BL3U

Performance of a High-Precision Transmission Gratingfor High-Resolution Soft X-Ray Emission Spectroscopy

H. Yamane and N. Kosugi
Dept of Photo-Molecular Science, Institute for Molecular Science, Okazaki 444-8585, JapanThe X-ray emission spectroscopy (XES) is tomeasure X-rays emitted as a photon-in/photon-out
process arising from the decay of the inner-shell
excitation and resonant Raman or inelastic scattering.
The XES technique combined with the synchrotron
radiation is a powerful method to study occupied
partial density of states and element-specificexcitations of materials. However, a serious problem
exists in XES using a reflection grating, that is, arather low efficiency in detecting emitted X-rays due
to (i) the limited number of X-ray focusing optics inthe Rowland circle mount (vertical direction only) and(ii) the low acceptance angle and low quantum
efficiency at charge coupled devices (CCD) detector
with the grazing-incident configuration in the soft
X-ray region. Such problems are in general solved by
the use of high brilliant X-ray beam of 1013
photons/sec. In this case, however, one has to takecare of radiation damage, in particular in soft matters.
In order to overcome the above problems, we havedeveloped a new X-ray emission spectrometer basedon a transmission grating (TG) at UVSOR BL3U.
The spectrometer adopts a novel optical design with a
Wolter type I mirror, a free-standing TG, and aback-illuminated CCD, which enables high efficiency
and high energy resolution due to an omnidirectional
focusing for emitted X-rays and the normal-incident
configuration for the CCD detector.
Very recently, we have installed a new TG, which
was fabricated by NTT-AT Co. Ltd. In order toachieve the high precision of the TG spectrometer, the
new TG is focused on the stability and accuracy of
the groove structure. In the high-precision TG, the
SiC thickness is about 540 nm, the groove density is5555 lines/mm (180 nm periods), and the size of theslit is 108 nm at the surface side and 64 nm at thebackside as illustrated in Fig. 1. In this report, weshow the result of evaluation of the performance forthe high-precision TG spectrometer in the energy
region of 60–300 eV.
Figure 2 shows an example of resonant XES in theelastic peak region of a LiF(100) single crystal,measured at an incident photon energy hȞ = 61.0 eV.
We observed a very sharp elastic peak in the Li KĮ
spectrum with the full-width at half-maximum of
about ǻEout = 0.02 eV, i.e., E/ǻEout ~ 3050. Such a
performance has been examined for various photon
energies using Si(111), InP(100), CdS(0001), h-BN,
and HOPG as summarized in Table 1. Note that, theemission intensity is reasonably high for all XES data,e.g., 0.5–1 spectrum/hr. As for the energy resolution,
the high precision TG spectrometer shows a highperformance below hȞ = 100 eV with the resolving
power of E/ǻEout ~ 3000, which is the advantage in
our XES system; e.g., the present spectrometer may
offer new insight into the field of Li-based functional
materials such as energy-storage devices.

900 nm

300 nm (supporting structure)

180 nm108 nmCross section

539 nm

SiC
SiN

64 nm

Si

Fig. 1. Scheme of the high-precision transmission
grating with the 5555 lines/mm groove density.

LiF(100)
Intensity / arb. units

hvin = 61.0 eV
E/'Ein = 4000
texp = 90 min.

'Eout ~ 20 meV

60.7

60.8

60.9

61

61.1

61.2

61.3

Emission Energy / eV

Fig. 2. Elastic peak of the Li KĮ XES spectrum (hȞ =61.0 eV) for the LiF(100) single crystal.
Table 1. Performance of the transmission grating
spectrometer with the 5555 lines/mm groove density.
hȞ / eV
61.0
100.0
139.6
164.1
196.0
285.5

