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Frans Snik Sterrewacht Leiden

10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden. Definitions. Polarimetric sensitivity Polarimetric accuracy Polarimetric efficiency Polarimetric precision. Polarimetric sensitivity.

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Frans Snik Sterrewacht Leiden

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  1. 10-3 versus 10-5 polarimetry: what are the differences?orSystematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden

  2. Definitions • Polarimetric sensitivity • Polarimetric accuracy • Polarimetric efficiency • Polarimetric precision

  3. Polarimetric sensitivity The noise level in Q/I, U/I, V/I above which a polarization signal can be detected. In astronomy: signals <1%  polarimetric sensitivity: 10-3 – 10-5 (or better)

  4. Polarimetric accuracy Quantifies how well the measured Stokes parameters match the real ones, in the absence of noise.

  5. Polarimetric accuracy polarization response of photometry transmission 1 polarization rotation Not a Mueller matrix, as it includes modulation and demodulation. instrumental polarization related to polarimetric efficiency cross-talk

  6. Polarimetric accuracy scale zero level >> 10-5 sensitivity level!

  7. Polarimetric efficiency Describes how efficiently the Stokes parameters Q, U, V are measured by employing a certain (de)modulation scheme. • 1/[susceptibility to noise in demodulated Q/I, U/I, V/I] del Toro Iniesta & Collados, Appl.Opt. 39 (2000)

  8. Polarimetric precision Doesn’t have any significance…

  9. Temporal modulation Advantages: • All measurements with one optical/detector system. Limitations: • Susceptible to all variability in time: • seeing • drifts Solution: Go faster than the seeing: ~kHz. • FLCs/PEM + fast/demodulating detector

  10. Temporal modulation Achievable sensitivity depends on: • Seeing (and drifts); • Modulation speed; • Spatial intensity gradients of target; • Differential aberrations/beam wobble. Usually >>10-5

  11. Spatial modulation Advantages: • All measurements at the same time. • beam-splitter(s)/micropolarizers Limitations: • Susceptible to differential effects between the beams. • transmission differences • differential aberrations • limited flat-fielding accuracy • Never better than 10-3

  12. Dual-beam polarimetry “spatio-temporal modulation” “beam exchange” Best of both worlds: Sufficient redundancy to cancel out degrading differential effects (to first order). • double difference • double ratio Can get down to 10-6

  13. Increasing sensitivity If • All noise-like systematic effects have been eliminated; • For each frame photon noise > read-out noise, then: total amount of collected photo-electrons • Adding up exposures; • Binning pixels (in a clever way); • Adding up spectral lines (in a clever way); • Better instrument transmission and efficiency; • Larger telescopes! = 1010 for 10-5 sensitivity!

  14. Increasing sensitivity HARPSpol ±10-5 Kochukhov et al. (2011) Snik et al. (2011)

  15. Calibration Create known polarized input: • rotating polarizer • rotating polarizer + rotating QWP • misalignment and wrong retardance can be retrieved with global least-squares method • standard stars

  16. Calibration • What does really limit calibration with calibration optics? • How to quantify calibration accuracy? • How often does one need to calibrate? • How to calibrate large-aperture telescopes? • How stable are standard stars? • How to efficiently combine with models/lab measurements?

  17. Systematic effects that (still) limit polarimetric performance • Polarized fringes • Polarized ghosts • Higher-order effects of dual-beam method • Surprising interactions • e.g.: coupling of instrumental polarization with bias drift and detector non-linearity • Polarized diffraction (segmented mirrors!) • System-specific effects (e.g. ZIMPOL detector)  Error budgeting approach

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