Roberto Casini

Polarization fringes in optical systems: a compendium

whawkins — Tue, 23 Dec 2025 21:58:07 +0000

Polarization fringes in optical systems: a compendium whawkins Tue, 12/23/2025 - 14:58

Author:

whawkins

Dec 23, 2025

Applied Optics: Spectral and spatial fringes in polarized light are produced by the interference of transmitted and reflected waves at the interface between materials with different indexes of refraction. These instrumental artifacts can affect the accuracy of optical designs conceived for high-sensitivity spectroscopy and polarimetry. We review the fundamental mechanisms that are responsible for these artifacts and the possible design pathways that allow us to mitigate them. In order to do so, we also present an approximate treatment of the problem of the transmission and reflection of light through (possibly absorptive) birefringent layers, relying on a few fundamental results that can be found in the already extensive literature on the subject. Unfortunately, many of these results remain the domain of a niche of investigators working in the field of thin films and optical coatings, and are often overlooked even by experienced designers of spectro-polarimetric instrumentation. The treatment presented in this work is limited to isotropic materials and uniaxial crystals, which are the most common types of optics employed in polarimetric instrumentation, and it fundamentally relies on the approximation of small birefringence for its implementation. An extensive set of modeling examples is provided to highlight the salient characteristics of polarization fringes, as well as to assess how approximate treatments such as this compare to exact but more computational expensive formulations of the problem such as Berreman's calculus.

Wavelength dependence between 200 and 1000 nm of the (intensity normalized) Mueller matrix of a PCM optimized for full-Stokes polarimetry between 400 and 1000 nm. The polarization fringes are calculated with a spectral resolution of 20000. The PCM design uses three compound retarders in the configuration MgF2-SiO2-MgF2, where the two MgF2 elements are identical. The gray curves plotted over the fringes represent the ideal Mueller matrix from the PCM design.

Using the Hanle Effect in Mg II k to Quantify the Open Flux above the Solar Poles

whawkins — Tue, 16 Sep 2025 20:00:52 +0000

Using the Hanle Effect in Mg II k to Quantify the Open Flux above the Solar Poles whawkins Tue, 09/16/2025 - 14:00

Author:

whawkins

Sep 16, 2025

The Astrophysical Journal: We test the use of the Mg II resonant lines for measurement of the magnetic field at the top of the chromosphere of polar coronal holes (CHs). The Hanle effect in the core of Mg II k enables access to a regime of field strengths where the Zeeman effect has little diagnostic value (especially at the solar poles, where most of the field is transverse to the line of sight). Synthetic Stokes spectra computed from a radiation magnetohydrodynamic simulation of a CH emulating a high viewing angle are inverted with the HanleRT Tenerife Inversion Code, which accounts for the physical processes that lead to scattering-induced polarization and its modification due to the magnetic field and other symmetry-breaking mechanisms. We find that, while degeneracies in the atmospheric model lead to poor inferences of the thermodynamical properties, the magnetic inferences are highly consistent with the model values. The mean magnetic field strength in the simulation cube is typically retrieved with a relative error of δB ∼ 20% and an absolute error of ΔB ∼ 2 G at the top of the chromosphere. This opens up an avenue for promising chromospheric constraints for magnetic extrapolation models that ingest photospheric magnetograms, whose biases and uncertainties are troublesome to the reconstruction of the heliospheric magnetic field.

Inversion of an average synthetic Mg II k spectrum averaged over a 5"x5" area of a quiet Sun coronal hole simulation. The inversion strategy optimizes the fit of the spectral line core to extract the magnetic information at the top of the chromosphere. The synthetic observations are represented by black dots and the best-fit spectrum achieved by the HanleRT-TIC inversion is shown with red lines in the top four panels; additionally, the top-left panel shows the contribution function of the spectral line in blue. The bottom row shows the retrieved magnetic field stratification (red line) and the formal error bars at the inversion nodes derived by HanleRT-TIC. The inversion results are compared to the ensemble of values from the MURaM model atmospheres (gray histogram) used to create the synthetic observation. The height range of formation of the line core is indicated by the vertical blue stripes. The inversion inferences of the magnetic field are compatible with the average values of the simulation within the blue stripes.

