise statistics and spatial resolution of the results significantly.Comment: A&A, accepte

SPICES: Spectro-Polarimetric Imaging and Characterization of Exoplanetary Systems

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 2020/22, 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 ($<$25 pc) with masses ranging from a few Jupiter masses to Super Earths ($\sim$2 Earth radii, $\sim$10 M$_{\oplus}$) as well as circumstellar disks as faint as a few times the zodiacal light in the Solar System

Stokes imaging polarimetry using image restoration at the Swedish 1-m Solar Telescope

Aims: We aim to achieve high spatial resolution as well as high polarimetric sensitivity, using an earth-based 1m-class solar telescope, for the study of magnetic fine structure on the Sun. Methods: We use a setup with 3 high-speed, low-noise cameras to construct datasets with interleaved polarimetric states, particularly suitable for Multi-Object Multi-Frame Blind Deconvolution image restorations. We discuss the polarimetric calibration routine as well as various potential sources of error in the results. Results: We obtained near diffraction limited images, with a noise level of approximately 10^(-3) I(cont). We confirm that dark-cores have a weaker magnetic field and at a lower inclination angle with respect to the solar surface than the edges of the penumbral filament. We show that the magnetic field strength in faculae-striations is significantly lower than in other nearby parts of the faculae.Comment: Accepted for publication in Astronomy & Astrophysics, 12 pages, 11 figure

Detection of Wind Turbines in Intertidal Areas Using SAR Polarimetry

The detection of wind turbines in a strong clutter background is analyzed at variance of polarimetric synthetic-aperture radar (SAR) configurations. The area of interest is the intertidal zone near Jiangsu, China and two detectors are used, the polarimetric notch filter (PNF) and a change detector that optimizes the ratio between covariance matrices. The detection performance is quantitatively analyzed using the receiver operating characteristic (ROC) curve, while the scattering mechanisms that characterize wind turbines are analyzed using the Yamaguchi decomposition. Experimental analysis shows that: 1) wind turbines result in a nontrivial scattering mechanism and 2) full-polarimetric measurements achieve the best detection performance independently of the two detectors

Design and construction of a snapshot full-Stokes polarimetric camera : seeing through fog

