Measurement of vector magnetic fields

Balasubramaniam, Krishnan; Venkatakrishnan, P.; Bhattacharyya, J. C.

doi:10.1007/bf00157317

Cited by 18 publications

(4 citation statements)

References 2 publications

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“…The polarimetric data was corrected for polarimeter response (for details see Nagaraju et al (2007)). The instrumental polarization introduced by the telescope was corrected by using the telescope model developed by Balasubramaniam et al (1985) and Sankarasubramanian (2000). The refractive index values for Aluminium coating used in the model are obtained from the catalog (Walter 1978).…”

Section: Correction For Polarimeter Response and Telescope Induced Cr...mentioning

confidence: 99%

On the Weakening of the Chromospheric Magnetic Field in Active Regions

Nagaraju

Sankarasubramanian

Rangarajan

2008

ApJ

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Simultaneous measurement of line-of-sight (LOS) magnetic and velocity fields at the photosphere and chromosphere are presented. Fe I line at λ6569 and H α at λ6563 are used respectively for deriving the physical parameters at photospheric and chromospheric heights. The LOS magnetic field obtained through the centerof-gravity method show a linear relation between photospheric and chromospheric field for field strengths less than 700 G. But in strong field regions, the LOS magnetic field values derived from H α are much weaker than what one gets from the linear relationship and also from those expected from the extrapolation of the photospheric magnetic field. We discuss in detail the properties of magnetic field observed in H α from the point of view of observed velocity gradients. The bisector analysis of H α Stokes I profiles show larger velocity gradients in those places where strong photospheric magnetic fields are observed. These observations may support the view that the stronger fields diverge faster with height compared to weaker fields.

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Section: Correction For Polarimeter Response and Telescope Induced Cr...mentioning

confidence: 99%

On the Weakening of the Chromospheric Magnetic Field in Active Regions

Nagaraju

Sankarasubramanian

Rangarajan

2008

ApJ

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“…We applied the formalism to the optical train of the VTT to derive a time-dependent model of its polarization properties for infrared and visible wavelengths. Polarization models for other solar telescopes have been presented by Balasubramaniam et al (1985), Capitani et al (1989), Skumanich et al (1997), and Bernasconi (1997). Horn et al (1996) present a Mueller matrix of the VTT for some fixed times (without a model), but it includes both telescope and polarimeter, which makes comparison difficult.…”

Section: Introductionmentioning

confidence: 99%

A polarization model for the German Vacuum Tower Telescope from in situ and laboratory measurements

Beck

Schlichenmaier

Collados³

et al. 2005

A&A

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It is essential to properly calibrate the polarimetric properties of telescopes, if one wants to take advantage of the capabilities of high precision spectro-polarimeters. We have constructed a model for the German Vacuum Tower Telescope (VTT) that describes its time-dependent polarization properties. Since the coelostat of the telescope changes the polarization state of the light by introducing cross talk among different polarization states, such a model is necessary to correct the measurements, in order to retrieve the true polarization as emitted from the Sun. The telescope model is quantified by a time-dependent Mueller matrix that depends on the geometry of the light beam through the telescope, and on material properties: the refractive indices of the coelostat mirrors, and the birefringence of the entrance window to the vacuum tube. These material properties were determined experimentally in-situ by feeding the telescope with known states of polarization (including unpolarized light) and by measuring its response, and from measurements of an aluminum-coated sample in the laboratory. Accuracy can in our case be determined only for the combination of telescope and spectro-polarimeter used; for the instrument POLIS at the VTT, we estimate an accuracy of ±4-5 × 10 −3 for the cross talk correction coefficients.

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“…The straightforward manner to calibrate the polarimetric effects of telescope optics is to introduce a well known polarization and see how it transforms after passage through the optics. Thus in solar telescopes it is a common practice to place in the entrance window polarizers and retarders, and to create a model of the telescope Mueller matrix with a few free parameters to be fit (Beck et al 2005;Skumanich et al 1997;Balasubramaniam et al 1985). Night telescopes can seldom afford such calibrations.…”

Section: Introductionmentioning

confidence: 99%

Polarimetric calibration of large mirrors

Ariste

2015

Preprint

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Aims. To propose a method for the polarimetric calibration of large astronomical mirrors that does not require use of special optical devices nor knowledge of the exact polarization properties of the calibration target Methods. We study the symmetries of the Mueller matrix of mirrors to exploit them for polarimetric calibration under the assumptions that only the orientation of the linear polarization plane of the calibration target is known with certainty. Results. A method is proposed to calibrate the polarization effects of single astronomical mirrors by the observation of calibration targets with known orientation of the linear polarization. We study the uncertainties of the method and the signal-to-noise ratios required for an acceptable calibration. We list astronomical targets ready for the method. We finally extend the method to the calibration of two or more mirrors, in particular to the case when they share the same incidence plane.

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Measurement of vector magnetic fields

Cited by 18 publications

References 2 publications

On the Weakening of the Chromospheric Magnetic Field in Active Regions

On the Weakening of the Chromospheric Magnetic Field in Active Regions

A polarization model for the German Vacuum Tower Telescope from in situ and laboratory measurements

Polarimetric calibration of large mirrors

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