Image instabilities and polarization cross-talk

whawkins — Tue, 28 Jan 2025 18:06:38 +0000

Image instabilities and polarization cross-talk whawkins Tue, 01/28/2025 - 11:06

Author:

whawkins

Jan 28, 2025

The Astrophysical Journal: We expand on our previous study of the impact of atmospheric seeing on polarization cross-talk, and show how the formalism that was developed in that work can be applied to treat the case of spatial modulators of polarization. Beside formally demonstrating how the problem of cross-talk is fully eliminated in such devices, we also gain insight on the meaning of polarimetric noise of temporal modulation schemes in the limit of very high modulation frequency. We also describe the problem of spectrograph instabilities, and how the spectral gradients that are naturally associated with a line spectrum feed into the problem of polarimetric errors induced by mechanical vibrations, thermal drifts, and pointing jitter. Finally, we show how this formalism can be used to estimate the contribution of polarization cross-talk to the errors on the elements of the 4$\times$4 Stokes response matrix, for the purpose of producing realistic error budgets for polarimetric instrumentation.

Figure 2. Top two rows: Example of spectrally resolved Stokes profiles of a narrow line (FWHM ∼6.3 pm), observed with a resolving power of 200 000 (2-px critical-sampling), and assuming maximum linear polarization of 10% and circular polarization of 30%. The spectral gradients reported inside the panels are obtained through Eq. (15), and must be used in Eq. (3) to estimate the corresponding polarimetric cross-talk errors. Bottom row: (left) modeled PSD of the image jitter at the detector in the spectral direction, delivering a target 1/8 px rms (de Wijn et al. 2022); (right) polarization cross-talk errors induced by the same PSD, for a spectrograph configuration representing the observation of the synthetic Stokes profiles shown at the top, assuming a total integration time of 10 s and a 10-state modulation cycle.)

Erratum: Analysis of Seeing-Induced Polarization Cross-Talk and Modulation Scheme Performance

whawkins — Thu, 01 Aug 2024 15:39:00 +0000

Erratum: Analysis of Seeing-Induced Polarization Cross-Talk and Modulation Scheme Performance whawkins Thu, 08/01/2024 - 09:39

Author:

whawkins

Aug 1, 2024

Corrected Figure 7 of Paper I, showing the polarimetric errors for a stepped waveplate with 150◦ retardance (8-state modulation scheme), with dual-beam analysis, and 10 s integration. For this example, we adopted a non-optimal demodulation scheme consisting of simple additions and subtractions of individual modulated signals.

The Astrophysical Journal: (Published 2012 September 4; submitting a "corrected" Figure 7 of Paper I) We analyze the generation of polarization cross-talk in Stokes polarimeters by atmospheric seeing, and its effects on the noise statistics of spectropolarimetric measurements for both single-beam and dual-beam instruments. We investigate the time evolution of seeing-induced correlations between different states of one modulation cycle and compare the response to these correlations of two popular polarization modulation schemes in a dual-beam system. Extension of the formalism to encompass an arbitrary number of modulation cycles enables us to compare our results with earlier work. Even though we discuss examples pertinent to solar physics, the general treatment of the subject and its fundamental results might be useful to a wider community.

A unifying polarization formalism for electric- and magnetic-multipole interactions

whawkins — Tue, 16 Jul 2024 21:00:47 +0000

A unifying polarization formalism for electric- and magnetic-multipole interactions whawkins Tue, 07/16/2024 - 15:00

Author:

whawkins

Jul 16, 2024

APJ: We extend the spherical tensorial formalism for polarization to the treatment of eletric- and magnetic-multipole transitions of any order. We rely on the spherical-wave expansion to derive the tensor form of the operator describing the interaction of the atomic system with a polarized radiation field, which naturally leads to the introduction of spherical tensors describing the polarization properties of the interacting field. As a direct application, the formalism is used to model the radiation anisotropy affecting the scattering of radiation in an electric-quadrupole transition, and the associated Hanle effect in the presence of a magnetic field.

