pushing the limits of astronomical polarimetry frans snik sterrekundig instituut utrecht bbl 710
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Pushing the limits of Astronomical Polarimetry Frans Snik Sterrekundig Instituut Utrecht BBL 710 [email protected]. Astronomical Polarimetry. Outline. Why polarization? What is polarization? Measurement principles. Instrumental limitations. Why polarization?. Astronomy: study of starlight. - PowerPoint PPT PresentationTRANSCRIPT
Pushing the limits of
Astronomical Polarimetry
Frans Snik
Sterrekundig Instituut Utrecht
BBL 710
Outline
• Why polarization?
• What is polarization?
• Measurement principles.
• Instrumental limitations.
Astronomical Polarimetry
Astronomy: study of starlight
Why polarization?
λ
Three measurable quantities:
• Intensity
• Wavelength:
Astronomy: study of starlight
Why polarization?
αλ
Three measurable quantities:
• Intensity
• Wavelength:
• Polarization:
Astronomy: study of starlight
Why polarization?
αλ
Three measurable quantities:
• Intensity
• Wavelength:
• Polarization:
… as a function of [x,y] and/or t
Polarization creation
• Polarization is created (and/or modified) wherever perfect spherical symmetry is broken:– Reflection/scattering– Magnetic/electric fields– Anisotropic materials
➔ Polarimetry provides information on the
symmetry-breaking process/event.
Why polarization?
Why polarization?
Polarimetric projects at SIU
• Circumstellar disks and exoplanets– WHT/ExPo, VLT/SPHERE, E-ELT/EPICS, SPICES
• Solar magnetic fields– S5T, SOLIS-VSM, Hinode SOT, EST
• Stellar magnetic fields– HARPSpol, VLT/X-shooter-pol
• Atmospheric aerosols– SPEX
• Detection of life– TreePol
Examples: degree of polarization
• LCD screen 100%
• 45o reflection off glass ~90%
• clear blue sky ~75%
• 45o reflection off mirror ~5%
• solar/stellar magnetic fields ~1%
• exoplanet in stellar halo ~10-5-10-6
• cosmic microwave background ~10-6-10-7
Why polarization?
Why NOT polarization?
• Technically challenging.
• Conflicting with imaging optics (like AO).
• Adds a lot of instrument complexity.
• Data difficult to interpret.
Why polarization?
Electromagnetic wave
• Polarization of an EM wave is a natural consequence of Maxwell’s equations
• “General” light:– Not monochromatic– Superposition of polarization of many photons
• Unpolarized light:– No preferred orientation of polarization
What is polarization?
Electromagnetic wave
• 100% linearly polarized light:
• Partially linearly polarized light:– Combination of unpolarized & 100% polarized
What is polarization?
α
Electromagnetic wave
• Circularly polarized light:– ¼ λ phase shift between orthogonal linear
polarization directions
• General case: elliptical
What is polarization?
Jones & Stokes formalisms
• Jones formalism– amplitude and phase of EM waves (radio regime)– 100% polarized– coherent sum (interference)
• Stokes formalism– differential photon fluxes (optical regime)– partial polarization– incoherent sum (no interference)
What is polarization?
Stokes vector
Q/I, U/I, V/I = normalized/fractional polarization
√(Q2+U2+V2)/I = polarization degree
V
U
Q
I
S
Q= U= V= -
--
I= = =
+++
: ½(I+Q): ½(I-Q): ½(I+U): ½(I-U): ½(I+V): ½(I-V)
What is polarization?
Measurement principles
• Polarimetry in the optical regime is the measurement of (part of) the Stokes vector.
• Essentially differential photometry.
• Susceptible to all kinds of differential effects!