43

ǻEout / eV
0.02
0.03
0.08
0.16
0.20
0.30

E/ǻEout
3050
3333
1745
1026
980
952

Material
LiF(100)
Si(111)
InP(100)
CdS(0001)
h-BN
HOPG

BL3B

New VIS-VUV Bending Magnet Beamline Design for BL3B
R. Ikematsu1, K. Fukui1, T. Ejima2, E. Nakamura3, M. Hasumoto3 and S. Kimura3
1
Dept. Elec. Elec. Engi, University of Fukui, Fukui 910-8507, Japan
2
IMRAM, Tohoku University, Sendai 980-8577, Japan
3
UVSOR, Institute for Molecular Science, Okazaki 444-8585, Japan
photon flux specification requires 40 mrad horizontal
acceptance angle. Then, a toroidal mirror (M1) has ~210 mm width. S (source point) – M1 to M1 – S1
(entrance slit) distance ratio is almost 1:1 to preventthe shape deformation of the focal spot. The M2 is aplane mirror reflects the light to the x direction (xyplane and y axis are defined as the plane of incidence
and the light axis). The roles of M2 are to give wide
space at the sample position (Q) and to cancel thesource point (S) horizontal movement under the
UVSOR feature version up. An off-plane Eagle type
monochromator consists of S1, G (gratings), and S2(exit slit). The focal length is 2.5 m satisfies both the
F number matching to the pre-mirror system (M0 –M2) and the resolution specification. Three gratings(G1 ~ G3) with the different glazing angles cover 2 ~24 eV region. The blazing angles and the additional
optical filters prevent the higher order lights. The
deviation angle of the G is 4 degree. The post-mirror
system with Kirkpatrick - Baez optics consists of bothspherical mirrors M3 and M4 which focus x and z
directions, respectively. The reflected light by the M4
becomes the parallel to the floor. Both the VIS – UV
and VUV spectrometers with CCD sensors for the PL
measurements will be installed into the sample
chamber at Q.
The ray tracing results of this optical configuration
are as follows; Photon flux (E/'E = 1000) : 1 ~ 4 u1010 photons/sec, Resolution (E/'E) : 12000 ~ 46000,
Beam spot size (H u V): 0.5 u 0.8 mm2, Linear
polarization degree (P) : 0.7 ~ 0.8. This beamline
will be constructed within FY2011.

The solid state optical devices in ultraviolet (UV)region will bring dramatic improvements in not only
the telecommunication applications but also theilluminating, environmental and medical applications.
To achieve both technological and scientificinvestigations of these device materials, the lightsources which cover wide photon energy region fromvisible (VIS) to vacuum ultraviolet (VUV) are
required.
Then,
BL3B
normalincidence
monochromator beamline has been planned forphotoluminescence (PL) studies in VIS – VUVregions. This beamline will be constructed as the
successor beamline of the BL1B (1-m Seya Namioka
type monochromator beamline) which will be
shutdown at the end of March 2011. In this report, we
present the optical design of a new 2.5-m Eagle type
monochromator beamline.
The design parameters for the beamline
monochromator dedicated to the PL are usually boththe brilliance and the linear polarization degree
together with the photon energy coverage, photon
flux, resolution and purity. These design
specifications of this beamline are as follows; Photon
energy coverage : 2 ~ 24 eV, Photon flux (E/'E =1000) : t 1010 photons/sec, Resolution (E/'E) : t
10000, Beam spot size (H u V) : d 0.8 u 0.8 mm2,
Linear polarization degree (P) : t 0.7.
Figures 1 and 2 show the schematic side view andtop view layouts of the BL3B, respectively. The firstplane mirror (M0) only reflects the synchrotron
radiation up to increase the design freedom of thisbeamline optics in a limited area with the radiation
shield wall which is located between M0 and M1. The

Fig. 1. Schematic layout of the BL3B (side view).

Fig. 2. Schematic layout of the BL3B (top view).

44

BL5B

Measurement for the Quantum Efficiencies of Microchannel Plates
1

K. Sakai1, K. Yoshioka2, G. Murakami1, T. Homma1 and I. Yoshikawa1
Department of Earth and Planetary Science, Graduate School of Science,
The University of Tokyo, Tokyo 113-0033, Japan2
Department of Physics, Rikkyo University, Tokyo 171-8501, Japan

