High Precision Full Stokes Spectropolarimetry of the Sun as a star-Instrument design aspects

TL;DR

This paper discusses the design considerations for a high-precision spectropolarimeter aimed at measuring the Sun's magnetic field on a global scale, focusing on instrument noise reduction and modulation techniques.

Contribution

It presents a detailed analysis of instrumental design aspects, including noise sources and modulation methods, for high-accuracy Sun-as-a-star spectropolarimetry.

Findings

Analysis of seeing-induced cross-talk noise

Evaluation of polarization modulation concepts

Assessment of disk-averaged Stokes signal levels

Abstract

The magnetic field plays a major role in governing the dynamics of the sun. Many interesting features like sunspots, flares, prominences, and Coronal Mass Ejections (CMEs) occur on its surface due to the dynamics associated with the magnetic fields. The magnetic activity exhibits spatial scales ranging from very fine scale (below the resolution limit of the current largest telescope) to large scale such as sunspots, active regions and the spatial scales as large as the sun itself. While the major efforts in building large telescopes is going on towards the goal of resolving smallest structure possible we propose here to measure the magnetic field on the global scale. For this purpose we propose an instrument to carryout high precision and high accurate spectropolarimetry of sun-as-a-star. In this thesis, we explore various instrumental design aspects that are necessary to make such observations. As part of the design consideration we have analysed a major noise source i.e. seeing induced cross-talk through simulation as well as using the measured scintillation data. Further, we have analysed full disk Stokes images from SOLIS/VSM and SDO/HMI for the purpose of getting a rough idea on the disk averaged Stokes signal level. The instrument design aspects are mainly constrained by the need to modulate the light before it enters the telescope and image the full sun as a point source. Modulating the light before it enters the telescope has the advantage of completely avoiding the instrumental polarization which will enable us to carry out high accurate polarimetric measurements. We have explored in this thesis various concepts of polarization modulator and spectral discriminator and worked out their suitability for the purpose of carrying out high precision and high accurate spectropolarimetry of sun-as-a-star at high spectral resolution.