Tesi amb menció internacionalA la portada: Centre for Sensors, Instruments and Systems Development (CD6-UPC). Faculty of Optics and Optometry of Terrassa (FOOT)(English) Polarization is one of the properties of light and it is often put aside. Human beings receive light beams and process only the intensity and wavelength information. Our optical system lacks the capability of ’seeing’ polarization in comparison to other animals. The vectorial nature of polarization is uncorrelated to the intensity and colour information and this can unveil additional information for improving the current technology. This Thesis aims to develop a camera capable of measuring the full polarization in a 2D scene. In particular, it focuses on the design and construction of a prototype that measures in the visible waveband the full-Stokes vector in a snapshot such that the acquisition time and noise equalization are balanced while reducing movement and registration artefacts. The Thesis starts with the revision of the current state of the art in the polarimetric imaging field. Based on this, the optomechanical design of the polarimetric camera is developed ensuring a faster acquisition of polarization since it integrates optimum states to immunize the system from Gaussian and Poisson noise. Accordingly, this Thesis proposes a general calibration methodology addressed to the radiometry of the sensor, the geometrical aberrations from optics and the polarization elements in the system to transform the intensity measurements into polarization information. Besides, this Thesis studies two imaging modes of polarization, Stokes imaging and Mueller matrix imaging, for different applications. The novelty of this system consists of the use of optimal polarization states in a division of aperture architecture for noise immunization. Finally, this Thesis studies the application of the system to improve detection in the real-world problem of seeing through the fog. Polarization information can improve the range of detection due to the polarization memory effect. This system could be employed inside a multimodal system to ensure detection when others are hampered due to external conditions.(Català) La polarització és una de les propietats de la llum i que sovint es prescindeix d’ella. Els éssers humans reben els raigs de llum i únicament processen la informació provinent de la intensitat i la longitud d’ona de la llum. El nostre sistema òptic no té la capacitat de "veure" la polarització en comparació a altres animals. La natura vectorial de la polarització està no correlada amb la informació donada per la intensitat i el color, i això pot revelar informació addicional per millorar la tecnologia actual. Aquesta Tesi té com a objectiu desenvolupar una càmera per mesurar la polarització d’una escena 2D. En particular, es centra en el disseny i construcció d’un prototip que mesuri a l’espectre visible el vector de Stokes complert en un sol tret de manera que el temps d’adquisició i la equalització del soroll siguin compensats a la vegada que es redueixen els artefactes causats pel moviment i pel registre. La Tesi comença revisant l’actual estat de l’art en el camp de la imatge polarimètrica. Arran d’això, es realitza el disseny optomecànic de la càmera polarimètrica garantint una adquisició ràpida, ja que el disseny implementa uns estats òptims de polarització per immunitzar el sistema de soroll gaussià i de Poisson. Per tant, aquesta Tesi proposa una metodologia general de calibratge dirigida a la radiometria del sensor, a les aberracions geomètriques de l’òptica i als elements de polarització en el sistema per transformar les mesures d’intensitat en informació polarimétrica. A més, aquesta Tesi estudia dos maneres d’imatge de la polarització, imatge de Stokes i imatge de la matriu de Mueller, per diferents aplicacions. La novetat d’aquest sistema radica en la utilització d’estats de polarització òptims basant-se en una arquitectura de divisió d’apertura per la immunització al soroll. Finalment, aquesta tesi estudia les aplicacions del sistema per millorar la detecció en un problema del món real com és veure a través de la boira. La informació de la polarització pot millorar el rang de detecció degut a l’efecte de memòria de la polarització. Aquest sistema, doncs, podria utilitzar-se dins d’un altre sistema multimodal per assegurar la detecció quan la resta de sistemes estiguin perjudicats per les condicions externes.(Español) La polarización es una de las propiedades de la luz y de la que a menudo se prescinde. Los seres humanos percibimos los rayos de luz y solo procesamos la información proveniente de la intensidad y de las longitudes de onda de la luz. Nuestro sistema óptico carece de la capacidad de "ver" la polarización en comparación con otros animales. La naturaleza vectorial de la polarización está no correlacionada con la información aportada por la intensidad y el color, y esto puede desvelar información adicional para mejorar la tecnología actual. Esta Tesis tiene como objetivo desarrollar una cámara para medir la polarización en una escena 2D. En particular, se centra en el diseño y construcción de un prototipo que mida en el espectro visible el vector de Stokes completo en un solo disparo de manera que el tiempo de adquisición y la ecualización del ruido estén compensados a la vez que se reduzcan los artefactos debidos al movimiento y al registro. La Tesis comienza revisando el actual estado del arte en el campo de imagen polarimétrica. A partir de esto, se realiza el diseño optomecánico de la cámara polarimétrica garantizando una adquisición rápida ya que el diseño implementa unos estados óptimos de polarización para inmunizar el sistema del ruido de gaussiano y de Poisson. Por consiguiente, esta Tesis propone una metodología general de calibración dirigida a la radiometría del sensor, a las aberraciones geométricas de la óptica y a los elementos de polarización en el sistema para transformar las medidas de intensidad en información polarimétrica. Además, esta Tesis estudia dos modos de imagen de la polarización, imagen de Stokes e imagen de la matriz de Mueller, para diferentes aplicaciones. La novedad de este sistema radica en la utilización de estados de polarización óptimos basándose en una arquitectura de división de apertura para la inmunización al ruido. Finalmente, esta Tesis estudia las aplicaciones del sistema para mejorar la detección en un problema del mundo real como es ver a través de la niebla. La información de la polarización puede mejorar el rango de detección debido al efecto de memoria de la polarización. Este sistema podría utilizarse dentro de un sistema multimodal para asegurar la detección cuando el resto de sistemas están perjudicados por las condiciones externas.DOCTORAT EN ENGINYERIA ÒPTICA (Pla 2013