Polarimetry

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 510-4
• Derived telescope polarization requirements
• lt 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 lt
  510-4 (to achieve polarization accuracy
  requirement)
• Instrumental polarization stability lt 510-4
  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

• lt1 instrumental polarization before modulation
  and less than 510-4 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