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

Introduction Danny Price Direct imaging correlators 2-PAD Correlator Holography

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

2-PAD Digital Backend Danny Price

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! Danny Price Image: Ant Holloway

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

Holography Primer Rectangle FunctionSinc Function Danny Price F.T.

Holography Primer Danny Price Reflector Antenna Horn Antenna

Holography Primer Danny Price Reflector Antenna Horn Antenna

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

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

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

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

Dish Holography Danny Price 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 Danny Price

Holography: Method Danny Price s t

Holography: Method Danny Price

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

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). Danny Price 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 Danny Price M. Morales, Enabling next generation dark energy and epoch of reionisation radio 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 Danny Price Weight by noise Grid by holography Fourier Transform

Direct Imaging Correlators Danny Price

Conclusion Danny Price Direct imaging correlators 2-PAD Correlator Holography

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! Danny Price

Supplementary Material Danny Price

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. Danny Price

OSKAR Simulator Danny Price Radiation Power PatternAperture Array

MOFF Maths Danny Price