[alma-config] a question about configuration with complete UV coverage

[Leonia Kogan]

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>From mholdawa at cv3.cv.nrao.edu Sun Feb  6 14:24 MST 2000
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Date: Sun, 6 Feb 2000 14:24:00 -0700 (MST)
From: Mark Holdaway <mholdawa at cv3.cv.nrao.edu>
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To: "[Leonia Kogan]" <lkogan at zia.aoc.NRAO.EDU>
cc: dwoody at caltech.edu, alma-config at zia.aoc.NRAO.EDU
Subject: Re: [alma-config] a question about configuration with complete UV
 coverage
In-Reply-To: <200002060120.SAA13546 at bonito.aoc.nrao.edu>
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> I have read your ALMA memo #270 "ALMA configurations with complete UV coverage".
> Of course the idea of configurations with complete UV coverage is very 
> attractive. At the same time I see some contradiction with a simmulation 
> carried out by me  comparing uniform ring array with a configuration 
> having minimum side lobes (my memo #247).
> My simmulation was carried out for snapshot experiment without 
> mosaic technick. 
> 
> The simmulation shows the obvious advantage of the array with minimum sidelobes 
> under the uniform ring array for the all three source model considered 
> in the memo. At the same time the uniform ring array can be considered 
> as a configuration with complete UV coverage for  snapshot observation 
> if the diameter of the ring is small enough.
> 
> I gess the demonstrated disadvantage of the uniform ring array is 
> explained by the fact that I did not use mosaic technick in my simmulation.
> Is it right explanation?
> 
> But still this example shoes that a configuration with complete UV coverage
> may be not optimum if observation does not use mosaic.
> 
> Now the question to the ALMA people:
> 
> Will all ALMA's observations use mosaick? 
> 

I think there is a general lack of understanding on the relationship
between complete Fourier Plane coverage and our imaging algorithms
and the quality of images they produce.  Keto's "most uniform coverage"
and Woody's arguments for "complete coverage" are conceptually very nice,
but do not necesarily lead to better images given the imaging algorithms
we use today.  

It has been estimated that between 25% and 75% of ALMA observations
will be mosaiced, depending on who you talk to.  While the complete
Fourier plane coverage is even more compelling for mosaicing observations
(because you have no support constraint -- ie, the field is full of
emission, so you need something like complete Fourier plane coverage),
it is still unclear that these configurations will produce superior
images for mosaicing.

One thing which I found helped the uniform coverage arrays was a
taper in the Fourier plane -- which made a more gaussian coverage,
and the PSF sidelobes were reduced, providing improved imaging -- but
to get the better imaging, you need to throw away a LOT of sensitivity and
resolution in the taper.


	-Mark




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