Accurate Polarimetry Research Articles

Neural network assisted high-spatial-resolution polarimetry with non-interleaved chiral metasurfaces

Polarimetry plays an indispensable role in modern optics. Nevertheless, the current strategies generally suffer from bulky system volume or spatial multiplexing scheme, resulting in limited performances when dealing with inhomogeneous polarizations. Here, we propose a non-interleaved, interferometric method to analyze the polarizations based on a tri-channel chiral metasurface. A deep convolutional neural network is also incorporated to enable fast, robust and accurate polarimetry. Spatially uniform and nonuniform polarizations are both measured through the metasurface experimentally. Distinction between two semblable glasses is also demonstrated. Our strategy features the merits of compactness and high spatial resolution, and would inspire more intriguing design for detecting and sensing.

Accurate Polarimetry of Hybrid K-Rb and 21Ne Atoms Based on Spin-Exchange Interactions

Hybrid spin-exchange optical pumping is widely used in precision measurements and tests of fundamental physics. In these researches and applications, accurate measurement of the spin polarizations of hybrid alkali atoms and noble gas atoms has a significant role. Here we propose a novel method of measuring the spin polarizations of hybrid K-Rb and $^{21}Ne$ atoms simultaneously. We study the mixture of the slowing-down factors of hybrid alkali atoms due to spin-exchange interactions between hybrid alkali atoms, and the resonance-frequency shift of alkali atoms due to spin-exchange interactions with noble-gas atoms. We investigate the relationship between the hybrid resonance frequency and the polarizations. The spin polarizations of alkali atoms and noble-gas atoms have been measured based on this method, which is from 0.082 to 0.756 and from 0.018 to 0.124 with uncertainties smaller than 10.4 % and 12.7 % respectively. This method is promising for the applications requiring simultaneous and real-time measurement of the polarizations of alkali and noble-gas atoms, such as inertial sensors and tests of fundamental physics.

Instrumental polarisation at the Nasmyth focus of the E-ELT

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.

Elliptically Polarized High-Order Harmonic Emission from Molecules in Linearly Polarized Laser Fields

We perform an accurate polarimetry measurement of high-order harmonic emission from aligned molecules. We find that harmonic emission from N2 can be strongly elliptically polarized even when driven by linearly polarized laser fields. These data have broad implications for understanding molecules in strong fields because they cannot be explained by simple theories based on the strong field approximation and single active electron models. Finally, this work also shows that it is possible to engineer the polarization properties of harmonic emission by carefully preparing a molecular medium.

Aperture synthesis polarimetry: Application to the Dominion Radio Astrophysical Observatory synthesis telescope

Aperture synthesis is a powerful technique for imaging the radio sky and can be used to make images in all four Stokes parameters, providing a complete measurement of the polarization state of the received radiation. In centimeter‐wavelength continuum astronomy the received signals are generally partially linearly polarized, with a negligibly small fraction of circular polarization. For this application the preferred antenna configuration receives both left‐ and right‐hand circular polarization. In this paper the effects of nonideal antenna performance are analyzed, and calibration and data correction procedures are described which allow precise measurement of the four Stokes parameters. Three levels of data correction are identified. Level 1, complex channel gain correction, is the standard calibration of amplitude and gain required in every synthesis telescope. Level 2, orthogonality correction, makes the instrument appear as a set of interferometers, identical to the level of approximation involved, which have two orthogonal, but not precisely circular, polarizations. Level 3 correction converts the telescope into an array of identical antennas with exactly circular polarization. Level 3 correction gives the most accurate polarimetry; for this level of correction the polarization characteristics of one antenna, which is used as a reference, must be determined. In the absence of a precise determination of the polarization characteristics of the reference antenna, a measurement which may be very difficult, there is no value in proceeding to Level 3 correction. If the cross‐hand contamination of the antennas is less than about 15%, then only level 1 and level 2 corrections are needed to achieve a sensitivity to polarized emission of 1% of the total intensity, an accuracy of measurement of polarized intensity of better than 10% and a measurement of the position angle of linearly polarized emission better than 5°. The implementation of polarimetry at 1420 MHz on the Dominion Radio Astrophysical Observatory synthesis telescope, Penticton, British Columbia, Canada, is described, and instrument performance to the above specifications is demonstrated. Observations of the supernova remnant DA530 are presented, demonstrating a usable field of view for polarimetry of at least 1.5°.

