[daip] [evlatests] UVW incorrect sign convention - AM889 deep integration

[Jim Ulvestad]

Would this be caused if somebody has the sign of
the sideband wrong somewhere?  I'm trying to figure
out in my brain how this could propagate to a (u,v,w)
error, and I don't think I see how, but I throw it
out as a dumb idea anyway.

Jim

> Rick,
>
>     Check to see if the image of the p-band field or any other extended
> source) is reflected about the origin.    That's what we are seeing with
> the deep Lband observations.
>
> Here is our report
>
> =========================================================================
>
> Neil Miller and I have reduced several deep field observations, code
> AH889, over the last month, including the July 1 modcomp-free
> observation.  The July 1 data were easy to calibrate, but the deep
> field image was strange looking.  First, the position of the sources
> over the 20'x20' field did not conform to what is in the sky, via the
> NVSS, and to sources in overlapping fields.  Second, the image was
> much more w-term distorted even with the image faceting that is
> generally used.
>
> Both of these problems are caused by:
>
> THE UVW COORDINATES WRITTEN IN THE EVLA ARCHIVE FILE HAVE SWITCHED
> SIGNS.
>
> This would not matter if you also flip the sign of the phase, and
> define north and east differently (or something like this), but only
> the sign of the uvw coordinates were changed. Here is the evidence:
>
> 1.  The listing of the u,v,w in the aips data base of a source at the
> same sideral time before and after modcomp-free change-over does show
> that the u,v,w have the opposite sign but the phases do not.
>
> 2.  Running UVFIX on the July 1 data (recalculates the u,v,w using the
> antenna file, the time, and the source location), does indeed flip the
> sign of u,v,w, without touching the phase---as it is supposed to.
>
> 3.  The comparison of the July 1 image before UVFIX and after UVFIX
> shows that:
>
> a.  The image before is an approximate reflection through the phase
> center of the 'after UVFIX' data.
>
> b.  The UVFIX data cleans up sources far from the phase center much
> better because the sign error in the w-term does not reflect the
> image, but sort of doubles the sky distortion.
>
> CONCLUSION:
>
> 1.  Change the signs of U,V,W calculated by on-line system and placed in
> the archive data
>
> 2.  Running UVFIX after calibration on the SPLIT files corrects for this
> error.
>
> We are still progressing with careful imaging on this field to see if we
> get down to the expected noise level, and if there are any
> correlator-offset residual errors.
>
> Cheers,  Ed
>
>
>
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