Optical spectropolarimetry

With optical spectropolarimetry, one can address with unprecedented detail a broad range of important issues in stellar physics, from stellar magnetic fields to extrasolar planets, from stellar surface inhomogeneities and surface differential rotation to activity cycles and magnetic braking, from microscopic diffusion to turbulence, convection and circulation in stellar interiors, from abundances and pulsations in stellar atmospheres to stellar winds and accretion discs, from the early phases of stellar formation to the late stages of stellar evolution, from extended circumstellar environments to distant interstellar medium.

A spectropolarimeter is composed of a standard spectrograph and a separated polarimeter. The polarimeter has to provide very achromatic polarisation analysis of the stellar light without producing spectral interference patterns or crosstalk effects. Crosstalk effects consist in the instrument measuring a spurious signal of circular polarization when the sources is linearly polarized, or vice versa. Space polarimeters are currently beeing designed for the Chinese Space Solar Telescope (SST) and the European Solar Orbiter. These instruments could serve as a basis for the spectropolarimeter discussed here.

The polarimeter is usually installed at the Cassegrain focus to avoid instrumental polarisation. It is composed of one or more waveplates, which create a phase delay between two beams of light, and an analyser or beamsplitter, which spatially separate the two states of orthogonal polarization. To measure Stokes V it is necessary to measure the two states (left and right) of circular polarization. To measure each circular polarization state, it is necessary to measure two linear polarization states (the ordinary and extraordinary beams).