Introduction
We are developing the space-based imagers forplasma emissions from the plasmasphere that is
torus-shaped dense region filled with plasmas flowing
upward from the earth’s ionosphere. The main targetsof our instruments are the He IIand O II ions in theplasmasphere. The He II and O II ions haveresonantly scattered emission lines at 30.4 nm and83.4 nm respectively. Column densities along thefield of view of instruments can be obtained because
intensities of emissions are proportional to themunder the assumption that observational targets are
optically thin. Therefore, the accuracies of thedetective efficiencies for the instruments are criticalfor estimation of the densities and to achieve thescience goals of our missions. The detective
efficiencies are absolutely calibrated at UVSOR.
Microchannel plates (MCPs) detect photons of UV
and high-energy particles with incident positions byusing position encoder, with extremely low noises.
MCPs produce single charge pulses of thousands ofelectrons via secondary emissions. They haveheritages as detectors of the space-based optical
instruments. It is essential to obtain the quantum
efficiencies (QE) of the MCP for the calculation of
the incident intensities. It is commonly used methodthat photocathodes are deposited to achieve highquantum efficiencies in the spectral range of theultraviolet. Even in this case, relative QEs of MCPs
with photoelectric materials to the bare one are easily
measured. Therefore, absolute QEs can be estimated.
In this experiment, the QE of one bare MCP at the
wavelength of 30.4 nm is measured. This result leads
calibrations of the detectors for various space-based
instruments.

is achieved at UVSOR with entrance filters.
With the pure lines, the QE of the MCP detector forthe 30.4 nm light is measured. The QEs are calculatedby the rate of the MCPs counts to the electron yield of
the photo diode which is absolutely calibrated. As aresult, the QE of the MCP detector for the 30.4 nm
light is 9.83% with the accuracy of 0.41%.
The QE of the MCPs for the 83.4 nm light has tobe measured as the future work in addition to the30.4 nm light. The O II ions have emission line at
83.4 nm and also known as one of main components
in the plasmasphere. The pure 83.4 nm light isessential for the measurements and is going to beachieved by using entrance filters and one more UV
diffraction grating.

Fig. 1. The transmittances of the Al/C sample filter
measured at UVSOR and at ISAS.

Measurement and result
The Al/Mg/Al (744 Å/3958 Å/747 Å) filter and theSn (1730Å) filter are installed on the entrance of thebeam line to achieve the pure 30.4 nm light. The
purity is investigated from the consistency between
the wavelength characteristics of the Al/C and theAl/Mg/Al sample filters for the light at UVSOR
dispersed by the gratings and for the emission line of
the RF excited helium gas light at the EUV facilities
of Institute of Space and Astronautical Science(ISAS). Fig. 1 shows the transmittances of the Al/C
sample filter measured at UVSOR and at ISAS, Fig. 2
shows those of the Al/Mg/Al filter. It is clear thatboth transmittances of the filter are consistent at 30.4
nm. Therefore, it is shown that the pure 30.4 nm light

Fig. 2. The transmittance of the Al/Mg/Al sample
filter. It is showed that the pure 30.4 nm light is at
UVSOR achieved through Al/C filter and the Sn filter
as the entrance filters.

