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Polarimetric Calibration for Nasmyth instrument HiCIAO

Polarimetric Calibration for Nasmyth instrument HiCIAO. Jun Hashimoto (National Astronomical Observatory of Japan) and SEEDS, HiCIAO, AO team. What is HiCIAO ?. Near-infrared high contrast imaging instrument with a linear polarimetor at the 8.2m Subaru telescope.

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Polarimetric Calibration for Nasmyth instrument HiCIAO

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  1. Polarimetric Calibration for Nasmyth instrument HiCIAO Jun Hashimoto (National Astronomical Observatory of Japan) and SEEDS, HiCIAO, AOteam

  2. 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 HiCIAO results PDS 70 （Thalmann et al. 2009） k And SAO206462 ~10-30MJ 100AU Hashimoto+12 52AU Muto+12 30AU Carson+12

  3. Nasmyth polarimetor HiCIAO • HiCIAO is mounted on Nasmyth platform. • Instrumental polarization comes from the telescope and instruments. • Correcting with Correction Matrix, not standard star. SUBARU unpolarized light HiCIAO polarized light 45º Nasmyth focus Tertiary mirror

  4. 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. Reflectance case of a bare silver mirror refractive index Mirror

  5. 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.

  6. 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 polarization is generated by Tertiary Mirror

  7. Instrumental polarization from Image Rotator Conceptual figure of 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 multi-reflection Phase Difference

  8. Cross talk (Q , U => V) in Image Rotator Observed linear polarization with a rotation angle • Most incident linear polarization is changed to circular polarization at 45 degrees Angle of Image rotator (degree)

  9. Correction • Construction of the matrix between observed values and true Stokes parameters • no need of observations for polarized standard star Incident Jones matrix Observed Jones matrix Incident Stokes parameters HiCIAO matrix linear combination of IQUV observed values Deriving incident stokes QU by linear transformation.

  10. Matrix between observed values and Stokes parameter observable incident Stokes parameters • p, q, u, v are functions of • refractive indecs of Tertiary • Mirror and Image Rotator • Constant Values • A, B, C, D are functions of • parallactic angle and • Telescope Elevation, Image • Rotator angle, and • wave-plate angle. • Variable values

  11. Stabilize instrumental polarization • Optimizing the tracking of half-waveplate to stabilize time-variable instrumental polarization • Two tracking mode • Sidereal tracking • Pupile tracking (ADI mode) star Sidereal time dependency of instrumental polarization ADI telescope spider Tracking half-waveplate Tracking half-waveplate Q/I Q/I No tracking half-waveplate No tracking half-waveplate

  12. Performance • Instrumental polarization is • corrected by our model. • 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 issigmaP < ~0.2%, sigmaθ < ~5º.

  13. 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

  14. 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!

  15. 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. 600 400 (number) 85 90 95 (degree)

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