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: This talk is about algorithms….

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  1 But I will not give recipes.

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Instantaneous FOV

Instantaneous FOV

Dynamic range concept

Dynamic range concept

Structure of an imaging algorithm

Non-coplanar baselines • Two-dimensional Fourier transform is only an approximation

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Baseline component towards source Equation for celestial sphere    Points far from the phase center are defocused Effect is important if 

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Strange requirement Not a problem at all if

w=0

Standard 2D reduction

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

Faceted approaches • Approximate integral by summation of 2D Fourier transforms

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   Can do in image plane or Fourier plane Fourier plane is better since it minimizes facet edge problems 3 

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Number of facets ~

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Faceted approach

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

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Fresnel scale 

B

• Fresnel scale = size of region of influence • If Fresnel scale > antenna diameter, measurements must be distorted

Baseline length

350 1000 3500 10000 35000 100000 350000 4

37 63 118 200 374 632 1183

1

Wavelength

0.21

19 32 59 100 187 316 592 9 14 27 46 86 145 271

0.06

5 8 14 24 46 77 145 Roughly the size of convolution function in pixels

W-projection

The convolution function Image plane phase screen

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W projected image

DR limitations

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

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

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

Mosaic example

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

Australia Telescope National Facility

Max Voronkov Software Scientist (ASKAP) Phone: 02 9372 4427 Email: [email protected]

Web: http://www.narrabri.atnf.csiro.au/~vor010

Thank you

Contact Us

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