ViSP Polarimetry

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ViSP Polarimetry

• David Elmore
• HAO/NCAR
• Elmore_at_ucar.edu

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Polarization Measurement

• The measured Stokes vector is the product of the
  polarimeter response matrix and telescope matrix
  acting upon the input Stokes vector from the Sun.
• To achieve the required accuracy, the uncertainty
  in XT must be less than some value, but not all
  elements need to be less than the polarimetric
  accuracy requirement.

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Requirement Matrix

• Uncertainty in a measured normalized Stokes
  parameter is due uncertainty in gain, a, and
  uncertainty in cross talk among Stokes
  parameters, b. For the ith Stokes parameter
• DPi aPi bPj?i .
• For cross talk among the polarization parameters
  the uncertainty in the measurement of the ith
  parameter due to the jth Stokes parameter, bPj?i
  must be less than the polarimeteric accuracy
  requirement, e.
• For cross talk into I, bPj?1 aPj?1.
• from Kioshi Ichimoto NAOJ

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Requirement Matrix

• Combine the four perterbation equations into a
  matrix and scale the cross talk elements by
  typical polarization signals
• Where pl and pv are typical normalized solar
  polarization signals, pI 1

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Requirement Matrix

• For ATST we have no gain requirement, but 1 is
  typical, therefore a 0.01
• The polarization accuracy requirement e 5 x
  10-4
• From experience with ASP, typical values for
  solar polarization are, pl pv 0.1.
  Substitution into the requirement matrix gives
• The requirement matrix gives the accuracy to
  which we need to know XT.

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Measurement of XT

• Use calibration optics for both X T (ASP at
  DST)
• Calibration polarizers between polarimeter and
  telescope for X (In the GOS for ATST)
• Calibration polarizer over the telescope for T
• Calibrate X, Model T
• Calibration polarizers between polarimeter and
  telescope for X
• Create a two mirror telescope model and use some
  external measure of unknowns in model such as of
  the complex refractive index of the mirror
  coating

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Measurement of XT

• Calibrate X, determine T using solar spectra
• Calibration polarizers between polarimeter and
  telescope for X
• Create a model T and use solar spectra to fit the
  unknows factors in the model
• Determine X and T from solar spectra
• Create a model of XT and use solar spectra to fit
  the unknown factors in the model
• Use different techniques for different XT
  matrix elements

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Cross talk into Intensity
Use flat field for 1,1. For remainder of first
row, a simple model of XT will suffice
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Cross talk from I to Q, U, and V

• Use solar continuum polarization (Standard
  operating procedure for ASP)

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Scale and phase

• Calibration polarizers between X and T. The
  effect of T is small so a simple model will
  suffice.

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Cross talk between U and V

• Use an accurate model of the telescope supported
  by external refractive index data or use solar
  spectra

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pau