Static spectropolarimeter concept adapted to space conditions and wide spectrum constraints

Abstract

The issues related to moving elements in space and instruments working in broader wavelength ranges lead to a need for robust polarimeters, efficient on a wide spectral domain, and adapted to space conditions. As part of the UVMag consortium, created to develop spectropolarimetric UV facilities in space, such as the Arago mission project, we present an innovative concept of static spectropolarimetry. We studied a static and polychromatic method for spectropolarimetry, applicable to stellar physics. Instead of modulating the polarization information temporally, as usually done in spectropolarimeters, the modulation is performed in a spatial direction, orthogonal to the spectral one. Thanks to the proportionality between phase retardance imposed by a birefringent material and its thickness, birefringent wedges can be used to create this spatial modulation. The light is then spectrally cross-dispersed, and a full-Stokes determination of the polarization over the whole spectrum can be obtained with a single-shot measurement. The use of Magnesium Fluoride wedges, for example, could lead to a compact, static polarimeter working at wavelengths from 0.115 µm up to 7 µm. We present the theory and simulations of this concept, as well as laboratory validation and a practical application to Arago.

Reflections on the discovery space for a large ultraviolet-visible telescope: inputs from the European-led EUVO exercise

“…The ARAGO mission is intended for this purpose. 52 Spectropolarimetric studies will benefit extraordinarily from the use of large collecting surfaces such as that intended for LUVOIR. The analysis of the polarized radiation requires the light beam to be split into two (or more) polarization states and optical elements such as polarizers are rather inefficient in the UV.…”

Innovative static spectropolarimeter concept for wide spectral ranges: tolerancing study

“…To create a static spectropolarimeter, we chose to modulate the polarization of the light in a spatial direction, orthogonal to the main dispersion direction of the spectrograph. To do so, we use birefringent wedges of magnesium fluoride (MgF2) as retardation plates, combined with a linear polarizer [2]. The thickness of the retardation plates varies linearly along its height and, therefore, the retardance imposed to the light also varies linearly along the wedge.…”

“…Using the system described in [2], the final retardance equation is the following, with n the birefringence of the material used (MgF2 in our case),  the apex angle of the first wedge, and x the height of the wedges (with x=0 on the optical axis):…”

“…Therefore, a classical least-squares solving code is established to solve the system Iout=A.S at every wavelength, where Iout is the input image, A is a vector composed with the trigonometric functions shown in Eq. 2, and S is the Stokes vector we want to determine [2].…”

“…The concept described in this proceeding is explained in more detail in the articles by Sparks et al (2012) [1] and Pertenais et al (2015) [2].…”