Assessing the Behavior of Modern Solar Magnetographs and Spectropolarimeters

TL;DR

This paper analyzes the design and performance of modern solar magnetographs and spectropolarimeters, deriving detection thresholds and stability requirements to improve measurement accuracy of solar magnetic fields and velocities.

Contribution

It provides a comprehensive framework for understanding and optimizing the physical and instrumental parameters affecting solar magnetograph measurements.

Findings

Derived formulas for detection thresholds based on SNR and polarimetric efficiency.

Established relationships between measurement inaccuracies and instrument stability.

Showed that liquid crystal magnetographs can achieve ideal polarimetric efficiencies.

Abstract

The design and later use of modern spectropolarimeters and magnetographs require a number of tolerance specifications that allow the developers to build the instrument and then the scientists to interpret the data accuracy. Such specifications depend both on device-specific features and on the physical assumptions underlying the particular measurement technique. Here we discuss general properties of every magnetograph, as the detectability thresholds for the vector magnetic field and the line-of-sight velocity, as well as specific properties of a given type of instrument, namely that based on a pair of nematic liquid crystal variable retarders and a Fabry-P\'erot etalon (or several) for carrying out the light polarization modulation and spectral analysis, respectively. We derive formulae that give the detection thresholds in terms of the signal-to-noise ratio of the observations and the polarimetric efficiencies of the instrument. Relationships are also established between inaccuracies in the solar physical quantities and instabilities in the instrument parameters. Such relationships allow, for example, to translate scientific requirements for the velocity or the magnetic field into requirements for temperature or voltage stability. We also demonstrate that this type of magnetograph can theoretically reach the optimum polarimetric efficiencies of an ideal polarimeter, regardless of the optics in between the modulator and the analyzer. Such optics induces changes in the instrument parameters that are calculated too.