Instrumental Challenges in Polarization Observations using Large - - PowerPoint PPT Presentation

Instrumental Challenges in Polarization Observations using Large Interferometry

Chau Ching Chiong (ASIAA)

SKA Science Workshop in East Asia, Nagoya, Jun. 7 2013

Outline

• Polarization observation overview
• Source of polarization leakage
• ALMA Band-1 front-end system
• Current receiver performance
• Current receiver performance
• Ways for improvement
• Summary

Polarization Observation

Probing Magnetic Field:

• Synchrotron radiation
• Faraday rotatoin
• Goldreich-Kylafis effect < 5%

Girart+ (2006)

• Goldreich-Kylafis effect < 5%
• AGN polarization: 5-10%
• Dust polarization: 1%
• Zeeman effect: 0.1%
• CMB polarization

Crutcher+ (1999)

Polarization Leakage in Interferometry

• For linear feed like ALMA. X/Y-signal leakage to Y/X-

feed.

• Stokes I signal goes into Q, U, and V, and generates

“instrumental polarization”.

• Neglecting higher order terms (small leakage),
• Neglecting higher order terms (small leakage),

G and D need to be determined

Parallactic angle

Characterization of Instrumental Effects

• Time-invariant

– Antenna – Optics – Feedhorn

OMT Feedhorn

– Feedhorn – Orthomode Transducer

• Time-variant

– Front-end (amplifier, mixer) gain – Back-end

RF LNA mixer LO LO IF LNA To back-end (correlator etc.)

ALMA Band-1 Project Overview

• Led by ALMA-EA (ASIAA)
• Cooperation with HIA

(Canada), NRAO (USA) and U Chile.

• Freq.: 35-52 GHz in SSB
• peration
• Receiver Noise Temp.: 25-32

K (expected) @ 15K

• PDR in July 2013
• CDR in 2014
• End of project: Summer 2019

ALMA Band-1 Polarimetry

• Unique science: Zeeman Measurement via

circular polarization (Stokes V)

• CCS(43-32) f= 45.379033 GHz
• Dense core
• Detect 1mG in

hours

1 hr integration 10 hr integration

Summary of Instrumental Leakage

• Antenna: geometry is given: beam squint is proportional

to the offset distance

• Optics + Feedhorn < -40 dB (1%) (band center); < -30

dB (3%) (band edge)

• OMT < -40 dB (< 1%)
• OMT < -40 dB (< 1%)
• Amplifier gain: < 0.1% (spec.) but sensitive to ambient

temperature drifting.

• Antenna deformation is not obvious.
• Maximum deviation in D-term (ALMA spec.): -12 dB (6%)

Typical Calibration Strategies

• Determine the Jones/Mueller matrix.
• Calibration source is slightly linear polarized.
• Using long observation on calibration source with

different parallactic angle. Source polarization is rotating with the sky but instrumental polarization is fixed.

– Time consuming.

• After calibration, error is of order of 0.1% of Stokes I.
• After calibration, error is of order of 0.1% of Stokes I.

From Johnston 2002

Current VLA and ALMA Receiver Performance

VLA:

• D ~ 1% at low frequency (C-, X-band), 5-6% at high

frequency (K-, Q-band)

• Not time-stable.

ALMA: ALMA:

• Nagai et al. (2012) measured the polarization

performance of current ALMA Band-3, 6 and 7.

• D-term in Band-3 < 3%, Band-6 < 5%.
• Each antenna has its own feature.
• Off- beam axis degradation. Best at phase center.
• Time variation of 10% over two days, mostly from front-

end electronics(?).

D-Term Time Variation in ALMA

From Nagai+ 2012

Different Date Frequent calibration required

Ways for Improvement (1)

• In ALMA, time-variant error

causes problem in high sensitivity polarization measurements.

• Adding correlated noise source.
• Adding half/quarter-wave plate to

provide higher flexibility in provide higher flexibility in calibration. – Half-wave plate exchanges the polarization state of two linear polarization. – Quarter-wave plate changes linear into circular feed (or vice versa).

Ways for Improvement (2)

• For ALMA Band-1 or future SKA, stable and powerful

calibration source with high signal-to-noise ratio and changeable polarization feature is possible.

• Photonic phase calibrator: one of the ALMA-EA

development item led by Dr. Hitoshi Kiuchi (NAOJ).

From Kiuchi

Summary

• The origins of the polarization leakage are

identified.

• D-term is in a few %, with off-beam center

degradation. degradation.

• There are two component corrupting the

polarization: time-invariant and time-variant.

• Time-invariant should be antenna based.
• Time-variant needs more investigations.
• Applying experience from ALMA to SKA.