Greg Taylor, Synthesis Imaging School, 18 June 2002
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Polarization in InterferometryGreg Taylor
• What is polarized light? • What fun science can be done with polarimetry?• How do interferometers measure polarization?• How do you calibrate and image in full polarization?
Greg Taylor, Synthesis Imaging School, 18 June 2002
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Greg Taylor, Synthesis Imaging School, 18 June 2002
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What is Polarized Light?• Light is oscillating electric and magnetic fields• Polarization is labeled by the shape of the trace of the tip of the E vector• Each polarization has an orthogonal state• Incoherent light can contain many polarization states
Stokes Parameters describe partially polarized light
Alternate representation:
• pol. angle (EVPA) = 0.5 atan (U/Q)• polarized intensity p = sqrt(Q2 + U2)• fractional linear m = p / I• fractional circular v = |V | / I
I = RR + LLQ = RL + LRU = i(LR – RL)V = LL - RR
For circular feeds
Greg Taylor, Synthesis Imaging School, 18 June 2002
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3C 31VLA @ 8.4 GHz
E-VectorsLaing (1996)
3 kpc
Greg Taylor, Synthesis Imaging School, 18 June 2002
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Cygnus AB-VectorsVLA @ 8.5 GHz
Perley & Carilli (1996)
10 kpc
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Attridge et al 1999
VLBA @ 5 GHz
1055+018
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R Aquirrii
Stellar SiO MasersBoboltz et al 1998
VLBA @ 43 GHz
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15 12 8 GHz
Taylor (1998)
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See Review of “Cluster Magnetic Fields” by Carilli & Taylor 2002 (ARA&A)
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Parallactic Angle
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Bad D-term solution Good D-term solution
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Compact Symmetric
Objects (CSOs)
VLBA @ 8.4 GHz
Peck & Taylor (2001)
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Practical VLBIpolarization anglecalibration
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1) Find a Calibrator
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2) Estimate _VLA
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2200+4203) Sum Q and U over VLBA image.
4) Compute VLBA = 0.5 atan (U/Q)
5) Rotate VLBA data by (_VLA – _VLBA)
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