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A 2-PAD Correlator, Holography, and Direct Imaging

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.

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A 2-PAD Correlator, Holography, and Direct Imaging

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  1. 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

  2. Introduction Direct imaging correlators Holography 2-PAD Correlator Danny Price

  3. Part I: The 2-PAD Correlator 2-PAD Correlator Danny Price

  4. 2-PAD Digital Backend Danny Price

  5. Adding a correlator Image: Ant Holloway • 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! Danny Price

  6. Part II: A Holography Primer Holography 2-PAD Correlator Danny Price

  7. Holography Primer Rectangle Function Sinc Function F.T. Danny Price

  8. Reflector Antenna Horn Antenna Holography Primer Danny Price

  9. Holography Primer Reflector Antenna Horn Antenna Danny Price

  10. Holography Primer Aperture Illumination F.F. Electric Field Pattern F.T. 2 |F| Radiation Power Pattern Danny Price

  11. Holography Primer Aperture Illumination F.F. Electric Field Pattern F.T. 2 |F| Radiation Power Pattern Danny Price

  12. 2-PAD Measurements Aperture Illumination F.F. Electric Field Pattern ? ? Radiation Power Pattern Photo: Ant Holloway Danny Price

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

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

  15. 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 Danny Price

  16. Holography: Method t s Danny Price

  17. Holography: Method Danny Price

  18. Part III: Direct Imaging Direct imaging correlators Holography 2-PAD Correlator Danny Price

  19. Direct Imaging Correlators Image: Ian Heywood • 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). Danny Price

  20. Direct Imaging Correlators M. Morales, Enabling next generation dark energy and epoch of reionisation radio observatories with the MOFF correlator, 2008 Image: Ian Heywood • 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 Danny Price

  21. Direct Imaging Correlators • Measure Electric field • Weight by noise & grid using holography • Spatial Fourier Transform • Square & integrate to produce an image Grid by holography Fourier Transform Weight by noise Danny Price

  22. Direct Imaging Correlators Danny Price

  23. Conclusion Direct imaging correlators Holography 2-PAD Correlator Danny Price

  24. Acknowledgements • SaschaSchediwy, Richard Armstrong, Kris Zarb-Adami, Jack Hickish, and Mike Jones • 2-PAD team • Oxford • Cambridge • Manchester • Pre-emptive acknowledgements • CASPER Workshop 2009 attendees! Danny Price

  25. Supplementary Material Danny Price

  26. 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 digitalbeamforming 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. Danny Price

  27. OSKAR Simulator Aperture Array Radiation Power Pattern http://wiki.oerc.ox.ac.uk/oskar Danny Price

  28. MOFF Maths Danny Price

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