0. Polarimetric Calibration for Nasmyth instrument HiCIAO
Jun Hashimoto
(National Astronomical Observatory of Japan)
and SEEDS, HiCIAO, AO team

1. 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: %
20-30 % polarization in disks
GJ758
HiCIAO results
（Thalmann et al. 2009）
PDS 70
k And
SAO206462
~10-30MJ
100AU
52AU
Hashimoto+12
30AU
Carson+12
Muto+12

2. 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

3. 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

4. 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.

5. Instrumental polarization from Tertiary mirror
Bare silver mirror
1.6um
Tertiary mirror rotates with the telescope elevation
Q/I (unpolarized light)
~0.3% instrumental polarization
is generated by Tertiary Mirror

6. 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
Phase Difference

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

8. Deriving incident stokes QU by linear transformation.
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
observed values
linear combination
of IQUV
Deriving incident stokes QU by linear transformation.

9. refractive indecs of Tertiary Mirror and Image Rotator Constant Values
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

10. 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

11. Performance Polarimetric standard star
Elias 16
4.22% %, 71.1º º (HiCIAO)
4.09% %, 71.0º º (Hough+88)
HDE
2.67% %, 63.0º º (HiCIAO)
2.59% %, 58.0º º (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.

12. 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

13. 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 º ,FWHM: 4.3º º
Instrumental Polarization is corrected.
The PI image of AB Aur is less affected by speckle noise.
(number)
(degree)