View
50
Download
0
Category
Tags:
Preview:
DESCRIPTION
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. - PowerPoint PPT Presentation
Citation preview
1/13
Polarimetric Calibration for Nasmyth instrument HiCIAO
Jun Hashimoto (National Astronomical Observatory of Japan)
and SEEDS, HiCIAO, AO team
2/13
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
• 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
3/13
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
4/13
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
5/13
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.
6/13
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 polarizationis generated by Tertiary Mirror
7/13
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
8/13
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)
9/13
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
10/13
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
11/13
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
12/13
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.
13/13
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
14/13
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!
15/13
16/13
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.
85 90 95 (degree)
600 400 (number)
Recommended