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 8

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 10

11. Faceted approaches Can do in image plane or Fourier plane
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 ~ 11

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 13

14. Roughly the size of convolution function in pixels
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 14

15. W-projection

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

17. W projected image 17

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 19

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 20

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
Mosaicing issues
errors of the primary beam
Wide bandwidth
Joint vs. individual deconvolution

24. Thank you Australia Telescope National Facility Max Voronkov
Contact Us
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Australia Telescope National Facility
Max Voronkov
Software Scientist (ASKAP)
Phone:
Web:
Thank you