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Polarimetry in Astronomy

Polarimetry in Astronomy. Or Do you know where your photons are coming from ?. Elizabeth Corbett AAO. Polarimetry: The Basics. Light be described in terms of two components:. Taken from Hecht (1987) “Optics”. Polarimetry: The Basics. In general light is elliptically polarised.

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Polarimetry in Astronomy

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  1. Polarimetry in Astronomy Or Do you know where your photons are coming from? Elizabeth Corbett AAO

  2. Polarimetry: The Basics Light be described in terms of two components: Taken from Hecht (1987) “Optics” Polarimetry in Astronomy

  3. Polarimetry: The Basics In general light is elliptically polarised Special cases: x = 0 or np - linearly polarised light x = p/2  np and Ex=Ey circularly polarised light Unpolarised light has a well-defined E which fluctuates rapidly, hence no net polarisation is measured. q Polarimetry in Astronomy

  4. Introducing: - The Stokes Vectors • Electromagnetic radiation can be described in terms of the Stokes Vectors; I, Q, U & V. • I - total intensity • Q & U - describing linear polarisation • V - circular polarisation • Polarisation PA: • Degree of Polarisation: • For linear polarisation V=0 Polarimetry in Astronomy

  5. Why Stokes Vectors? • Easy to describe polarisations: • Additive - e.g. Polarimetry in Astronomy

  6. Sources of Polarised Emission • Synchrotron: • dominant radiation mechanism in the optical - radio continua of the blazar class of AGN, also seen in SNR, pulsars • emitted by charged particles, generally electrons accelerated by a magnetic field • produces a high degree of linear polarisation (up to 45% in some blazars) • polarisation position angle is aligned with the E vector perpendicular to the local magnetic field Polarimetry in Astronomy

  7. Dichroic Absorption: • also known as interstellar polarisation • dichroic absorbers preferentially absorb radiation with one polarisation state and transmit the orthogonal state • due to anisotropic dust grains aligning in the presence of a magnetic field • radiation passing through such a cloud becomes polarised with an E vector parallel to the magnetic field Polarimetry in Astronomy

  8. Scattering: • Light can be scattered by electrons or dust • High degrees of linear polarisation can result • Polarisation PA is perpendicular to the scattering plane • Degree of polarisation depends on the scattering angle,c • Circular polarisation can result from multiple scatters from dust 0% 60% c 100% Polarimetry in Astronomy

  9. Part IV: Polarimeters • Polaroid-type polarimeters • Dual-beam polarimeters Polarization of Light: Basics to Instruments

  10. Part IV: Polarimeters, polaroid-type Polaroid-type polarimeterfor linear polarimetry (I) • Use a linear polarizer (polaroid) to measure linear polarization ... [another cool applet]Location:http://www.colorado.edu/physics/2000/applets/lens.html • Polarization percentage and position angle: Polarization of Light: Basics to Instruments

  11. Part IV: Polarimeters, polaroid-type Polaroid-type polarimeterfor linear polarimetry (II) • Advantage: very simple to make • Disadvantage: half of the light is cut out • Other disadvantages: non-simultaneous measurements, cross-talk... • Move the polaroid to 2 positions, 0º and 45º (to measure Q, then U) Polarization of Light: Basics to Instruments

  12. Part IV: Polarimeters, polaroid-type Polaroid-type polarimeterfor circular polarimetry • Polaroids are not sensitive to circular polarization, so convert circular polarization to linear first, by using a quarter-wave plate • Polarimeter now uses a quarter-wave plate and a polaroid • Same disadvantages as before Polarization of Light: Basics to Instruments

  13. Part IV: Polarimeters, dual-beam type Dual-beam polarimetersPrinciple • Instead of cutting out one polarization and keeping the other one (polaroid), split the 2 polarization states and keep them both • Use a Wollaston prism as an analyzer • Disadvantages: need 2 detectors (PMTs, APDs) or an array; end up with 2 ‘pixels’ with different gain • Solution: rotate the Wollaston or keep it fixed and use a half-wave plate to switch the 2 beams Polarization of Light: Basics to Instruments

  14. Part IV: Polarimeters, dual-beam type Dual-beam polarimetersSwitching beams • Unpolarized light: two beams have identical intensities whatever the prism’s position if the 2 pixels have the same gain • To compensate different gains, switch the 2 beams and average the 2 measurements Polarization of Light: Basics to Instruments

  15. rotate by 180º Part IV: Polarimeters, dual-beam type Dual-beam polarimetersSwitching beams by rotating the prism Polarization of Light: Basics to Instruments

  16. Part IV: Polarimeters, dual-beam type Dual-beam polarimetersSwitching beams using a ½ wave plate Rotated by 45º Polarization of Light: Basics to Instruments

  17. Part IV: Polarimeters, example of circular polarimeter A real circular polarimeterSemel, Donati, Rees (1993) Quarter-wave plate, rotated at -45º and +45º Analyser: double calcite crystal Polarization of Light: Basics to Instruments

  18. Part IV: Polarimeters, example of circular polarimeter A real circular polarimeterfree from gain (g) and atmospheric transmission () variation effects • First measurement with quarter-wave plate at -45º, signal in the (r)ight and (l)eft beams: • Second measurement with quarter-wave plate at +45º, signal in the (r)ight and (l)eft beams: • Measurements of the signals: Polarization of Light: Basics to Instruments

  19. Part IV: Polarimeters, example of circular polarimeter A real circular polarimeterfree from gain and atmospheric transmissionvariation effects • Build a ratio of measured signals which is free of gain and variable atmospheric transmission effects: average of the 2 measurements Polarization of Light: Basics to Instruments

  20. Part IV: Polarimeters, summary Polarimeters - Summary • 2 types: • polaroid-type: easy to make but ½ light is lost, and affected by variable atmospheric transmission • dual-beam type: no light lost but affected by gain differences and variable transmission problems • Linear polarimetry: • analyzer, rotatable • analyzer + half-wave plate • Circular polarimetry: • analyzer + quarter-wave plate • 2 positions minimum • 1 position minimum Polarization of Light: Basics to Instruments

  21. l/4 plate Polarimeter To TV guider l/2 plate Calcite To spectrograph or imager Tilted slit/dekker Analyser Arc lamp Polarimetry in Astronomy

  22. Summary • Polarimetry provides information on where your photons originated • Have they been scattered? • Have they been through dust? • Have they (perhaps) come from a jet? • Important for inclination dependent systems - eg AGN, YSO • “Not as hard as it used to be” - easy data reduction • But - very “photon hungry” • so for a dP~0.1% you need SNR ~1400 or 2E6 photons! Polarimetry in Astronomy

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