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10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden
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10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Dec 16, 2015

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Page 1: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

10-3 versus 10-5 polarimetry: what are the differences?

orSystematic approaches to deal with

systematic effects.

Frans SnikSterrewacht Leiden

Page 2: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Definitions

• Polarimetric sensitivity• Polarimetric accuracy• Polarimetric efficiency• Polarimetric precision

Page 3: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Polarimetric sensitivity

The noise level in Q/I, U/I, V/I above which a polarization signal can be detected.

In astronomy: signals <1% polarimetric sensitivity:

10-3 – 10-5 (or better)

Page 4: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Polarimetric accuracy

Quantifies how well the measured Stokes parameters match the real ones, in the absence of noise.

rS meas = (X+ ΔX)⋅

r S in

Page 5: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Not a Mueller matrix, as it includes modulation and demodulation.

Polarimetric accuracy

X =

I →I Q →I U →I V →I

I →Q Q →Q U →Q V →Q

I →U Q →U U →U V →U

I →V Q →V U →V V →V

⎜ ⎜ ⎜ ⎜

⎟ ⎟ ⎟ ⎟

transmission 1

instrumental polarization

cross-talk

polarization rotation

related topolarimetric efficiency

polarization responseof photometry

Page 6: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Polarimetric accuracy

ΔX≤

− 10−2 10−2 10−2

10−3 10−2 10−2 10−2

10−3 10−2 10−2 10−2

10−3 10−2 10−2 10−2

⎜ ⎜ ⎜ ⎜

⎟ ⎟ ⎟ ⎟

zero level>> 10-5 sensitivity level!

scale

ΔP ≤ 0.001+ 0.01⋅ P

Page 7: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Polarimetric efficiency

Describes how efficiently the Stokes parameters Q, U, V are measured by employing a certain (de)modulation scheme.

1/[susceptibility to noise in demodulated Q/I, U/I, V/I]

del Toro Iniesta & Collados, Appl.Opt. 39 (2000)

Page 8: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Polarimetric precision

Doesn’t have any significance…

Page 9: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Temporal modulation

Advantages:• All measurements with one optical/detector system.

Limitations:• Susceptible to all variability in time:

– seeing– drifts

Solution:Go faster than the seeing: ~kHz.

• FLCs/PEM + fast/demodulating detector

Page 10: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Temporal modulation

Achievable sensitivity depends on:• Seeing (and drifts);• Modulation speed;• Spatial intensity gradients of target;• Differential aberrations/beam wobble.

Usually >>10-5

Page 11: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Spatial modulation

Advantages:• All measurements at the same time.

– beam-splitter(s)/micropolarizers

Limitations:• Susceptible to differential effects between the

beams.– transmission differences– differential aberrations– limited flat-fielding accuracy

Never better than 10-3

Page 12: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Dual-beam polarimetry

“spatio-temporal modulation”“beam exchange”

Best of both worlds:Sufficient redundancy to cancel out degrading

differential effects (to first order).– double difference– double ratio

Can get down to 10-6

Page 13: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Increasing sensitivityIf• All noise-like systematic effects have been eliminated;• For each frame photon noise > read-out noise,

then:

σ QI ,U

I , VI( ) =

N

N=

1

N total amount of collected photo-electrons

• Adding up exposures;• Binning pixels (in a clever way);• Adding up spectral lines (in a clever way);• Better instrument transmission and efficiency;• Larger telescopes!

= 1010 for 10-5 sensitivity!

Page 14: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Increasing sensitivityHARPSpol

Kochukhov et al. (2011)Snik et al. (2011)

±10-5

Page 15: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Calibration

Create known polarized input:• rotating polarizer• rotating polarizer + rotating QWP

–misalignment and wrong retardance can be retrieved with global least-squares method

• standard stars

Page 16: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Calibration

• What does really limit calibration with calibration optics?

• How to quantify calibration accuracy?• How often does one need to calibrate?• How to calibrate large-aperture telescopes?• How stable are standard stars?• How to efficiently combine with models/lab

measurements?

Page 17: 10 -3 versus 10 -5 polarimetry: what are the differences? or Systematic approaches to deal with systematic effects. Frans Snik Sterrewacht Leiden.

Systematic effects that (still) limit polarimetric performance

• Polarized fringes• Polarized ghosts• Higher-order effects of dual-beam method• Surprising interactions

– e.g.: coupling of instrumental polarization with bias drift and detector non-linearity

• Polarized diffraction (segmented mirrors!)• System-specific effects (e.g. ZIMPOL detector) Error budgeting approach