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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 f.snik@uu.nl. Astronomical Polarimetry. Outline. Why polarization? What is polarization? Measurement principles. Instrumental limitations. Why polarization?. Astronomy: study of starlight.

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

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  1. Pushing the limits of Astronomical Polarimetry Frans Snik Sterrekundig Instituut Utrecht BBL 710 f.snik@uu.nl

  2. Astronomical Polarimetry Outline • Why polarization? • What is polarization? • Measurement principles. • Instrumental limitations.

  3. Why polarization? Astronomy: study of starlight Three measurable quantities: • Intensity

  4. Why polarization? Astronomy: study of starlight Three measurable quantities: • Intensity • Wavelength: λ

  5. Why polarization? Astronomy: study of starlight Three measurable quantities: • Intensity • Wavelength: • Polarization: α λ

  6. Why polarization? Astronomy: study of starlight Three measurable quantities: • Intensity • Wavelength: • Polarization: … as a function of [x,y] and/or t α λ

  7. 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.

  8. Why polarization? Example - Military

  9. Why polarization? Example - Military

  10. Why polarization? Example - Astronomy Scattering polarization:

  11. Why polarization? Example - Astronomy

  12. 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

  13. Why polarization? Polarimetric projects at SIU EST

  14. Why polarization? Polarimetric projects at SIU E-ELT

  15. Why polarization? 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

  16. Why polarization? Why NOT polarization? • Technically challenging. • Conflicting with imaging optics (like AO). • Adds a lot of instrument complexity. • Data difficult to interpret.

  17. 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

  18. What is polarization? Electromagnetic wave • 100% linearly polarized light: • Partially linearly polarized light: • Combination of unpolarized & 100% polarized α

  19. What is polarization? Electromagnetic wave

  20. What is polarization? Electromagnetic wave • Circularly polarized light: • ¼ λ phase shift between orthogonal linear polarization directions • General case: elliptical

  21. What is polarization? Electromagnetic wave

  22. 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)

  23. : ½(I+Q) - Q= : ½(I-Q) - U= : ½(I+U) - V= : ½(I-U) : ½(I+V) : ½(I-V) What is polarization? Stokes vector Q/I, U/I, V/I = normalized/fractional polarization √(Q2+U2+V2)/I = polarization degree + I= + = + =

  24. 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!

  25. Measurement principles Multidimensional data • General case: S(x, y, l) • But detectors are only two-dimensional…

  26. Measurement principles Multidimensional data • General case: S(x, y, l) • Combining l:  Imaging polarimetry Separate images of the Stokes vector elements

  27. Measurement principles Multidimensional data • General case: S(x, y, l) • Combining x, y:  Spectropolarimetry Separate spectra of the Stokes vector elements

  28. Measurement principles General polarimeter set-up • … • modulator = retarder • … • analyzer = (fixed) polarizer • … • detector (demodulator)

  29. Measurement principles Polarizers • wire grid

  30. Measurement principles Polarizers • wire grid

  31. Measurement principles Polarizers • stretched polymer (dichroism)

  32. Measurement principles Polarizers • cube beam-splitter

  33. Measurement principles Polarizers • birefringent crystal no & ne Savart plate

  34. Measurement principles Retarders • introduction of phase difference half wave plate quarter wave plate

  35. Measurement principles Retarders • introduction of phase difference half wave plate quarter wave plate

  36. Measurement principles Retarders • Crystal wave plates Chromatic and temperature sensitive for birefringent crystal plates.

  37. fast fast slow fast slow slow fast slow V=0 V V<0 V>0     = < m a x m a x Measurement principles Retarders – Liquid crystals Liquid Crystal Variable Retarders (LCVRs) Ferroelectric Liquid Crystals (FLCs) ~20 ms ~100 s

  38. Measurement principles Retarders – Fresnel rhomb • Phase difference through total internal reflections

  39. Measurement principles Retarders – PEMs • Piezo-Elastic Modulators • Birefringence induced in normal glass by stress. • Resonance frequency: fast variation of retardance (~10 kHz).

  40. Measurement principles Mueller matrices

  41. Measurement principles Modulation • Spatial • Measuring different polarization states at different locations • Temporal • Measuring different polarization states at different times • Spectral

  42. Measurement principles Spatial modulation + Strictly simultaneous measurements. - Different (parts of) detectors. • Differential alignment / aberrations. • Limited detector gain calibration. - 2 to 6 beams.

  43. 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.

  44. Measurement principles Temporal modulation • Rotating waveplate + polarizer analyzer + demodulating detector.  Intensity measurements are linear combinations of I with Q, U and V

  45. I+Q l l 0 0 Measurement principles Temporal modulation • 2 LCVRs + polarizer

  46. I-Q l l 0 1/2 Measurement principles Temporal modulation

  47. I+V l l 0 1/4 Measurement principles Temporal modulation

  48. I-V l l 0 3/4 Measurement principles Temporal modulation

  49. I+U l l 1/4 1/4 Measurement principles Temporal modulation

  50. Measurement principles Temporal modulation I-U l l 1/4 3/4 • Also complicated 4-fold modulation scheme.

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