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

• 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)

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

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

V

U

Q

I

V

U

Q

I

998422.0049915.00.00.0

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Gregorian Wavelength Dependence

f/2 instrumental polarization and crosstalk

0.0001

0.001

0.01

0.1

0 2000 4000 6000 8000 10000

wavelength (nm)

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I to Q

V to Q

Science I,V to Q Requirement

Gregorian I to Q Requirement

Gregorian Time Dependence

Coudé Time Dependence

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

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

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

Turret at Gregorian Focus

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

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)

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

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