Technique for accurate stellar polarimetry using CCD cameras

Using a technique for CCD polarimetry, we have obtained stellar polarization data with the 0.9-m telescope on Kitt Peak and with the Tek 2048 CCD camera. Measurements of stars viewed through holes in polarizing filters serve to correct for variations in atmospheric transparency. For the brightest stars the uncertainties in the measured polarization during a single, hour-long, polarimetric sequence are 0.3-0.5%, because of the residual effects of the variations in transparency. For fainter stars we are Poisson noise limited. Our technique provides some advantages over other techniques for CCD polarimetry, primarily because it can be easily integrated into existing photometric systems.

Two-dimensional polarimeter with a charge-coupled-device image sensor and a piezoelastic modulator

We present the first measurements and scientific observations of the solar photosphere obtained with a new two-dimensional polarimeter based on piezoelastic modulators and synchronous demodulation in a CCD imager. This instrument, which is developed for precision solar-vector polarimetry, contains a specially masked CCD that has every second row covered with an opaque mask. During exposure the charges are shifted back and forth between covered and light-sensitive rows synchronized with the modulation. In this way Stokes I and one of the other Stokes parameters can be recorded. Since the charge shifting is performed at frequencies well above the seeing frequencies and both polarization states are measured with the same pixel, highly sensitive and accurate polarimetry is achieved. We have tested the instrument in laboratory conditions as well as at three solar telescopes.

The Near-Infrared Capabilities of LEST

The Large Earth-based Solar Telescope (LEST) will be a powerful, next-generation telescope with unprecedented angular resolution, capable of highly accurate polarimetry of the Sun, covering the optical spectral range from about 300 nm into the near infrared to about 2.5 μm.The telescope is a 2.4-m aperture, “polarization-free” concept based on a modified Gregorian optical system. A fast polarization modulator will be located close to the secondary focus of the system. An actively controlled NTT-type main mirror, a high precision pointing and tracking system, a helium-filled light path and a thin entrance window, together with an integrated adaptive optics system, will give the telescope near diffraction-limited performance in the visible. LEST will be sited on La Palma, in the Canary Islands, near the caldera rim on the Roque de los Muchachos Observatory, which often offers excellent seeing. A frequently occurring seeing parameter of ro = 15–20 cm in the visible will correspond to ro ≥ 1 m in the near IR.The construction of LEST will begin in 1993, and the telescope is to be ready for “first light” in 1997. The telescope facility will accommodate a large number of focal plane instruments on a spacious instrument table. LEST will be made available for near-IR instrumentation from the start of its regular operation.

Accurate null polarimetry for measuring the refractive index of transparent materials

Simple null polarimetry is introduced to measure the refractive index n of transparent and semitransparent materials. For a linearly polarized incident light, the reflected polarization can be used to determine n with the Fresnel equations. This method is fast, convenient, and versatile enough to provide accurate results on small laboratory samples. Possible errors in experiments are analyzed, and ways to eliminate these errors are given. With our instrument, the uncertainty in the deduced n is ±0.0004. The accuracy is still within 0.1% for materials with high n and with the extinction coefficient of the order of 0.001. The method is also sufficiently accurate for characterizing the homogeneity of transparent materials.