Hanle diagrams for the 90-deg scattering of resonant radiation in the E2 transition (Jl , Ju) = (0, 2) (top row) and in the E1 transition (Jl , Ju)=(0, 1) (bottom row). Two inclinations (30 deg and 90 deg) of the magnetic field from the local vertical are shown (respectively two leftmost and two rightmost panels). Each curve spans a range of magnetic Larmor frequencies, normalized by the inverse lifetime of the excited level, between 0.1 and 10 (range of criticality of the Hanle effect). Each colored curve is for different field azimuths ranging from -180 to +150 deg in steps of 30 deg.

EUV polarimetric diagnostics of the solar corona: the Hanle effect of Ne viii 770 A

whawkins — Tue, 30 Apr 2024 16:16:15 +0000

EUV polarimetric diagnostics of the solar corona: the Hanle effect of Ne viii 770 A whawkins Tue, 04/30/2024 - 10:16

Author:

whawkins

Apr 30, 2024

Top row: Rising phase (CR2104) of SC24: (a) PSIMAS model map of magnetic field, (b) LOS-integrated Stokes L/I in the presence of magnetic fields, (c) LOS-integrated linear polarization azimuth (relative to radial direction on the plane-of-sky), and (d) synthesized ratio between LOS integrated L/I in presence and (L/I)0 in absence of model magnetic fields. Only on-disk information within 1 Rsun is shown here. Middle row and bottom row illustrate the same maps, but during the maximum phase (CR2171) and the minimum phase (CR2225), respectively. Contours of a particular color in a given map represent iso-curves of the depicted physical quantity shown in logarithmic scale. Note that collisional excitation has been included here.

Astrophysical Journal: Magnetic fields are the primary driver of the plasma thermodynamics in the upper solar atmosphere, especially in the corona. However, magnetic field measurements in the solar corona are sporadic, thereby limiting us from the complete understanding of physical processes occurring in the coronal plasma. In this paper, we explore the diagnostic potential of a coronal emission line in the extreme-ultraviolet (EUV), i.e., Ne viii 770 AA to probe the coronal magnetic fields. We utilize 3D 'Magneto-hydrodynamic Algorithm outside a Sphere' (MAS) models as input to the FORWARD code to model polarization in Ne viii line produced due to resonance scattering, and interpret its modification due to collisions and the magnetic fields through the Hanle effect. The polarization maps are synthesized both on the disk as well as off-the-limb. The variation of this polarization signal through the different phases of solar cycle 24 and the beginning phase of solar cycle 25 is studied in order to understand the magnetic diagnostic properties of this line owing to different physical conditions in the solar atmosphere. The detectability of the linear polarization signatures of the Hanle effect significantly improves with increasing solar activity, consistently with the increase in the magnetic field strength and the intensity of the mean solar brightness at these wavelengths. We finally discuss the signal-to-noise ratio (SNR) requirements by considering realistic instrument designs.

First Observation of Chromospheric Waves in a Sunspot by DKIST/ViSP

whawkins — Tue, 04 Apr 2023 16:33:16 +0000

First Observation of Chromospheric Waves in a Sunspot by DKIST/ViSP whawkins Tue, 04/04/2023 - 10:33

The Anatomy of an Umbral Flash

Author:

whawkins

Apr 4, 2023

Astrophysical Journal Letters: The Visible Spectro-Polarimeter of the NSF Daniel K. Inouye Solar Telescope collected its Science Verification data on 2021 May 7-8. The instrument observed multiple layers of a sunspot atmosphere simultaneously, in passbands of Ca II 397 nm (H line), Fe I 630 nm, and Ca II 854 nm, scanning the region with a spatial sampling of 0.04 arcsec1 and an average temporal cadence of 7.76 s, for a duration of 38.8 minutes. The slit moved southward across the plane of sky at 3.83 km s-1. The spectropolarimetric scans exhibit prominent oscillatory "ridge" structures that lie nearly perpendicular to the direction of slit motion (north to south). These ridges are visible in the maps of line intensity, central wavelength, line width, and both linear and circular polarization. Contemporaneous Atmospheric Imaging Assembly observations indicate that these ridges are purely temporal in character and are likely attributed to the familiar chromospheric 3 minute umbral oscillations. We observe in detail a steady umbral flash near the center of the sunspot umbra. Although bad seeing limited the spatial resolution, the unique high signal-to-noise ratio data enable us to estimate the shock Mach numbers (~2), propagation speeds (~9 km/s), and their impacts on the longitudinal magnetic field (dB ~ 50 G), gas pressure, and temperature (dT/T ~ 0.1) of subshocks over 30 s. We also find evidence for rarefaction waves situated between neighboring wave train shocks. The Ca II 854 nm line width is fairly steady throughout the umbral flash, except for a sharp 1.5 km/s dip immediately before, and a comparable spike immediately after, the passage of the shock front. This zigzag in line width is centered on the subshock and extends over 0.4 arcsec.

