Pushing the limits of
Astronomical Polarimetry
Frans Snik
Sterrekundig Instituut Utrecht
BBL 710
f.snik@uu.nl
Astronomical Polarimetry
Outline
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•
•
•
Why polarization?
What is polarization?
Measurement principles.
Instrumental limitations.
Why polarization?
Astronomy: study of starlight
Three measurable quantities:
• Intensity
Why polarization?
Astronomy: study of starlight
Three measurable quantities:
• Intensity
• Wavelength:
λ
Why polarization?
Astronomy: study of starlight
Three measurable quantities:
• Intensity
• Wavelength:
λ
α
• Polarization:
Why polarization?
Astronomy: study of starlight
Three measurable quantities:
• Intensity
… as a function of
[x,y] and/or t
• Wavelength:
λ
α
• Polarization:
Why polarization?
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?
Example - Military
Why polarization?
Example - Military
Why polarization?
Example - Astronomy
Scattering polarization:
Why polarization?
Example - Astronomy
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
Why polarization?
Polarimetric projects at SIU
EST
Why polarization?
Polarimetric projects at SIU
E-ELT
Why polarization?
Examples: degree of polarization
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•
•
•
•
•
•
LCD screen
45o reflection off glass
clear blue sky
45o reflection off mirror
solar/stellar magnetic fields
exoplanet in stellar halo
cosmic microwave background
100%
~90%
~75%
~5%
~1%
~10-5-10-6
~10-6-10-7
Why polarization?
Why NOT polarization?
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•
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Technically challenging.
Conflicting with imaging optics (like AO).
Adds a lot of instrument complexity.
Data difficult to interpret.
What is 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:
α
• Partially linearly polarized light:
– Combination of unpolarized & 100% polarized
What is polarization?
Electromagnetic wave
What is polarization?
Electromagnetic wave
• Circularly polarized light:
– ¼ λ phase shift between orthogonal linear
polarization directions
• General case: elliptical
What is polarization?
Electromagnetic wave
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
What is polarization?
Stokes vector
I 
 Q 
S 
U 
 
V 
Q=
U=
V=
-
I=
=
=
+
+
+
: ½(I+Q)
: ½(I-Q)
: ½(I+U)
: ½(I-U)
: ½(I+V)
: ½(I-V)
Q/I, U/I, V/I = normalized/fractional polarization
√(Q2+U2+V2)/I = polarization degree
Measurement principles
The basics
• Polarimetry in the optical regime is the
measurement of (part of) the Stokes vector.
• Essentially differential photometry.
• Susceptible to all kinds of differential
effects!
Measurement principles
Multidimensional data
• General case: S(x, y, l)
• But detectors are only two-dimensional…
Measurement principles
Multidimensional data
• General case: S(x, y, l)
• Combining l:
 Imaging polarimetry
Separate images of the Stokes vector elements
Measurement principles
Multidimensional data
• General case: S(x, y, l)
• Combining x, y:  Spectropolarimetry
Separate spectra of the Stokes vector elements
Measurement principles
General polarimeter set-up
1.
2.
3.
4.
5.
6.
…
modulator = retarder
…
analyzer = (fixed) polarizer
…
detector (demodulator)
Measurement principles
Polarizers
• wire grid
Measurement principles
Polarizers
• wire grid
Measurement principles
Polarizers
• stretched polymer (dichroism)
Measurement principles
Polarizers
• cube beam-splitter
Measurement principles
Polarizers
• birefringent crystal
no & ne
Savart plate
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

2dn o  n e 
l
Chromatic and temperature sensitive

for birefringent crystal plates.
Measurement principles
Retarders – Liquid crystals
Liquid Crystal Variable Retarders (LCVRs)
fast
fast
slow
~20 ms
slow
fast
slow
V=0
 =  max
Ferroelectric Liquid Crystals (FLCs)
fast
slow
V
 <  max
V<0
V>0
~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


Sout  M n  M n1  ...  M 2  M1  Sin
M pol
M ret
1

1 1
 
2 0

0

1

0

0

0

1 0 0

1 0 0
0 0 0

0 0 0 
0
0
1
0
0 cos 
0 sin 
0 

0 
 sin  

cos  
M rot
0
1

 0 cos 2

0  sin 2

0
0

0
sin 2
cos 2
0
0

0
0

1 
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
• 2 LCVRs + polarizer
I+Q
0l
0l
Measurement principles
Temporal modulation
I-Q
0l
1/2 l
Measurement principles
Temporal modulation
I+V
0l
1/4 l
Measurement principles
Temporal modulation
I-V
0l
3/4 l
Measurement principles
Temporal modulation
I+U
1/4 l
1/4 l
Measurement principles
Temporal modulation
I-U
1/4 l
3/4 l
• Also complicated 4-fold modulation scheme.
Measurement principles
Temporal modulation
• Temporal modulation
faster than seeing
(~ 1 kHz)
 special demodulating
camera
ZIMPOL
10-5 polarimetric
sensitivity
Measurement principles
Temporal modulation
S5T
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
return beam is not blocked
56 mm
rotating waveplates
Foster prism
(modified
Glan-Thompson)
CaF2 channeling
prism
(compensates for
focal shift)
cylindrical lens
(compensates for
crystal astigmatism)
fiber 1
fiber 2
HWP
QWP
rotated by
one actuator
on a belt
existing slider
HARPSpol
Measurement principles
Beam-exchange method
HARPSpol
Instrumental limitations
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
• 45 Al mirror (very common in telescopes!)
M mir
0
0 
 1.000 0.028


0
0 
 0.028 1.000

0
0
 0.983  0.180 


 0

0
0
.
180

0
.
983


• Also effect due to growing Al2O3 layer.
Instrumental limitations
Other issues
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photon noise (fundamental:  2  I )
read (electronics) noise
seeing
guiding errors
scattered light 
instrumental polarization
(polarized) fringes & ghosts
differential aberrations
chromatism
temperature dependence
stress birefringence
polarization optics misalignment
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!
Astronomical Polarimetry
Questions?