Spectral modulation for full linear polarimetry

Snik, Frans; Karalidi, Theodora; Keller, Christoph U.

doi:10.1364/ao.48.001337

Cited by 138 publications

(95 citation statements)

References 14 publications

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“…For this study, a spectropolarimeter designed for multi-angle ground-based observations based on the spectral modulation technique (Snik et al, 2009) is used. The key idea behind this technique is that the intensity of the incoming light (e.g.…”

Section: Methodsmentioning

confidence: 99%

“…The corresponding modulation amplitude is proportional to the DLP, and the modulation phase is related to the angle of linear polarization (ALP). The DLP and the ALP of the incoming radiation can be derived from the spectrally modulated intensity through a demodulation algorithm as prescribed by Snik et al (2009). This measurement concept eliminates the need for moving parts in the polarization measurement device.…”

Section: Methodsmentioning

confidence: 99%

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Use of neural networks in ground-based aerosol retrievals from multi-angle spectropolarimetric observations

et al. 2015

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Abstract. In this paper, the use of a neural network algorithm for the retrieval of the aerosol properties from groundbased spectropolarimetric measurements is discussed. The neural network is able to retrieve the aerosol properties with an accuracy that is almost comparable to that of an iterative retrieval. By using the outcome of the neural network as first guess in the iterative retrieval scheme, the accuracy of the retrieved fine-and coarse-mode optical thickness is further improved, while for the other parameters the improvement is small or absent. The resulting scheme (neural network + iterative retrieval) is compared to the original one (look-up table + iterative retrieval) on a set of simulated ground-based measurements, and on a small set of real observations carried out by an accurate ground-based spectropolarimeter. The results show that the use of a neural-networkbased first guess leads to an increase in the number of converging retrievals, and possibly to more accurate estimates of the aerosol effective radius and complex refractive index.

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Use of neural networks in ground-based aerosol retrievals from multi-angle spectropolarimetric observations

et al. 2015

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“…Modulators include liquid crystals (LCs), piezo-elastic modulators (PEMs), rapidly rotating components and other techniques (Gandorfer 1999;Elmore et al 1992;Skumanich et al 1997;Gandorfer et al 2004;Lin et al 2004). Modulation can be chromatically balanced, tuned or optimized for various observing cases (Povel 1995;Gisler et al 2003;Tomczyk et al 2010;de Wijn et al 2010de Wijn et al , 2011Snik et al 2009;López Ariste & Semel 2011;Nagaraju et al 2007). At modulation rates faster than 1kHz most instrument effects and even atmospheric seeing fluctuations can be suppressed (Keller et al 1994;Stenflo 2007;Hough et al 2006;Hanaoka 2004;Rodenhuis et al 2012;Xu et al 2006).…”

Section: Spectropolarimetric Instruments Errors and Suppression Tecmentioning

confidence: 99%

Calibrating and stabilizing spectropolarimeters with charge shuffling and daytime sky measurements

Harrington

Kuhn

Nevin

2015

A&A

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Well-calibrated spectropolarimetry studies at resolutions of R > 10 000 with signal-to-noise ratios (S/Ns) better than 0.01% across individual line profiles, are becoming common with larger aperture telescopes. Spectropolarimetric studies require high S/N observations and are often limited by instrument systematic errors. As an example, fiber-fed spectropolarimeters combined with advanced line-combination algorithms can reach statistical error limits of 0.001% in measurements of spectral line profiles referenced to the continuum. Calibration of such observations is often required both for cross-talk and for continuum polarization. This is not straightforward since telescope cross-talk errors are rarely less than ∼1%. In solar instruments like the Daniel K. Inouye Solar Telescope (DKIST), much more stringent calibration is required and the telescope optical design contains substantial intrinsic polarization artifacts. This paper describes some generally useful techniques we have applied to the HiVIS spectropolarimeter at the 3.7 m AEOS Telescope on Haleakala. HiVIS now yields accurate polarized spectral line profiles that are shot-noise limited to 0.01% S/N levels at our full spectral resolution of 10 000 at spectral sampling of ∼100 000. We show line profiles with absolute spectropolarimetric calibration for cross-talk and continuum polarization in a system with polarization cross-talk levels of essentially 100%. In these data the continuum polarization can be recovered to one percent accuracy because of synchronized charge-shuffling model now working with our CCD detector. These techniques can be applied to other spectropolarimeters on other telescopes for both night and daytime applications such as DKIST, TMT, and ELT which have folded non-axially symmetric foci.

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“…In addition, temporal modulation provides successive rather than simultaneous measurements of the Stokes Q, U and V parameters. Instead of temporal modulation, polarization can be modulated spectrally [7] or spatially using channeled polarimetry or division of amplitude.…”

Section: B2 Modulation Of the Polarization Statementioning

confidence: 99%

Static spectropolarimeter concept adapted to space conditions and wide spectrum constraints

et al. 2015

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The issues related to moving elements in space and instruments working in broader wavelength ranges lead to a need for robust polarimeters, efficient on a wide spectral domain, and adapted to space conditions. As part of the UVMag consortium, created to develop spectropolarimetric UV facilities in space, such as the Arago mission project, we present an innovative concept of static spectropolarimetry. We studied a static and polychromatic method for spectropolarimetry, applicable to stellar physics. Instead of modulating the polarization information temporally, as usually done in spectropolarimeters, the modulation is performed in a spatial direction, orthogonal to the spectral one. Thanks to the proportionality between phase retardance imposed by a birefringent material and its thickness, birefringent wedges can be used to create this spatial modulation. The light is then spectrally cross-dispersed, and a full-Stokes determination of the polarization over the whole spectrum can be obtained with a single-shot measurement. The use of Magnesium Fluoride wedges, for example, could lead to a compact, static polarimeter working at wavelengths from 0.115 µm up to 7 µm. We present the theory and simulations of this concept, as well as laboratory validation and a practical application to Arago.

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Spectral modulation for full linear polarimetry

Cited by 138 publications

References 14 publications

Use of neural networks in ground-based aerosol retrievals from multi-angle spectropolarimetric observations

Use of neural networks in ground-based aerosol retrievals from multi-angle spectropolarimetric observations

Calibrating and stabilizing spectropolarimeters with charge shuffling and daytime sky measurements

Static spectropolarimeter concept adapted to space conditions and wide spectrum constraints

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