Polarimetry Overview

1
Polarimetry Overview

• David Elmore
• HAO/NCAR
• Elmore_at_ucar.edu

2
Polarimeter Guidelines

• Time multiplexed polarization modulation and
  analysis
• Issues are understood
• Versatile
• Calibration optics precede highly polarizing
  reflections (Calibration station at Gregory
  focus)
• Polarization modulators precede highly polarizing
  reflections (Modulator turret at Gregory focus)
• Seeing induced errors reduced at high modulation
  frequency (kHz)
• Seeing induced errors reduced using dual beam
  analyzer

3
Polarimeter Concept (HPP)

• High Precision Polarimeter
• Modulator at Gregory
• High speed Piezo-elastic (PEM)
• Ferroelectric liquid crystal (FeLC)
• Analyzer Linear polarizer at Gregory
• Advantages
• kHz modulation frequency rejects residual seeing
• No highly polarizing optics between modulator and
  analyzer
• Disadvantages
• Narrow wavelength range, but tunable
• Charge caching photo-detector required

4
Polarimeter Concept (FAP)

• Fast Achromatic Polarimeter
• Modulator Rapidly rotating achromatic retarder
  at Gregory (but not as fast as FeLC or PEM)
• Analyzer at detector Linear polarizer for
  8-state modulation or high frequency FeLC
  linear polarizer for 4-state modulation
• Advantages
• Hundreds of Hz modulation frequency helps reject
  residual seeing
• Wavelength diversity possible
• Disadvantages
• Highly polarizing optics between modulator and
  analyzer
• Time varying polarimeter response matrix
• Calibration intensive data collection and
  reduction
• Charge caching photo-detector required

5
Polarimeter Concept (SAP)

• Slow Achromatic Polarimeter
• Modulator Achromatic slowly rotating retarder at
  Gregory
• Analyzer Polarizing beam splitter at detectors
• Advantages
• Wavelength diversity possible
• Conventional CCD or IR detectors can be mixed
  with charge caching photo-detectors (that use
  rapidly chopped FeLC before analyzer)
• Disadvantages
• Highly polarizing optics between modulator and
  analyzer
• Time varying polarimeter response matrix
• Calibration intensive data collection and
  reduction
• Charge caching detector needed for mixed scheme

6
Polarimeter Performance
Depends upon modulator type and particular
lines Additional study required
7
Further Analysis Fast Retarder

• Fast modulation using a retarder
• Up to 1600Hz frame rate gt 200Hz to 400Hz
  polarization modulation
• 8 state demodulation
• 4 state demodulation using FeLC at the analyzer

8
Further Analysis Fast Retarder
9
Further Analysis Slow Retarder Fast Analyzer

• Slow modulation using a retarder
• Typically 50 Hz frame rate gt 12.5Hz 25Hz
  polarization modulation
• 8 state demodulation
• Rapidly chopped analysis using FeLC at the
  analyzer

10
Further Analysis Slow Retarder Fast Analzer
     
11
Phils Analysis
     
12
Bottom Line
     
requirement
requirement
13
Further Analysis Conclusions

• Slow modulation using a retarder (50hz frame
  rate)
• Does not meet accuracy requirement
• Superimposition of rapidly chopped analyzer does
  not help
• Fast modulation using a retarder
• Meets accuracy requirement for dual beam but not
  single
• 4 state chopped analyzer helps V, but not Q.
  Good for reducing number of modulation states
• How does FeLC or PEM perform?