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Polarimetry

Polarimetry. Christoph Keller. Polarimetry Requirements. Polarization sensitivity : amount of fractional polarization that can be detected above a (spatially and/or spectrally) constant background, a relative measurement: 10 -5

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Polarimetry

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  1. Polarimetry Christoph Keller

  2. Polarimetry Requirements • Polarization sensitivity: amount of fractional polarization that can be detected above a (spatially and/or spectrally) constant background, a relative measurement: 10-5 • Polarization accuracy: absolute error in measured fractional polarization, an absolute measurement: 5·10-4 • Derived telescope polarization requirements: • < 1% instrumentally induced polarization at all wavelengths before polarization modulation (to keep second-order effects small enough to achieve required polarization sensitivity) • Instrumental polarization calibration error: < 5·10-4 (to achieve polarization accuracy requirement) • Instrumental polarization stability: < 5·10-4 within 15 min (to achieve polarization accuracy requirement)

  3. Side-Note: 2nd Order Effects • Taking into account first-order effects only, polarimetric sensitivity better than 10-4 is difficult to achieve • Influence of seeing: mostly I to Q,U,V and Q,U,V to Q,U,V cross-talk • Influence of camera non-linearity, dark-current and bias fluctuations • Influence of (polarized) scattered light

  4. Gregorian Focus • Instrumental polarization due to off-axis optics • Aluminum coating at 400 nm • Polarization effects depend on wavelength, field of view, coating properties and age • Instrumental polarization fixed with respect to telescope • Instrumental polarization rotates with respect to image

  5. Gregorian Wavelength Dependence Gregorian I to Q Requirement Science I,V to Q Requirement

  6. Gregorian Time Dependence

  7. Coudé Time Dependence

  8. Temporal Polarization Change • Gregorian: up to 0.05 in 15 minutes around noon in coordinate system fixed with image, but constant in telescope coordinate system • Coudé: up to 0.5 in 15 minutes around noon • Only Gregorian focus in telescope coordinate system fulfills specifications

  9. Distributed Polarimetry • <1% instrumental polarization before modulation and less than 5·10-4 change in 15 minutes  polarization modulation close to Gregorian focus • Only a single beam can be sent to coudé because AO cannot handle two beams • Strongly polarizing transfer optics  Calibration polarizers close to Gregorian • Coronal instruments: compact, no need for adaptive optics, fast beam  at Gregorian instrument station • On-disk instruments: large, need for AO correction, slow beam  on coudé platform

  10. Polarization Optics in Gregorian • Polarization calibration (rotating polarizers and retarders for different wavelength ranges) • Focal masks for alignment and tests • Polarization modulators (and analyzers) for different wavelength ranges, space for visiting polarimeter • Telescope optics will be adjusted according to inserted optics

  11. Turret at Gregorian Focus

  12. Telescope Polarization Issues • Telescope polarization rotates with respect to solar image • Telescope polarization depends on field position • Telescope polarization depends on wavelength • Optical properties of coatings will change (slowly) in time • Coatings might not be uniform across mirror surface(s) • Must calibrate telescope polarization accurately enough to meet science specifications • Based on experience with existing strongly polarizing telescopes, we expect that ATST telescope will meet polarization science requirements

  13. Measuring Telescope Polarization • Find a way to measure instrumental polarization with sufficient accuracy to meet science requirements at all wavelengths and at all times • Study coating uniformities on large telescopes • Estimate complexity of such measurements (required instrumentation and time)

  14. Realistic Mueller-Matrix Model • Create Mueller-matrix model of all-reflective telescope that includes aluminum coatings with thin aluminum-oxide overcoat • Measure instrumental polarization to determine free parameters of model • Determine required IP measurement frequency • Determine most suitable wavelength(s) for IP measurements • Test model and measurement approaches

  15. Polarimetry Error Budget • Classical error tree approach does not work because ‘leaves’ are non-linearly coupled • Example: non-linearity of CCD camera and telescope polarization couple multiplicatively • No good approach available yet • Will work out a potential approach and apply it to telescopes and instruments

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