45

BL6U

Two-Dimensional Electron Spectroscopy on BL6U

E. Shigemasa1, H. Iwayama1, E. Nakamura1, N. Kondo1, T. Horigome1 and M. Nagasaka2
1
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan2
Department of Photo-Molecular Science, Institute for Molecular Science,
Okazaki 444-8585, JapanA new project for constructing the undulator
beamline BL6U has been started since 2007. A
variable-included-angle Monk-Gillieson mounting
has been selected, in order to cover a wide photon
energy region (30-500 eV) with one single grating. It
has been confirmed through its performance tests thatthe monochromator designed can cover the photon
energy ranging from 40 to 400 eV with the resolving
power higher than 5000 and the photon flux morethan 1011 photons/sec, when the storage ring is
operated in the top-up mode.
A new electron spectrometer for gas phase
spectroscopy has successfully been installed, inparallel with the construction program of BL6U.
It is well known that high-resolution electron
spectroscopy is a powerful tool to investigate
electronic structures of atoms and molecules,
especially when high-resolution electron spectra andtheir polarization dependences are measured as afunction of photon energy in high-resolution mode.
The ability of this two dimensional (2D) electron
spectroscopy has been demonstrated in our recent
work at SPring-8 [1, 2], where special attention is paid
to detect slow electrons following core excitations.
In order to apply high-resolution 2D electron
spectroscopy to the investigation of the L-shell
excitations of the second row elements, a newexperimental setup for BL6U has been designed andconstructed. As a high performance hemisphericalelectron energy analyzer, MBS-A1, developed by the
MB Scientific AB company, has been selected.
In order to realize 2D electron spectroscopy,
software development for controlling both thebeamline monochromator and MBS-A1 analyzer has
been performed. For the beamline monochromator,
not only its output but the gap of the undulator should
be controlled. After careful optimizations for theundulator gaps, 2D electron spectroscopy on BL6U
has become feasible, thanks to the stable operation of
the UVSOR-II storage ring.
Figure 1 and 2 demonstrate the 2D maps for thede-excitation spectra following the double excitations
near the carbon 1s photoionization threshold in CO,
measured in the horizontal and vertical directions,
respectively, as examples of successful measurements.
The exit slit opening of the monochromator was set at
30 Pm, which corresponds to the photon energy
resolution of about 60 meV. The pass energy and slit
width of the MBS-A1 analyzer were set to 100 eV
and 0.2 mm, which results in the electron energy
resolution of about 60 meV. The straight lines with aslope of 1 in the high kinetic energy (KE) region ineach 2D map are due to the valence photoelectrons
with vibrational structures. The vertical lines around
KE of 275 eV in Fig. 2 are assigned to the atomic
Auger line from oxygen atoms after the dissociationof CO molecules. Some island-like structures are seenin the KE range of 267-270 eV in Fig. 1 and Fig. 2,
which seem to be specific to the decay processes ofthe double excitations. The complicated photon
energy dependences of the structures may indicatethat the de-excitation processes of the doubly excitedstates are not so simple.
[1] T. Kaneyasu, Y. Hikosaka, P. Lablanquie, F.
Penent, L. Andric, G. Gamblin, J.H.D. Eland, Y.
Tamenori, T. Matsushita and E. Shigemasa, Phys. Rev.
Lett. 101 (2008) 183003.
[2] E. Shigemasa, T. Kaneyasu, T. Matsushita, Y.
Tamenori and Y. Hikosaka, New J. Phys. 12 (2010)
063030.

Fig. 1. 2D map of de-excitation spectra following the
double excitations around the carbon K-edge of CO
measured in horizontal direction.

Fig. 2. 2D map of de-excitation spectra following the
double excitations around the carbon K-edge of CO
measured in vertical direction.

46

BL7B

Reflection Measurement on [SiC/W/Co]2 and SiC/W/Ir/Ti
Sub-Quaterwave Multilayers for Use in 50110 nm Region

T. Ejima1, H. Iwai2, K. Fukui2, M. Yanagihara1, M. Watanabe3,
J. Zhu4, H. Li4, Q. Huang4 and Z. Wang41
IMRAM, Tohoku University, Sendai 980-8577, Japan
2
Faculty of Engineering, University of Fukui, Fukui 910-8507, Japan
3
Tohoku University (Prof. Emeritus), Sendai 980-8577, Japan
4
Institute of Precision Optical Engineering, Tongji University, Shanghai 200092, China
Traditional multilayers consisting of high- andlow-Z materials alternatively having high normal
incidence reflectance with broad bands in 50110 nm
region have been developed [1]. In this region,
however, because of the strong absorption of allmaterial, the reflectance is rather low. In order toimprove this situation, a theory of sub-quarterwave
multilayer has been developed [2], basing on themulti-material combination. The layer thicknesses of
sub-quarterwave multilayer are less than a quarter ofthe wave-length. Comparing with traditional
multilayer, sub-quarterwave multilayer provides moreinterfaces per constant optical path. This provides ahigh reflectance with a broad band by the interfaces
close to the outer surface, thus reducing absorption.
Preliminary measurement of reflectance on such
multilayers of [Si/W/Co]2 and [Si/W/C]2 had been
performed [3]. The subscript “2” means 2 trilayers ofdifferent layer thicknesses. In the present study, themeasurements on the multilayers, [SiC/W/Co]2,[SiC/W/Ir]2, SiC/W/Ir/Co and SiC/W/Ir/Ti have been
performed. The multilayers were designed to havehigh reflectance of 4050% in this region at incident
angle of 20° for p-polarization. In the multilayers, thetop layers were SiC layers and the substrates were Si
wafers. Here we report the reflectance measurementon the [SiC/W/Co]2 and SiC/W/Ir/Ti multilayers. In
the former, the designed value of layer thickness wasrespectively 10.59, 11.06, 12.67, 17.51, 10.02 and8.69 nm from the top SiC layer, and in the latter,
10.42, 5.97, 9.49 and 9.18 nm. They were depositedby using magnetron sputtering method in Tongji
University. Near normal incidence reflectance wasmeasured at BL7B, which is equipped with a 3 m
McPherson type monochromator. The p-reflectance
was measured at incident angles of 20°.
The measured reflectances are shown in Figs.1 and
2 with the calculated results. It can be seen that all themeasured reflectances are lower than those of thecalculated ones. The possible reasons are as follows.
(1) In design, the optical constants used were thosefrom the optics handbook. There is a possibility thatthe optical constants used in the calculation are
different from the actual ones. (2) During and after
deposition, the materials reacted with atmosphere andoxidized. Some improvement should be required innext research, such as higher base vacuum conditionand capping layer to protect the multilayers fromoxidization when they are exposed to air.

