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Wide-field imaging

Wide-field imaging. Max Voronkov (filling up for Tim Cornwell) Software Scientist – ASKAP 1 st October 2010. General information. This presentation is heavily based on the original presentation by Tim Cornwell. Further info in the White book and Tim’s presentation. In this talk:.

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Wide-field imaging

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  1. Wide-field imaging Max Voronkov (filling up for Tim Cornwell) Software Scientist – ASKAP 1st October 2010

  2. General information This presentation is heavily based on the original presentation by Tim Cornwell Further info in the White book and Tim’s presentation In this talk: This talk is about algorithms…. But I will not give recipes.

  3. Instantaneous FOV

  4. Instantaneous FOV

  5. Dynamic range concept

  6. Dynamic range concept

  7. Structure of an imaging algorithm

  8. Non-coplanar baselines Two-dimensional Fourier transform is only an approximation Baseline component towards source Equation for celestial sphere • Points far from the phase center are defocused • Effect is important if • Not a problem at all if w=0 Strange requirement

  9. Standard 2D reduction

  10. Non-coplanar baselines • Point sources away from the phase center are distorted • Bad for long baselines, large field of view, and long wavelengths • Fix: use faceted or w projection deconvolution

  11. Faceted approaches Approximate integral by summation of 2D Fourier transforms • Can do in image plane or Fourier plane • Fourier plane is better since it minimizes facet edge problems • Number of facets ~

  12. Faceted approach

  13. Origin of non-coplanar baselines effect If we had measured on plane AB then the visibility would be the 2D Fourier transform of the sky brightness Since we measured on AB’, we have to propagate back to plane AB, requiring the use of Fresnel diffraction theory since the antennas are in each others near field

  14. Fresnel scale Fresnel scale = size of region of influence If Fresnel scale > antenna diameter, measurements must be distorted Roughly the size of convolution function in pixels

  15. W-projection

  16. The convolution function Image plane phase screen Fourier plane convolution function

  17. W projected image

  18. DR limitations

  19. Sources outside the field of view Sidelobes from sources outside the antenna primary beam fall into the field of view Can deconvolve if the convolution equation is obeyed BUT probably not…. Due to…. Non-symmetry of primary beam Non-isoplanatism Likely to be a limitation for wideband telescopes Can probably correct Some problems doing so

  20. Rotating primary beam Primary beam is not rotationally symmetric e.g. antenna feed legs As it rotates on the sky, sources low in the primary beam are modulated in amplitude Can be 100% modulation

  21. ASKAP 3-axis antenna mount • 3-axis mount allows us to keep beam pattern fixed on the sky

  22. Mosaic example

  23. This was just a tip of an iceberg Bandwidth and Time-average smearing Reobserve with a better spectral or time resolution Ionosphere (non-isoplanatism) For small baselines can fit Zernike polynomial phase delay screen Pointing errors Wide bandwidth effects Polarization of the primary beam Second order effects which may/will be significant for SKA e.g. see my presentation from the last synthesis school http://www.narrabri.atnf.csiro.au/people/vor010/presentations/MVoronkovSynthSchool2008.pdf Mosaicing issues errors of the primary beam Wide bandwidth Joint vs. individual deconvolution

  24. Contact Us Phone: 1300 363 400 or +61 3 9545 2176 Email: enquiries@csiro.au Web: www.csiro.au Thank you Australia Telescope National Facility Max Voronkov Software Scientist (ASKAP) Phone: 02 9372 4427 Email: maxim.voronkov@csiro.au Web: http://www.narrabri.atnf.csiro.au/~vor010

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