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Polarimetric Calibration for Nasmyth instrument HiCIAO

Jun Hashimoto (National Astronomical Observatory of Japan)

and SEEDS, HiCIAO, AO team

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What is HiCIAO ?• Near-infrared high contrast imaging instrument with a

linear polarimetor at the 8.2m Subaru telescope.• Targets: exoplanets and proto-planetary disks

• We use linear polarization for the detection of disks• Required polarimetric accuracy: 0.1-0.3 %

– 20-30 % polarization in disks

GJ758（ Thalmann et al. 2009 ）

~10-30MJ

30AU100AUMuto+12

SAO206462

PDS 70

Hashimoto+12

k And

Carson+1252AU

HiCIAO results

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Nasmyth polarimetor HiCIAO• HiCIAO is mounted on Nasmyth platform.

• Instrumental polarization comes from the telescope and instruments.

• Correcting with Correction Matrix, not standard star.

HiCIAO

Nasmyth focus

SUBARU

Tertiary mirror

45º

unpolarized light

polarized light

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What is instrumental polarization ?• The amplitude and phase of incident light are

changed at mirrors (Fresnel reflection).– Rs and Rp: the reflection coefficient of perpendicular

and parallel wave to the plane of incidence.

Ref

lect

ance

case of a bare silver mirror

Mirror

refractive index

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Conceptual figure of the optical path

• Instrumental polarization in HiCIAO mainly comes from Tertiary Mirror and Image Rotator.– Rotating during tracking objects, thus, time variable.

• Other optics generate static instrumental polarization.– Corrected by flat-fielding uging non-polarized light.

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Instrumental polarization from Tertiary mirror• Bare silver mirror

– n=0.3+11.7i @ 1.6um

• Tertiary mirror rotates with the telescope elevation

Q/I (unpolarized light)

~0.3% instrumental polarizationis generated by Tertiary Mirror

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Instrumental polarization from Image Rotator• Three silver mirrors

to derotate images

• Over coating (SiO2)– multi-reflection

• Wavelength dependency• Linear polarization is

changed to circular polarization at 1.6 um

Conceptual figure of Image Rotator

Conceptual figure of multi-reflection

Pha

se D

iffe

renc

e

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Cross talk (Q , U => V) in Image Rotator

• Most incident linear polarization is changed to circular polarization at 45 degrees

Observed linear polarization with a rotation angle

Angle of Image rotator (degree)

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Correction• Construction of the matrix between observed

values and true Stokes parameters– no need of observations for polarized standard star

Incident Jones matrixObserved Jones matrix

HiCIAO matrix

observed values

Incident Stokes parameters

Deriving incident stokes QU by linear transformation.

linear combinationof IQUV

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Matrix between observed values and Stokes parameter

observable incidentStokes parameters

• p, q, u, v are functions of refractive indecs of Tertiary Mirror and Image Rotator Constant Values

• A, B, C, D are functions ofparallactic angle and Telescope Elevation, Image Rotator angle, and wave-plate angle. Variable values

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Stabilize instrumental polarization • Optimizing the tracking of half-waveplate to stabilize

time-variable instrumental polarization• Two tracking mode

– Sidereal tracking– Pupile tracking (ADI mode)

Q/I Q/I

Sidereal

ADI

star

telescope spider

time dependency of instrumental polarization

No tracking half-waveplate

Tracking half-waveplate

No tracking half-waveplate

Tracking half-waveplate

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Performance• Polarimetric standard star

– Elias 16 • 4.22% +- 0.07%, 71.1º +- 0.5º (HiCIAO)• 4.09% +- 0.02%, 71.0º +- 1.0º (Hough+88)

– HDE 283809 • 2.67% +- 0.13%, 63.0º +- 3.1º (HiCIAO)• 2.59% +- 0.07%, 58.0º +- 1.0º (Whittet+92)

• Photometric standard star (not nearby star, may have interstellar polarization)– SAO93770

• <P>=0.70%, <sigmaP>=0.20%– GSC01804

• <P>=0.52%, <sigmaP>=0.13%

• Accuracy of polarization in HiCIAO PDI is 　　　　　　　　 sigmaP < ~0.2%, sigmaθ < ~5º.

• Instrumental polarization is corrected by our model.

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Known problem• Large wavelength dependency in Image Rotator

• Blue or red sources may have large uncertainty.

• Tracking Image Rotator around ‘0’ degree.

MKO H band filter

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Conclusion

• HiCIAO is the Nasmyth polarimetor at the Subaru telescope.

• Instrumental polarization including cross talk is corrected by Correction Matrix.

• For better correction, half-waveplate is tracked to stabilize the variable instrumental polarization

• Accuracy: a few 0.1% Thank you!

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Performance• Polarization vector pattern of AB Aur.

– Detecting polarization at the vicinity of the mask edge (r >0.15" : 22 AU)

– Center: 90.1º +- 0.2º ,FWHM: 4.3º +- 0.4º

• Instrumental Polarization is corrected.• The PI image of AB Aur is less affected by speckle

noise.

85 90 95 (degree)

600 400 (number)