Fig. 1. Calculated and measured p-reflectances of[SiC/W/Co]2 multilayer at incident angle of 20°.

Fig. 2. Calculated and measured p-reflectances of
SiC/W/Ir/Ti multilayer at incident angle of 20°.
[1] Y. Kondo et al., Nucl. Instr. Meth. A 467-468(2001) 333.
[2] J. I. Larruquert, J. Opt. Soc. Am. A 18 (2001)
2617.
[3] T. Ejima et al., UVSOR Activity Report 36 (2009)
130.

47

BL7B

Diffraction Efficiency Measurement on Sub-Quarterwave-Multilayer-Coated Grating for Seya-Namioka Mount in 50110 nm Region

Y. Liu1, J. Zhu2, S. He1, H. Li2, Q. Huang2, Z. Wang2, S. Fu1,
T. Ejima3, H. Iwai4, K. Fukui4, M. Yanagihara3 and M. Watanabe51National Synchrotron Radiation Laboratory, University of Science and Technology of China,
Anhui 230029, China
2
Institute of Precision Optical Engineering, Tongji University, Shanghai 200092, China
3
IMRAM, Tohoku University, Sendai 980-8577, Japan
4
Faculty of Engineering, University of Fukui, Fukui 910-8507, Japan
5
Tohoku University (Prof. Emeritus), Sendai 980-8577, JapanThe multilayer-coated gratings can provide highdiffraction efficiency. They have been widely used in
X-ray region and vacuum-UV (VUV) region below
50 nm. However, multilayer coated gratings havebeen precluded in the VUV region of 50–110 nm
because of the strong absorption of materials in thisregion. As reported in this issue sub-quarterwave
multilayers having high normal incidence reflectancewith broad bands in 50110 nm region have beendeveloped. In this study, one of these multilayers,
SiC/W/Ir/Ti designed to enhance diffraction
efficiency in the 50–110 nm region for Seya-Namioka
mount at the 1st order is coated on a grating. Its
efficiency was compared with those of the gratingscoated with SiC and Ir monolayers. In Seya-Namioka
mount, the deviation angle between incident anddiffracted rays is constant and about 70°.
The gratings of laminar type were designed to havehigh efficiency in the 50110 nm region using themodified integral method [1]. The gratings were
fabricated holographically by ion-beam etching. Theirline density was 1200/mm. The SiC/W/Ir/Ti
multilayer, and SiC and Ir monolayers were depositedon the gratings using magnetron sputtering at Tongji
University. Figure 1 shows the surface profile of themultilayer-coated grating. It can be seen that thegrooves are still sharp after multilayer deposition.
The diffraction efficiency for p-polarization wasmeasured at BL7B equipped with a 3 m McPherson
type monochromator in the 45120 nm. The
measurement was made keeping the deviation angleof Seya-Namioka mount.
The measured diffraction efficiency spectra of thegratings coated with SiC/W/Ir/Ti multilayer, and SiC
and Ir monolayers are shown in Fig. 2 with calculated
ones. The relative trend of the measured spectrum ofeach grating agrees with that of calculated one.
However, the measured efficiencies are lower thanthe calculated ones except the result at 56 nm for thegrating coated with Ir. The plausible reasons of thisfact are the deviation of the actual groove depth fromthe designed one, which causes a wavelength shift ofefficiency peak and the deviation of performance ofthe coating materials. In spite of this, the diffraction

efficiency of the grating coated with SiC/W/Ir/Ti
multilayer is more than 10% in the 55-100 nm region.
This value seems to be satisfactory for practical use in
VUV range. Further analysis and experiments arenecessary to find the factors that affect the efficiency
and to obtain higher efficiency.