The basics
Measurement principles
• General case: S(x, y, )
• But detectors are only two-dimensional…
Multidimensional data
Measurement principles
• General case: S(x, y, )
• Combining Imaging polarimetry
Multidimensional data
Separate images of the Stokes vector elements
Measurement principles
• General case: S(x, y, )
• Combining x, y: Spectropolarimetry
Multidimensional data
Separate spectra of the Stokes vector elements
General polarimeter set-up
1. …
2. modulator = retarder
3. …
4. analyzer = (fixed) polarizer
5. …
6. detector (demodulator)
Measurement principles
Retarders
– introduction of phase difference
half wave plate quarter wave plate
Measurement principles
Retarders
– introduction of phase difference
half wave plate quarter wave plate
Measurement principles
Retarders
• Crystal wave plates
Chromatic and temperature sensitive for birefringent crystal plates.€
δ =2πd no − ne( )
λ
Measurement principles
Retarders – Liquid crystals
Liquid Crystal Variable Retarders (LCVRs)
fast
slow
fast
slow
V
<δ δmax
V=0δ δ=
max
~20 ms
fast
slow
slow
fast
V<0 V>0
Ferroelectric Liquid Crystals (FLCs)
~100 s
Measurement principles
Retarders – Fresnel rhomb
• Phase difference through total internal reflections
Measurement principles
Retarders – PEMs
• Piezo-Elastic Modulators– Birefringence induced in normal glass by
stress.– Resonance frequency: fast variation of
retardance (~10 kHz).
Measurement principles
Mueller matrices
innnout SMMMMS
121 ...
1000
02cos2sin0
02sin2cos0
0001
rotM
0000
0000
0011
0011
2
1polM
δδδδ
cossin00
sincos00
0010
0001
retM
Measurement principles
Modulation
1.Spatial
• Measuring different polarization states at different locations
2.Temporal
• Measuring different polarization states at different times
3.Spectral
Measurement principles
Spatial modulation
+ Strictly simultaneous measurements.
- Different (parts of) detectors.
- Differential alignment / aberrations.
- Limited detector gain calibration.
- 2 to 6 beams.
Measurement principles
Temporal modulation
+ All measurements with same detector.
- Image motion / seeing / variability issues.
- Requires active component.
- Fast modulation and demodulation desirable but often not possible.
Measurement principles
Temporal modulation
• Rotating waveplate + polarizer analyzer + demodulating detector.
Intensity measurements are linear combinations of I with Q, U and V
Measurement principles
Temporal modulation
• Temporal modulation faster than seeing (~ 1 kHz)
special demodulating camera
ZIMPOL10-5 polarimetric
sensitivity
Measurement principles
Beam-exchange method
Best of both worlds: combining spatial and (fast) temporal modulation
Measurement principles
Beam-exchange method
Best of both worlds: combining spatial and (fast) temporal modulation
• All differential effects drop out to first order.
• Achievable sensitivity: ~10-6
– Hough et al. (2006)– Semel et al. (1993)
Measurement principles
Beam-exchange method
Measurement principles
Foster prism(modified Glan-Thompson)
HWP QWProtating waveplates
cylindrical lens(compensates for crystal astigmatism)
CaF2 channeling prism(compensates for focal shift) existing slider
fiber 1 fiber 2
return beam is not blocked
rotated by one actuator on a belt
56 m
m
HARPSpol
Instrumental polarization
• Every reflection polarizes...
• Every piece of glass is birefringent...
... to some degree.
So one has to be very careful that the measured polarization is not due to the instrument itself!
Instrumental limitations
Polarization cross-talk
983.0180.000
180.0983.000
00000.1028.0
00028.0000.1
mirM
• 45 Al mirror (very common in telescopes!)
• Also effect due to growing Al2O3 layer.
Instrumental limitations
Other issues
• photon noise (fundamental: )• read (electronics) noise• seeing• guiding errors• scattered light• instrumental polarization• (polarized) fringes & ghosts• differential aberrations• chromatism• temperature dependence• stress birefringence• polarization optics misalignment
€
σ 2 ∝ I
Instrumental limitations
Mitigation strategies
• Deep understanding of the measurement issues: different observational goals require different polarimeter designs.
• Polarimetric modulation as far upstream as possible.
• Careful instrument design.– rotationally symmetric– 90 compensations
• Calibration!
Instrumental limitations