A 2-PAD Correlator, Holography, and Direct Imaging
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Dual Polarisation All Digital Aperture Array Danny Price, University of Oxford CASPER Workshop 2009, Cape Town SA A 2-PAD Correlator, Holography, and Direct Imaging
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A 2-PAD Correlator, Holography, and Direct Imaging. Danny Price, University of Oxford CASPER Workshop 2009, Cape Town SA. Dual Polarisation All Digital Aperture Array. Introduction. Direct imaging correlators. Holography. 2-PAD Correlator. Part I: The 2-PAD Correlator. 2-PAD Correlator. - PowerPoint PPT Presentation
Transcript of 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
Introduction
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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
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
Dish Holography
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J. Baars et. al., Near-Field Radio Holography of Large Reflector Antennas, 2007
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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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
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
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
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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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