Fig. 1. AFM-measured profile of the grating after
SiC/W/Ir/Ti multilayer deposition.

Fig. 2. Measured and calculated ҟ1st order efficiency
spectra of the gratings coated with three kinds ofmaterials (G1#: SiC/W/Ir/Ti, red squares; G8#: SiC,
green circles; G15#: Ir, blue upper triangles).
[1] L. I. Goray and J. F. Seely, Appl. Opt. 41 (2002)
1434.

48

BL7B

Development and Verification of CLASP Polarimeter

H. Watanabe1, N. Narukage2, T. Bando2, M. Kubo2, R. Kano2, T. Kobiki2, R. Ishikawa2,
K. Ueda2, D. Song3 and S. Tsuneta2
1
Kwasan and Hida observatories, Kyoto University, Yamashina-ku, Kyoto 607-8417 Japan
2
National Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo 181-8588, Japan3
Department of Physics and Astronomy, Seoul National University, Seoul 151-742, Korea
We
developedapolarimeterforthe“Chromospheric Lyman-Alpha Spectro-Polarimeter(CLASP)” rocket experiment, which is aninternational project in collaboration with solar
researchers in U.S., Spain, Norway, and Japan.
CLASP will detect the weak linear polarization signal
produced by the Hanle effect [1] and directly measure
the chromospheric magnetic field using Lyman-alphaemission line (121.567 nm) for the first time. Since
no practical polarimeter for Lyman-alpha has beendeveloped, we designed and developed thepolarimeter, and verified its system performance withan engineering model.
To measure the linear polarization with a highaccuracy of 0.1%, we will take the spectra of two
orthogonal polarization states simultaneously with arotating half waveplate and two MgF2 plates at
Brewster's angle. The first MgF2 plate ("beamsplitter" in Fig.1) is a polarization beam splitter thatreflects pure s-polarized light to Channel 1. The
transmitted light is purified to be p-polarized by thesecond MgF2 plate ("polarization analyzer" in Fig.1)
and reflected to Channel 2. The rotating waveplateallows measurement of any direction of linear
polarization with the fixed beam splitter.
Based on the measurements of reflectivity,transmissivity, and retardation for MgF2 plate
performed in 2009 at UVSOR, components of thepolarimeter are designed as follows: The half
waveplate is composed by stacking two MgF2 plateswith slightly different thicknesses (14.7 Pm) in such aconfiguration that the slow axes are perpendicular toeach other. The beam splitter is a 2-mm thickness
MgF2 plate with its slow axis parallel to thes-polarized light. The reflectivity for s-polarization is
22% and the transmissivity for p-polarization is 78%.
The polarization analyzer at Channel 2 is a 15-mm
thickness MgF2 plate with its slow axis parallel to thep-polarized light, i.e., perpendicular to that of thebeam splitter.
Using these components described above, theengineering model of the CLASP polarimeter wasdeveloped as shown in Fig. 1. By using a pair of
autocollimated theodolites, we confirmed that boththe beam splitter and the polarization analyzer were
mounted at the Brewster’s angle (59 degree) to thebeam within 3 arcmin accuracy.
The engineering model of the CLASP polarimeter
was tested with the synchrotron radiation at the
UVSOR BL7B beamline. We input a highly linearlypolarized light to the polarimeter and observed theoutput signal at both channels with a CCD camera.
The expected results of the designed polarimeter are
the followings: (1) the output signals show a cos4ș
modulation with respect to the rotation angle of the
waveplate, and (2) the phases of modulations at
Channel 1 and Channel 2 are shifted by a half cycle
each other, i.e., the maximum at Channel 1
corresponds to the minimum at Channel 2. We indeed
observed such modulation patterns as shown in Fig. 2,
and verified that the polarimeter for CLASP has beendeveloped.
We note that the wavelength of BL7B beam wascalibrated with the absorption lines of molecularoxygen (O2) around Lyman-alpha (121.567 nm), at apressure of 0.3 Torr and with a beam line slit width of
50 Pm. This method can determine the wavelength
within 0.1 nm accuracy.

Fig. 1. Photo of the polarimeter engineering modelFig. 2. Observed modulation patterns in Channel 1(black) and Channel 2 (orange).
[1] J. Trujillo Bueno and A. Asensio Ramos,
Astrophysical Journal 655 (2007) 642.

49