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

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Presentation on theme: "10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden."— Presentation transcript:

1 10 -3 versus polarimetry: what are the differences? or Systematic 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: – (or better)

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

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

6 Polarimetric accuracy zero level >> sensitivity level! scale

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! = for sensitivity!

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

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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