Vacuum ultraviolet spectropolarimeter design for precise polarization measurements

Noriyuki Narukage; Frédéric Auchère; Ryohko Ishikawa; Ryouhei Kano; Saku Tsuneta; Amy R. Winebarger; Ken Kobayashi

doi:10.1364/AO.54.002080

Applied Optics
Vol. 54,
Issue 8,
pp. 2080-2084
(2015)
•https://doi.org/10.1364/AO.54.002080

Vacuum ultraviolet spectropolarimeter design for precise polarization measurements

Noriyuki Narukage, Frédéric Auchère, Ryohko Ishikawa, Ryouhei Kano, Saku Tsuneta, Amy R. Winebarger, and Ken Kobayashi

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Noriyuki Narukage, Frédéric Auchère, Ryohko Ishikawa, Ryouhei Kano, Saku Tsuneta, Amy R. Winebarger, and Ken Kobayashi, "Vacuum ultraviolet spectropolarimeter design for precise polarization measurements," Appl. Opt. 54, 2080-2084 (2015)

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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.

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Item	Requirement
Observing wavelength	Lyman- $α$ (121.567 nm)
Observing polarization	Linear polarization (Stokes $I$ , $Q$ , $U$ )
Polarization sensitivity	0.1% at line core
	(121.567 nm $\pm 0.02 nm$ )
Spectral resolution	0.01 nm
Spatial resolution	$< 10 arcsec$

Telescope
Entrance pupil		$ϕ 270 mm$
Effective focal length		2614.0 mm
F value		9.68
Plate scale at slit		$1.03 arcsec / 13.0 μm$
Spectropolarimeter
[Slit]
Slit width		$18.4 μm (= 1.45 arcsec)$
Slit length		$5.1 mm (= 400 arcsec)$
Distance to grating		905.7 mm
[Grating]
Groove density		$3000 grooves / mm$
Line spacing		Constant
Surface figure		Sphere ( $K = 0$ )
Curvature radius		1749.3 mm
Distance to camera mirror^a		450.0 mm
[Off-axis Parabolic Camera Mirror]
Surface figure		Paraboloid ( $K = - 1.0$ )
Off-axis value		180.0 mm
Curvature radius		1655.0 mm
Distance to CCD^b		827.5 mm
CCD Pixel size		13.0 μm
Spatial plate scale		$1.11 arcsec / 13.0 μm$
Spectral plate scale		$0.0048 nm / 13.0 μm$

Fixed parameters
Spectral plate scale of the spectrograph	$Δ λ$
CCD pixel size	$p$
Clear aperture of the grating	$D_{g}$
Diameter of the entrance pupil	$D_{e}$
Groove density of the grating	$g$
Design parameters
Curvature radius of the grating	$R_{g}$
Distance between grating and camera mirror	$d_{c}$
Off-axis value of camera mirror	$O_{c}$
Dependent parameters
Distance between camera mirror and CCD	$d_{d}$
Distance between slit and grating	$d_{g}$
F value of the telescope	$F$
Focal length of the telescope	$f$
Spatial plate scale of the telescope	$Δ x_{t}$
Spatial magnification of the spectrograph	$m$
Spatial plate scale of the spectrograph	$Δ x$

Item	Requirement
Observing wavelength	Lyman- $α$ (121.567 nm)
Observing polarization	Linear polarization (Stokes $I$ , $Q$ , $U$ )
Polarization sensitivity	0.1% at line core
	(121.567 nm $\pm 0.02 nm$ )
Spectral resolution	0.01 nm
Spatial resolution	$< 10 arcsec$

Telescope
Entrance pupil		$ϕ 270 mm$
Effective focal length		2614.0 mm
F value		9.68
Plate scale at slit		$1.03 arcsec / 13.0 μm$
Spectropolarimeter
[Slit]
Slit width		$18.4 μm (= 1.45 arcsec)$
Slit length		$5.1 mm (= 400 arcsec)$
Distance to grating		905.7 mm
[Grating]
Groove density		$3000 grooves / mm$
Line spacing		Constant
Surface figure		Sphere ( $K = 0$ )
Curvature radius		1749.3 mm
Distance to camera mirror^a		450.0 mm
[Off-axis Parabolic Camera Mirror]
Surface figure		Paraboloid ( $K = - 1.0$ )
Off-axis value		180.0 mm
Curvature radius		1655.0 mm
Distance to CCD^b		827.5 mm
CCD Pixel size		13.0 μm
Spatial plate scale		$1.11 arcsec / 13.0 μm$
Spectral plate scale		$0.0048 nm / 13.0 μm$

Abstract

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Applied Optics