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

Dive into the world of astronomical polarimetry to understand the principles, limitations, and applications in astronomy and beyond. Explore how polarization of light offers valuable insights into celestial bodies and how advanced instruments push the boundaries of knowledge.

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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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