Imaging 16cm hybrid array data - w term

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Imaging 16cm hybrid array data - w term

Postby Mark.Wieringa » Thu Sep 19, 2013 4:58 pm

Investigating a recent query about mysterious position offsets in 16cm ATCA data taken in the h214 array I found that contrary to my expectations the w-term can be significant for the Compact Array. According to 'Synthesis imaging in Radio Astronomy II' the effect is of the order:
theta^2*sin(z)/(2*206265) arcsec,
where theta is the distance to the field centre in arcsec and z is the zenith distance of the field.
For a 16cm observation at dec=-60 this gives about a 2" error in the position of a source at the edge of the beam, rising to 8" at 1 primary beamwidth from the field centre.
For long observations the distant sources get quite distorted and don't clean well especially when antenna 6 baselines are included in the image.
The solution is to use casapy clean with gridmode='widefield', wprojplanes=256. This restores the sources to their correct positions and removes the distortion and clean issues.

The following simulation images show the difference between miriad and casapy for h214 data at 16cm and a short E-W array.

Cheers,

Mark

PS the colour wedge used below is the newly added cubehelix scheme.

ewclean.png
ew214 data imaged with miriad
ewclean.png (120.51 KiB) Viewed 693 times

wclean.png
h214 data imaged with miriad
wclean.png (233.48 KiB) Viewed 693 times

wclean.casa.png
h214 data imaged with casa
wclean.casa.png (207.94 KiB) Viewed 693 times
Last edited by Mark.Wieringa on Tue Jul 05, 2016 2:45 pm, edited 1 time in total.
Reason: replace missing attachments
Mark.Wieringa
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Posts: 257
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