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? Three measurable quantities: Intensity

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

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

Astronomy: study of starlight Why polarization? α λ 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?

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 –S 5 T, 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

Examples: degree of polarization LCD screen100% 45 o reflection off glass~90% clear blue sky~75% 45 o reflection off mirror~5% solar/stellar magnetic fields~1% exoplanet in stellar halo~ cosmic microwave background~ 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 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 –incoherent sum (no interference) What is polarization?

Stokes vector Q/I, U/I, V/I = normalized/fractional polarization √ (Q 2 +U 2 +V 2 )/I = polarization degree 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

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 n o & n e Savart plate Measurement principles

Retarders –introduction of phase difference half wave platequarter wave plate Measurement principles

Retarders –introduction of phase difference half wave platequarter wave plate Measurement principles

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

Retarders – Liquid crystals Liquid Crystal Variable Retarders (LCVRs) ~20 ms 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 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 I+Q LCVRs + polarizer Measurement principles

0 1/2 I-Q Measurement principles Temporal modulation

0 1/4 I+V Measurement principles

Temporal modulation 0 3/4 I-V Measurement principles

Temporal modulation 1/4 1/4 I+U Measurement principles

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

Temporal modulation Temporal modulation faster than seeing (~ 1 kHz)  special demodulating camera ZIMPOL polarimetric sensitivity Measurement principles

Temporal modulation Measurement principles S5TS5T

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) HWPQWP rotating waveplates cylindrical lens (compensates for crystal astigmatism) CaF 2 channeling prism (compensates for focal shift) existing slider fiber 1fiber 2 return beam is not blocked rotated by one actuator on a belt 56 mm HARPSpol

Beam-exchange method Measurement principles 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 45  Al mirror (very common in telescopes!) Also effect due to growing Al 2 O 3 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 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

Questions? Astronomical Polarimetry