Polarization Calibration of HMI

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Polarization Calibration of HMI

• A. Norton, J. Schou, D. Elmore,
• J. Borrero, S. Tomczyk, G. Card
• High Altitude Observatory
• National Center for Atmospheric Research

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Polarization Modulation Schemes
Mod A - 4 observations mix linear circular (80
sec) I Q U V

I Q U V Mod C - 6 obs separate linear
circular IQ, IU, IV (120 sec) Can
correct for velocity during Mod C.
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Polarization Calibration Unit (PCU) G. Card, D.
Elmore, S. Tomczyk, K. Streander
Linear Polarizer a 1/4 Waveplate
Moved
in/out of beam (1.25 ? resolution)
Rotate optics (0.001
resolution) Inputs fixed states of polarization
into HMI for calibration purposes
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PCU
HMI Tel
HMI Polarizer
Sout?
Sin?
1/2 1/4 1/2 Lin WP WP WP Pol
Lin 1/4 Pol WP
HMI Tel Ret (?,?)
17 Unknowns in optical path 3 - Input light -
Slamp (1.0, q, u, v) 4 - PCU -
transmissions, retardation error and mount error
of WP 2 - HMI tel - retardation and angle 6 -
HMI Polarizer - mount errors and retardation
errors for WPs, 2 - CCD - gain, bias
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Calculating theT-Matrix Polarimeter Response
Matrix

• Calibration sequences selected to constrain
  unknowns
  With PCU elements in/out of beam
  and at various angles, step through a variety of
  HMI waveplate orientations.
  
  Short sequence - 69 exposures, includes Mod A
  Long sequence - 140
  exposures, includes Mod C.
• Forward model light path.
• Use least squares fitting routine to solve for
  unknowns.
• Combine for Telescope matrix Polarimeter
  Response Matrix.

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Monte Carlo Simulations - Photon Noise
Calibration requirement are that uncertainties
in XT lt 0.1
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Calibration in Progress
Polarization data taken Feb 9-10 With PCU in
beam. Work in progress. Please check HAO website
for updates http//web.hao.ucar.edu/hmi
1st magnetogram 2/7/06 courtesy of J. Schou