Pushing the limits of Astronomical Polarimetry Frans Snik Sterrekundig Instituut Utrecht BBL 710

Pushing the limits of

Astronomical Polarimetry

Frans Snik

Sterrekundig Instituut Utrecht

BBL 710

[email protected]

Outline

• Why polarization?

• What is polarization?

• Measurement principles.

• Instrumental limitations.


Astronomy: study of starlight

Why polarization?

Three measurable quantities:

• Intensity


Why polarization?

λ


• Intensity

• Wavelength:


Why polarization?

αλ


• Intensity

• Wavelength:

• Polarization:


Why polarization?

αλ


• 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?

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?


EST

Why polarization?


E-ELT

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?


• 100% linearly polarized light:

• Partially linearly polarized light:– Combination of unpolarized & 100% polarized


α


• Circularly polarized light:– ¼ λ phase shift between orthogonal linear

polarization directions

• General case: elliptical


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)


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)


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


• General case: S(x, y, )

• But detectors are only two-dimensional…

Multidimensional data



• Combining Imaging polarimetry


Separate images of the Stokes vector elements



• Combining x, y: Spectropolarimetry


Separate spectra of the Stokes vector elements

General polarimeter set-up

1. …

2. modulator = retarder

3. …

4. analyzer = (fixed) polarizer

5. …

6. detector (demodulator)


Polarizers

• wire grid


Polarizers

• stretched polymer (dichroism)


Polarizers

• cube beam-splitter


Polarizers

• birefringent crystal

no & ne

Savart plate


Retarders

– introduction of phase difference

half wave plate quarter wave plate


Retarders

• Crystal wave plates

Chromatic and temperature sensitive for birefringent crystal plates.€

δ =2πd no − ne( )

λ


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


Retarders – Fresnel rhomb

• Phase difference through total internal reflections


Retarders – PEMs

• Piezo-Elastic Modulators– Birefringence induced in normal glass by

stress.– Resonance frequency: fast variation of

retardance (~10 kHz).


Mueller matrices

innnout SMMMMS

121 ...

1000

02cos2sin0

02sin2cos0

0001

rotM

0000

0000

0011

0011

2

1polM

δδδδ

cossin00

sincos00

0010

0001

retM


Modulation

1.Spatial

• Measuring different polarization states at different locations

2.Temporal

• Measuring different polarization states at different times

3.Spectral


Spatial modulation

+ Strictly simultaneous measurements.

- Different (parts of) detectors.

- Differential alignment / aberrations.

- Limited detector gain calibration.

- 2 to 6 beams.


Temporal modulation

+ All measurements with same detector.

- Image motion / seeing / variability issues.

- Requires active component.

- Fast modulation and demodulation desirable but often not possible.


Temporal modulation

• Rotating waveplate + polarizer analyzer + demodulating detector.

Intensity measurements are linear combinations of I with Q, U and V


Temporal modulation

I+Q

0 0

• 2 LCVRs + polarizer


0 1/2

I-Q


Temporal modulation

Temporal modulation

0 1/4

I+V


Temporal modulation

0 3/4

I-V


Temporal modulation

1/4 1/4

I+U


Temporal modulation

1/4 3/4

I-U

• Also complicated 4-fold modulation scheme.


Temporal modulation

• Temporal modulation faster than seeing (~ 1 kHz)

special demodulating camera

ZIMPOL10-5 polarimetric

sensitivity


Temporal modulation


S5T

Beam-exchange method

Best of both worlds: combining spatial and (fast) temporal modulation



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)




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



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.


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


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!


Questions?


Pushing the limits of Astronomical Polarimetry Frans Snik Sterrekundig Instituut Utrecht BBL 710

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