Polarimetric Calibration for
Nasmyth instrument HiCIAO
Jun Hashimoto
(National Astronomical Observatory of Japan)
and SEEDS, HiCIAO, AO team
1/13
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
GJ758
（Thalmann et al. 2009）
HiCIAO results
k And
SAO206462
~10-30MJ
30AU
52AU Carson+12
PDS 70
Muto+12
100AU
Hashimoto+12
• We use linear polarization for the detection of disks
• Required polarimetric accuracy: 0.1-0.3 %
– 20-30 % polarization in disks
2/13
Nasmyth polarimetor HiCIAO
• HiCIAO is mounted on Nasmyth platform.
SUBARU
unpolarized light
HiCIAO
polarized light
45º
Nasmyth focus
Tertiary mirror
• Instrumental polarization comes from the telescope and
instruments.
• Correcting with Correction Matrix, not standard star.
What is instrumental polarization ?
3/13
• The amplitude and phase of incident light are
changed at mirrors (Fresnel reflection).
Reflectance
– Rs and Rp: the reflection coefficient of perpendicular
and parallel wave to the plane of incidence.
refractive index
Mirror
case of a bare silver mirror
4/13
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.
5/13
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
6/13
Instrumental polarization from Image Rotator
• Three silver mirrors
to derotate images
• Over coating (SiO2)
Conceptual figure of Image Rotator
– multi-reflection
Phase Difference
• Wavelength dependency
• Linear polarization is
changed to circular
polarization at 1.6 um
Conceptual figure of multi-reflection
7/13
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
8/13
Correction
• Construction of the matrix between observed
values and true Stokes parameters
– no need of observations for polarized standard star
Observed Jones matrix
HiCIAO matrix
observed values
Incident Jones matrix
Incident Stokes
parameters
linear combination
of IQUV
Deriving incident stokes QU by linear transformation.
9/13
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
10/13
Stabilize instrumental polarization
• Optimizing the tracking of half-waveplate to stabilize
time-variable instrumental polarization
star
• Two tracking mode
Sidereal
– Sidereal tracking
– Pupile tracking (ADI mode)
time dependency of
instrumental polarization
Q/I
ADI
telescope spider
Tracking
half-waveplate
Tracking
half-waveplate
Q/I
No tracking
half-waveplate
No tracking
half-waveplate
11/13
Performance
• Polarimetric standard star
– Elias 16
• Instrumental polarization is
corrected by our model.
• 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.
12/13
Known problem
• Large wavelength dependency in Image Rotator
MKO H band filter
• Blue or red sources may have large uncertainty.
• Tracking Image Rotator around ‘0’ degree.
13/13
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!
14/13
15/13
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)