Alfred de Wijn

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.

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

Derivation of Instrument Requirements for Polarimetry using Mg, Fe, and Mn lines between 250 and 290 nm

whawkins — Tue, 14 Feb 2023 18:45:32 +0000

Derivation of Instrument Requirements for Polarimetry using Mg, Fe, and Mn lines between 250 and 290 nm whawkins Tue, 02/14/2023 - 11:45

Author:

whawkins

Feb 14, 2023

Judge et al. (2021) recently argued that a region of the solar spectrum in the near-UV between about 250 and 290 nm is optimal for studying magnetism in the solar chromosphere due to an abundance of Mg II, Fe II, and Fe I lines that sample various heights in the solar atmosphere. In this paper we derive requirements for spectropolarimetric instruments to observe these lines. We derive a relationship between the desired sensitivity to magnetic field and the signal-to-noise of the measurement from the weak-field approximation of the Zeeman effect. We find that many lines will exhibit observable polarization signals for both longitudinal and transverse magnetic field with reasonable amplitudes.

IRIS map used to calculate the longitudinal and transverse sensitivity factors for Mg II and Mn I lines. Left panel: intensity in the core of the Mg II k line. Center panel: longitudinal field sensitivity factor Γ. Right panel: transverse field sensitivity factor X.

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.

Solar Transition Region UltraViolet Explorer (STRUVE) requirements flow down to design

whawkins — Tue, 23 Aug 2022 19:11:52 +0000

Solar Transition Region UltraViolet Explorer (STRUVE) requirements flow down to design whawkins Tue, 08/23/2022 - 13:11

Author:

whawkins

Aug 23, 2022

Graph showing the flow-down of requirements from the three science questions (SQ 1.A, SQ 1.B, and SQ 2.A) to Level 1 requirements, and to selected Level 2 requirements.

The Solar Transition Region UltraViolet Explorer (STRUVE) is a proposed miniature satellite (CubeSat) conceived to study the magnetic field in the solar atmosphere currently funded by NASA for a concept study. Magnetism in the solar atmosphere is responsible for "space weather" through explosive processes called flares and coronal mass ejections. Our ability to understand these processes is limited by a lack of observations. STRUVE is designed to observe magnetic field in the solar atmosphere from the deepest observable layers up to the base of the heliosphere that connects the magnetic field of the sun to the earth. To do this, the STRUVE instrument measures polarization in spectral lines in a region of the near-UV that contains the well-known Mg II h and k lines as well as a number of Fe I and Fe II lines that sample many heights in the atmosphere. This paper aims to illustrate the flow down of requirements from the mission science objectives to design requirements while also giving an overview of the design developed from the concept study. The derived requirements from the science objectives are the primary functions STRUVE will need to perform to capture the needed data for the mission. The overview of the mission design will delve into the mission operations and give an overview of the STRUVE instrument and spacecraft. Links between the requirements and mission design are made, pointing out how critical requirements are being met. Throughout the paper, trade studies are presented showing the rationale behind many of the critical design choices made in developing STRUVE for the concept study.

A Rare Transit of Venus Across the Sun

kolinski — Tue, 23 Nov 2021 18:52:21 +0000

A Rare Transit of Venus Across the Sun kolinski Tue, 11/23/2021 - 11:52

Design of a Highly Efficient Polychromatic Full-Stokes Polarization Modulator for the CRISP Imaging Spectrometer

kolinski — Tue, 16 Nov 2021 21:14:23 +0000

Design of a Highly Efficient Polychromatic Full-Stokes Polarization Modulator for the CRISP Imaging Spectrometer kolinski Tue, 11/16/2021 - 14:14

Publication: ApJ; First HAO Author: Alfred de Wijn; Authors as listed in article: A.G. de Wijn, J. de la Cruz Rodríguez, G.B. Scharmer, G. Sliepen, and P. Sütterlin

Author:

kolinski

Nov 16, 2021

Example spectro-polarimetric data of Active Region 12471 in the Fe I lines at 630.2 nm. The observation was recorded on 2019-05-10 around 09:10 UT. Top row: images of the Stokes parameter in the blue wing of the 630.25 nm line at −90 mÅ from line center. Spectra at the black, red, and blue crosses are shown in the bottom row. The locations of the spectral sampling are indicated by crosses.

In 2014 HAO built a polarimetric modulator for the CRISP instrument on the Swedish 1-m Solar Telescope in La Palma, using the principles of polychromatic modulation previously developed by HAO scientists. This paper discusses the processes by which the modulator was designed, built, and tested. The modulator was installed and had been in use since 2015. It is working at close to optimal efficiency and has produced considerable scientific output.

Link to paper: Design and Performance Analysis of a Highly Efficient Polychromatic Full Stokes Polarization Modulator for the CRISP Imaging Spectrometer

Alfred de Wijn

Image instabilities and polarization cross-talk

Recent News

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

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

Derivation of Instrument Requirements for Polarimetry using Mg, Fe, and Mn lines between 250 and 290 nm

Recent News

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

Recent News

Solar Transition Region UltraViolet Explorer (STRUVE) requirements flow down to design

Recent News

A Rare Transit of Venus Across the Sun

Recent News

Design of a Highly Efficient Polychromatic Full-Stokes Polarization Modulator for the CRISP Imaging Spectrometer

Recent News