[daip] Advice please

[Jim Ulvestad]

Looks like you have C array X-band data.

On 1829, you have either a large-scale halo or a confusing
source.  Try making an image with 512 pixels by 0.8 arcsec,
natural weighting (robust=5) and see what you find.  This 
image will cover the entire primary beam and should tell
you whether you have a background source or lots of diffuse
flux around your source.

The other source is quite strong, is it really supposed to
be 5 Jy?  You resolve the source strongly at about 10 kilolambda,
so that looks like some kind of 20" halo.  If you look
closely at the points between 0 and 10 klambda, they
bounce around a lot rather than showing a smooth trend.
This probably means that the resolution is dependent on
position angle; you have different correlated fluxes 
depending on the position angles of two baselines of
about the same length.  For this reason, the low points
at short spacings are not necessarily bad.  They may just
be taken at a position angle where you almost completely
resolve the source.

You also have uv minima at 35 and 70 kilolambda, so it
looks like you have a small double source.

So, if you image this source you should see two compact
components of about 100-200 mJy at a separation of a
few arcseconds, and an arcminute-scale asymmetric halo
with a few jansky in it.

The way to tell if the low points are bad would be to 
make an image that's cleaned fairly deeply, subtract
the clean components of that image from your data set
using UVSUB, then use UVPLT to plot the residual uv data.
If your map is a good representation of the source, the
UVPLT should show mostly data that looks like noise.
However, since you have incomplete sampling of the short
baselines and clean doesn't represent the large structures
very well, you probably still will see a fair amount of 
structure left over on the shortest spacings.

ju

Johan van der Walt wrote:
> 
> Let's try getting the attachments to you again.
> 
> On Fri, 18 Aug 2000, Jim Ulvestad wrote:
> 
> >Date: Fri, 18 Aug 2000 07:08:49 -0600
> >From: Jim Ulvestad <julvesta at aoc.nrao.edu>
> >To: Johan van der Walt <johan at fskdjvdw.puk.ac.za>
> >Cc: daip at zia.aoc.NRAO.EDU
> >Subject: Re: [daip] Advice please
> >
> >Johan van der Walt wrote:
> >>
> >> Attached are the uv plots of two of my program sources. In both cases
> >> the calibration has been applied. I have the following two questions:
> >>
> >> (1) 18290-0924: How should I interpret the larger fluxes at the
> >> very small uv distances compared to what seems like noise at larger uv
> >> distances? To me it seems to be too systematic to be bad data. I get
> >> the same type of behaviour also for another source.
> >>
> >> (2) 18056-1954: (a) It appears as if this distribution indicates a source.
> >> Is that correct? (b) The flux density falls rapidly with increasing uv
> >> distance. Between 0 and 10 there are quite a lot of points that fall
> >> below what is the general decrease of flux density with increasing uv
> >> distance. Are these bad points that need to be flagged?
> >>
> >> Thanks
> >>
> >> Johan
> >
> >I received no attachments ...
> >
> >(1) This certainly sounds like resolution/halo.  Or possibly a confusing
> >source.  What is your observing band, what is your configuration, and
> >is this a snapshot or a lot of data?
> >
> >(2) Can't make much of a response to this without seeing a plot.
> >
> >jim ulvestad
> >
> 
>   ------------------------------------------------------------------------
>                               Name: 18290-0924.PS.gz
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>                               Name: 18134-1942.PS.gz
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>                               Name: 18056-1954.ps.gz
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