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Some Comments to the W-projection algorithm.

Some Comments to the W-projection algorithm. L. Kogan National Radio AstronomyObservatory Socorro, NM USA.

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Some Comments to the W-projection algorithm.

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  1. Some Comments to the W-projection algorithm. L. Kogan National Radio AstronomyObservatory Socorro, NM USA

  2. 1.W PROJECTION: A NEW ALGORITHM FOR NON-COMPLANAR BASELINEST.J. Cornwell, K. Golap, and S. Bhatnagar, EVLA memo 67, December 2003 “MEMO”2. The non-complanar baselines effect in radio interferometry:W-projection algorithmT.J. Cornwell, K. Golap, and S. BhatnagarJuly 2008 “PAPER”

  3. . The visibilities with W 0 can be recalculated to visibilities with W=0 with the help of convolution. Frater R.H. was the first one discussing this idea (“Image Formation from Coherence Functions in Astronomy”, edited by C. Van Schooneveld, 1978)He considered the unpractical case of full UV coverage at each W layer. W-projection is implementation of this idea for the general case including the poor UV coverage in some W layers.

  4. The projection of the measured visibilities at W 0 to the desired visibilities at W = 0

  5. How wide should be the gridding function ? This question is very important because the wider it is the more computation time is required. The W projection memo says: “The support the W-dependent gridding function grows with both W and the field of view typically up to a largest value of about 70 by 70 pixels.” I evaluated the convolution (gridding) function analytically to verify this statement in particular.

  6. Analytical evaluation of the convolution function

  7. The full width of the convolution function for the 4 VLA configurations at wavelength 4m

  8. But the relation (*) is correct only if the measured visibilities are available everywhere. • In reality only few points of can be available at one W layer especially for large W!!!

  9. The computation time comparison • The authors of the W-projection declare its advantage in computation times 10-50 times. • But I heard this number as several times only! • The W-projection paper compares the computation time with uvw-space facets algorithm existed inside of CASA. • The big number can reflect the bad quality of uvw-space facets algorithm but not the good quality of the W-projection algorithm ! • It’d be more fare to compare with image-plane facet algorithm existed at AIPS.

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