Vacuum ultraviolet spectropolarimeter design for precise polarization measurements

Abstract

Precise polarization measurements in the vacuum ultraviolet (VUV) region provide a new means for inferring weak magnetic fields in the upper atmosphere of the Sun and stars. We propose a VUV spectropolarimeter design ideally suited for this purpose. This design is proposed and adopted for the NASA-JAXA chromospheric lyman-alpha spectropolarimeter (CLASP), which will record the linear polarization (Stokes Q and U) of the hydrogen Lyman-α line (121.567 nm) profile. The expected degree of polarization is on the order of 0.1%. Our spectropolarimeter has two optically symmetric channels to simultaneously measure orthogonal linear polarization states with a single concave diffraction grating that serves both as the spectral dispersion element and beam splitter. This design has a minimal number of reflective components with a high VUV throughput. Consequently, these design features allow us to minimize the polarization errors caused by possible time variation of the VUV flux during the polarization modulation and by statistical photon noise.

Indication of the Hanle Effect by Comparing the Scattering Polarization Observed by CLASP in the Lyα and Si iii 120.65 nm Lines

“…CLASP has two optically symmetric channels, which provide two simultaneous measurements of orthogonal polarization states with two cameras (Narukage et al 2015(Narukage et al , 2017. The two cameras recorded the modulated intensity every 0.3 s in synchronization with the waveplate rotation.…”

Polarization Calibration of the Chromospheric Lyman-Alpha SpectroPolarimeter for a 0.1% Polarization Sensitivity in the VUV Range. Part II: In-Flight Calibration

“…The 400"-length and 1.45"-width slit can be seen at the center of the slit-jaw image. The plate scales are 1.03" (slit-jaw spatial), 1.11" (spectro-polarimeter spatial) and 0.0048 nm (spectro-polarimeter spectral) on the 13 µm pixel CCD cameras (Narukage et al, 2015). From the optical alignment of the instrument, a ∼3" spatial resolution is expected on both the slit-jaw and the spectropolarimeter.…”

“…A unique pre-flight polarization calibration experiment was conducted at Lyman-α under vacuum condition (Giono et al, 2016a), which determined the instrument's response matrices X (Elmore, 1990) for each of the two channels inside the spectro-polarimeter (design presented in Narukage et al, 2015). This response matrix is defined as S =XS, where S is the incoming polarization signal expressed with its Stokes vector as (I,Q,U ) and S the measured polarization signal: (I ,Q ,U ) .…”

In-flight performance of the polarization modulator in the CLASP rocket experiment

“…2). PMU was attached to the telescope side on the baseplate between the telescope and spectropolarimeter (the detailed optical design is shown by Narukage et al 8 ). The waveplate holder mounted on PMU was blackened to reduce stray light.…”