COSMO Large Coronagraph Preliminary Design Review

The High Altitude Observatory (HAO) at the National Center for Atmospheric Research (NCAR)The National Center for Atmospheric Research is sponsored by the National Science Foundation.

Any opinions, findings and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

COSMO Large CoronagraphPreliminary Design Review

Birefringent Filter Design and PolarimetrySteven Tomczyk

National Center for Atmospheric Research

Boulder, Colorado – Nov 16, 2015

COSMO PDR (November 16-17, 2015) Birefringent Filter Design and Polarimetry

Birefringent Filter Design

Relevant documents:

COSMOLC-DE-7001 (Tunable Filter Design and Development)

COSMO TN 22 Birefringent Filter Crystal Requirements


Birefringent Filter Requirements

• COSMO LC 1 degree FOV, 1.5m aperture

Sets system étendue at 1.39 m2 deg2

Conservation of étendue - 100mm filter needs 15° full FOV

• Fabry-Perot won’t work over such steep angles

• Spectrograph requires too many slit positions

• Birefringent filter selected

• Spectral resolution set by need to resolve Stokes V signature


Optimal Tunable FilterFollowing Babcock (1953), S/N in magnetic measurement

SN∝

∫λ

V ( λ , w ) F ( λ , Δλ , d ) dλ

{∫λ

[ I ( λ ,w )+B ] F ( λ , Δλ ,d ) dλ}1 /2

w = line width Δλ = filter widthd = displacementB = background

Optimal FilterFeXIII 1074.7nmFWHM = 0.14 nm

d = 0.1 nm


Filter Resolution RequirementRequired resolution (dots) is about 8000 over the COSMO Filtergraph wavelength range


Wide-field Birefringent Filter

Δλλ

=− 14 no

2 ( ne − no

ne)sin2 θ= 1

𝑅 ,

For a wide-field birefringent filter, the wavelength shift is given by (Title and Rosenberg, 1979):

Ω=4 π sin2 θ2

(sr ) ,E=A Ω,

E ∙ R=πAF

, where F= 14no

2 ( ne − no

ne)

For COSMO we need E∙R ≥ 1.1∙104 m2deg2


Crystal Selection

Crystal no ne ne-no F Diameter (mm)

πA/F Length (mm)(m2deg2)

MgF2 1.384 1.396 0.012 1.12E-03 120 1.04E+05 605

LiNbO3 2.286 2.203 -0.086 1.80E-03 100 4.49E+04 84

SiO2 (quartz) 1.543 1.552 0.009 6.09E-04 50 3.33E+04 807

KDP 1.494 1.46 -0.034 2.61E-03 99 3.04E+04 214

TeO2 2.26 2.142 -0.118 2.70E-03 50 7.51E+03 62

BaB2O4 1.658 1.584 -0.073 4.25E-03 50 4.77E+03 99

YVO4 1.993 2.215 0.222 6.31E-03 38 1.85E+03 33

CaCO3 (calcite) 1.656 1.485 -0.171 1.05E-02 40 1.23E+03 42

TiO2 (rutile) 2.583 2.865 0.282 3.69E-03 25 1.37E+03 26

E ∙ R=πAF ≥ 1.1∙ 104 m2 deg2


Birefringent Filter Design• Lithium Niobate (LN)• Thickest elements 22 mm• Super-achromatic waveplates (5 plastic elements)• Tilted wire grid polarizers• Electro-optically tuned


Birefringent Filter Design


Birefringent Filter Components5 element, super-achromatic waveplate

Predicted transmission 27% @ 1075 nmCoMP achieved 29%

Transmission of filter is limited by polarizers - Meadowlark VL1


Birefringent Filter ComponentsNeed crystals with uniformity of birefringence

44 crystals tested, 0.5 to 25 mm thick, 75 – 100 mm diameterCommercially available crystals are uniform enough (just)


Electro-optical TuningWork led by Shibu K. Mathew (USO)


Electro-optical Tuning

Need +/- 9kV at 1074nm, +/- 3kV at 530nm

Single crystal shift, x2 for split element


Electro-optical Tuning Speed

Tests limited by response of power supplyPotential for very fast tuning


PolarimetryNoise scales inversely with polarimeter modulation efficiency (see del Toro Iniesta and Collados, 2000)

Will use efficient 2-element polymer polychromatic modulator

𝜎 𝐼 ,𝑄 ,𝑈 ,𝑉=𝜎

𝜀 𝐼 ,𝑄 ,𝑈 ,𝑉

𝜎=√𝑁


Polarimetry

Requirement: 1 G magnetic field in 900 s using FeXIII 1074 nm

In weak field limit 𝑉=−𝑘𝐵𝐿𝑂𝑆𝜕 𝐼𝜕 𝜆

𝑉𝐼0

(𝑎𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 )=1.3 ⋅10−4 𝐵𝐿𝑂𝑆(G)


Polarimetry

𝑋=[ 𝐼>𝐼 𝑄> 𝐼 𝑈>𝐼 𝑉 >𝐼𝐼>𝑄 𝑄>𝑄 𝑈>𝑄 𝑉 >𝑄𝐼>𝑈 𝑄>𝑈 𝑈>𝑈 𝑉 >𝑈𝐼>𝑉 𝑄>𝑉 𝑈>𝑉 𝑉>𝑉 ]

∆ 𝑋=[− 𝑎 /𝑝𝐿 𝑎 /𝑝𝐿 𝑎 /𝑝𝐶

𝜀 𝑎 𝜀 /𝑝𝐿 𝜀 /𝑝𝐶𝜀 𝜀 /𝑝𝐿 𝑎 𝜀 /𝑝𝐶

𝜀 𝜀 /𝑝𝐿 𝜀 /𝑝𝐿 𝑎 ]Following Ichimoto et al., 2008, we need to calibrate the response matrix elements to an accuracy of

Response matrix relates observed and actual Stokes vector

ε = 10-4 (to meet 1 G magnetic field error),a = 0.05 (following Ichimoto et al.),PL = maximum linear polarization = 0.1,PC = maximum circular polarization = 10-3 (corresponding to 10 G).

∆ 𝑋=[ − 0.50 0.50 50.010−4 0.05 10− 3 0.1010−4 10− 3 0.05 0.1010−4 10− 3 10− 3 0.05]

,


Polarimetry

COSMO LC needs 10-4 precision in V/I, not accuracyV is anti-symmetric wrt line center, need amplitude to determine magnetic field (Lin, Kuhn, Coulter, 2004)

Can calibrate I > Q,U,V using data itself, look in continuum where polarization is absent (e.g. Lites and Ichimoto, 2013)


Calibration

Insert calibration optics with known Mueller matrixPolarizerRetarderPolarizer + RetarderRotate them

,

References:Elmore, D.F., “A polarization calibration technique for the advanced stokes polarimeter”, 1990, NCAR Technical Note NCAR/TN-355+STR, NCAR, Boulder, Colorado.Ichimoto, et al., “Polarization Calibration of the Solar Optical Telescope onboard Hinode”, 2008, Solar Phys. 249, 233.


CalibrationFor COSMO LC, calibration optics cannot calibrate O1

But studies show O1 polarization should be very small

This will be verified

COSMO Large Coronagraph Preliminary Design Review

Documents

Transcript of COSMO Large Coronagraph Preliminary Design Review