Large Earth-based Solar Telescope—LEST

The Large Earth-based Solar Telescope (LEST) will be a powerful, next-generation solar telescope with unprecedented angular resolution and highly accurate polarimetry, that will serve the World's community of solar scientists into the next century. The project is run by the LEST Foundation which has its seat at the Royal Swedish Academy of Sciences in Stockholm. Being initially a European project ( Germany, Israel, Italy, Norway, Spain, Sweden, and Switzerland) the LEST has grown to include also Australia and USA. The LEST design is underway. The optical design is a 2.4 m aperture, “polarization-free” concept based on a modified Gregorian system. An actively controlled NNT-type main mirror, a high precision pointing and tracking system, a helium-filled light path and thin entrance window, together with an integrated Adaptive Optics system, will provide near diffraction-limited performance of the telescope. LEST will be placed on La Palma, Canary Islands, near the Caldera rim on the Roque de los Muchachos observatory. This site offers superb seeing conditions which will enable LEST to reach its scientific goals. The construction of LEST will begin early 1993, and the telescope is ready for “first light” in 1996. LEST marks a new era of international cooperation in solar physics and astrophysics.

Ultraviolet Polarimetry

AbstractInterstellar polarization and polarization of the sky background is predicted to be significantly smaller in the ultraviolet than in the optical region of the spectrum; while intrinsic polarization is expected to be greater. For these reasons alone ultraviolet polarimetry should provide a powerful diagnostic tool. To date such polarization measurements have not been undertaken. The Wisconsin Ultraviolet Photopolarimeter Experiment (WUPPE) will be the first to carry out a program of UV spectropolarimetry. WUPPE is one of the four ASTRO-1 payloads. It consists of a half meter aperture Cassegrain telescope feeding a spectropolarimeter capable of measuring spectral energy distribution and linear and circular polarization in the spectral region from about 3300 Å to 1400 Åwith spectral resolutions from about 4 Å to 40 Å, depending upon the mode of observation and nature of the source. The effective area of WUPPE is approximately 100 cm2 at 2000 Å and the faint stellar limit for measurement of all four Stokes parameters is about 16 magnitude. The design provides for a large dynamic range and the high precision required for accurate polarimetry. Launch of the first ASTRO Mission is expected in May of 1990.The science goals for WUPPE include studies of ultraviolet interstellar polarization, intrinsic polarization of a wide variety of stellar types and measurements of polarization of selected extragalactic objects. A description of WUPPE and simulations of expected data will be presented. The paper will also discuss a wide field imaging survey polarimeter (WISP) now undergoing development for a sounding rocket payload at the University of Wisconsin. (This work is supported by NASA contract NAS5-26777).

The miniblazar in 3C 273

The compact core of 3C 273 has been studied for over two years using optical photometry and accurate polarimetry. A wealth of polarization behavior, including variable polarization, observed at a level of under 1 percent is described. The results indicate that 3C 273 harbors a weak continuum component having all the characteristic properties of blazars. It is predicted that other compact radio sources with low optical polarization will also be found to harbor miniblazars. The detection of variable polarization in 3C 273 strengthens the link between superluminal motion at radio wavelengths and blazar activity at optical wavelengths.

The variability of polarized standard stars

view Abstract Citations (71) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Variability of Polarized Standard Stars Bastien, P. ; Drissen, L. ; Menard, F. ; Moffat, A. F. J. ; Robert, C. ; St-Louis, N. Abstract The linear polarization of 11 out of a total of 13 standard polarized stars has been found to be variable. This implies that these 11 stars, mostly giants and supergiants but also four early-type main-sequence stars, exhibit some level of small, variable, intrinsic polarization. Therefore, they are not suitable as polarized standards for accurate polarimetry. The problem of finding suitable candidates for standard polarized stars is therefore nontrivial and basically still unsolved. The general increase in the level of polarization variability with mean polarization may be partly interstellar in origin, analogous to radioflux scintillations seen in extragalactic radio sources. Publication: The Astronomical Journal Pub Date: March 1988 DOI: 10.1086/114688 Bibcode: 1988AJ.....95..900B Keywords: Early Stars; Extragalactic Radio Sources; Giant Stars; Linear Polarization; Signal To Noise Ratios; Variable Stars; Computational Astrophysics; Error Analysis; Gallium Arsenides; Telescopes; Astrophysics; STARS: MAGNETIC; STARS: PULSATION full text sources ADS | data products SIMBAD (18)