A 2-PAD Correlator, Holography, and Direct Imaging

Dual Polarisation All Digital Aperture Array

Danny Price, University of OxfordCASPER Workshop 2009, Cape Town SA

A 2-PAD Correlator, Holography, and Direct Imaging

danny.price@astro.ox.ac.uk

Introduction

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Part I: The 2-PAD Correlator

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2-PA

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2-PAD Digital Backend

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Adding a correlator Why tack on a correlator?

A1: Engineering challenge A2: Two element interferometer A3: Tile level calibration A4: Holography experiment

Station level calibration

What sort of correlator? 2-element, 2-polarisation (full stokes) FX correlator Freq range? Bandwidth? Pocket Correlator

Griffin Foster Would like help with this here in Cape Town!

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Image: Ant Holloway

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Part II: A Holography Primer

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Holography Primer

Rectangle Function Sinc Function

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F.T.

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Holography Primer

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Reflector Antenna Horn Antenna

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Holography Primer

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Reflector Antenna Horn Antenna

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Holography Primer

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Aperture Illumination F.F. Electric Field Pattern

Radiation Power Pattern

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Holography Primer

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Aperture Illumination F.F. Electric Field Pattern

Radiation Power Pattern

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2-PAD Measurements

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Aperture Illumination F.F. Electric Field Pattern

Radiation Power PatternPhoto: Ant Holloway

? ?

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Dish Holography

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Charles E. Mayer et. al., A holographic surface measurement of the Texas 4.9m antenna at 86GHz, 1983

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Dish Holography

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J. Baars et. al., Near-Field Radio Holography of Large Reflector Antennas, 2007

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Aperture Array Holography Why would we want a AA hologram?

A1: Measuring 2-PAD beam pattern A2: Calibration A3: Direct Imaging Correlator

Can we get a good enough hologram to calibrate 2-PAD? A: Not sure yet – but if we have the beam pattern, it’s

a simple FT to see the aperture

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Holography: Method

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Holography: Method

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Part III: Direct Imaging

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Direct Imaging Correlators

Direct imaging correlators do a spatial Fourier Transform, instead of electric field FT (FX / XF).

Rarely used because: Antennas must be in a grid Images produced suck Calibration is hard Individual visibilities are never formed

=> deconvolution is hard

Largest implementation is an 8x8 array at Waseda Uni, Japan (Dashido 2000).

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Image: Ian Heywood

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Direct Imaging Correlators

Morales shows that holographic data can drastically improve direct imaging correlators (2008):

Antennas must be in a grid Images produced suck Calibration is ??? hard Individual visibilities are never formed

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M. Morales, Enabling next generation dark energy and epoch of reionisationradio observatories with the MOFF correlator, 2008

Image: Ian Heywood

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Direct Imaging Correlators

Measure Electric field Weight by noise & grid

using holography Spatial Fourier Transform Square & integrate to

produce an image

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Weight by noise

Grid by holography

Fourier Transform

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Direct Imaging Correlators

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Conclusion

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Acknowledgements

Sascha Schediwy, Richard Armstrong, Kris Zarb-Adami, Jack Hickish, and Mike Jones

2-PAD team Oxford Cambridge Manchester

Pre-emptive acknowledgements CASPER Workshop 2009 attendees!

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Supplementary Material

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What is 2-PAD? The purpose of 2-PAD is to design and implement a Dual

Polarisation All Digital Phased Array for radio astronomy. In particular, 2-PAD aims to show the feasibility of ultra-wideband digital beamforming for radio astronomy purposes.

2-PAD is designed as a sandbox for testing different digital beamforming algorithms for radio astronomy. As such, the 2-PAD team is undertaking a variety of different approaches to digitisation, channelisation and beamforming.

CASPER beamforming approach headed by Richard Armstrong of The University of Oxford, DAQ card / offline / supercomputer based beamformer approach headed by The University of Manchester.

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OSKAR Simulator

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Radiation Power PatternAperture Arrayhttp://wiki.oerc.ox.ac.uk/oskar

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MOFF Maths

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