Detrended intensity, line width, azimuth, P, and B LOS cross sections, and raw Doppler cross sections, along the dashed cyan and magenta lines. The vertical dashed lines show the approximate locations of the Ca II 854 nm intensity peaks.

First Observation of Chromospheric Waves in a Sunspot by DKIST/ViSP

whawkins — Wed, 15 Mar 2023 16:08:51 +0000

First Observation of Chromospheric Waves in a Sunspot by DKIST/ViSP whawkins Wed, 03/15/2023 - 10:08

Effects of spectral resolution on simple magnetic field diagnostics of the Mg II h & k lines

whawkins — Fri, 23 Sep 2022 21:17:25 +0000

Effects of spectral resolution on simple magnetic field diagnostics of the Mg II h & k lines whawkins Fri, 09/23/2022 - 15:17

Author:

whawkins

Sep 23, 2022

The Astrophysical Journal: Rebecca Centeno, Matthias Rempel, Roberto Casini, and Tanausu del Pino Aleman study the effects of finite spectral resolution on the magnetic field values retrieved through the weak field approximation (WFA) from the cores of the Mg II h&k lines. The retrieval of the line-of-sight (LOS) component of the magnetic field, Blos, from synthetic spectra generated in a uniformly magnetized FAL-C atmosphere are accurate when restricted to the inner lobes of Stokes V. As we degrade the spectral resolution, partial redistribution (PRD) effects that more prominently affect the outer lobes of Stokes V, are brought into the line core through spectral smearing, degrading the accuracy of the WFA and resulting in an inference bias, which is more pronounced the poorer the resolution. When applied to a diverse set of spectra emerging from a sunspot simulation, we find a good accuracy in the retrieved Blos when comparing it to the model value at the height where the optical depth in the line core is unity. The accuracy is preserved up to field strengths of B~1700 G. Limited spectral resolution results in a bias toward weaker retrieved fields. The WFA for the transverse component of the magnetic field is also evaluated. We find the best estimates when the WFA is evaluated in the core of the line. Reduced spectral resolution degrades the accuracy of the inferences because spectral mixing results in the line effectively probing deeper layers of the atmosphere.

Scatter density plots of the retrieved value of Blos retrieved from Mg II h using the weak field approximation, against their MURaM model counterparts for the case of infinite spectral resolution (left), R = 45,000h (middle) and R = 30,000 (right). The model values are taken at the height where the core of the Mg II h line reaches optical depth unity. The darker the grey-level, the higher the number of samples in the bin. The bin size is 40 G. The red line represents the ideal solution and the blue line shows a linear fit through the data.

Optimal spectral lines for measuring chromospheric magnetic fields

kolinski — Mon, 11 Jul 2022 20:33:01 +0000

Optimal spectral lines for measuring chromospheric magnetic fields kolinski Mon, 07/11/2022 - 14:33

Roberto Casini

Polarization fringes in optical systems: a compendium

Recent News

Using the Hanle Effect in Mg II k to Quantify the Open Flux above the Solar Poles

Recent News

Image instabilities and polarization cross-talk

Recent News

Erratum: Analysis of Seeing-Induced Polarization Cross-Talk and Modulation Scheme Performance

Recent News

A unifying polarization formalism for electric- and magnetic-multipole interactions

Recent News

EUV polarimetric diagnostics of the solar corona: the Hanle effect of Ne viii 770 A

Recent News

First Observation of Chromospheric Waves in a Sunspot by DKIST/ViSP

Recent News

First Observation of Chromospheric Waves in a Sunspot by DKIST/ViSP

Recent News

Effects of spectral resolution on simple magnetic field diagnostics of the Mg II h & k lines

Recent News

Optimal spectral lines for measuring chromospheric magnetic fields

Recent News