[alma-config] Re: Near-in sidelobes

[John Conway]

hi,

> 
> As Holdaway suggested, Morita-san's report on the near-in sidelobe
> for the zoom spiral may be wrong -- I hope Morita-san can clear up
> this for us.  The old comparison John mentioned is outdated
> since Morita-san is using his latest topo-friendly zoom spiral.
> The new donut arrays also have rather different characteristics 
> from the old ones.  Of course, all I wanted was to include all of
> the latest strawperson configurations in the analysis Morita-san is
> conducting at the moment.
>

1) It wasn't my new topo frriendly version, just another version
that obtained some shorter spacings in a different way - my topo
friendly version is not on my web site yet.
 
> Using a smaller Gaussian beam to get around the problem of weak 
> sources on the Gaussian wing of a brighter source is not a real
> answer -- we are all familiar with problems associated with
> super-resolution, and this ad hoc answer goes against what makes
> the Gaussian dirty beam attractive.  The VLA example I mentioned 
> was just an illustration of the problem I was trying to describe.  
> As Leonia will discuss further in his own e-mail, the dirty beam
> from zoom spiral has a large wing around it, most of which also
> has to be negative (to have zero net power under the beam, the
> true dirty beam should have a negative DC offset).  I suspect the
> ring of near-in sidelobes in the donut arrays will ultimately
> limit its imaging capability at some point, but the Gaussian
> wing, which is negative, is also of some concern.
> 

2) OK I'll be interested to read Leonia comments - I'm glad 
we agree at least that uv coverage does after all influence 
the shape of the near-in sidelobes.

     John.