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

[Leonia Kogan]

Sorry for the previous emty message I sent erroniously

Mark, Dave,

I found another possible reason of some contradiction with a simmulation 
 carried out by me  comparing uniform ring array with a configuration 
 having minimum side lobes.

In my simmulation I had 36 element on the ring of diameter 95 meters.
Therefore the spacing between antennas is ~8m. As I understand the memo 
#270 the ring array will give complete UV coverage if diameter of the array's 
antenna is approximately equal to the spacing.
If the source model should be smaller than the primary beam. If not than
the simulation corresponds to the smaller diameter of the antenna and 
it will not be a case of the complete coverage.
So I checked the size of the field of my three models I used in my memo 
#247.

Model#1  Lambda = 0.1mm Primary beam width = lmabda/8meter = 2.5"
         The model size = 2.5"x2.5"

Model#2  Lambda = 0.32mm Primary beam width = lmabda/8meter = 8"
         The model size = 6"x10"

Model#3  Lambda = 32mm Primary beam width = lmabda/8meter = 13'
         The model size = 3'x7'

For the models 1 and 2 the model size is not less (but equal) to
the primary beam of the 8 meter antenna.

For the model 3 the model size is obviousy less than
the primary beam of the 8 meter antenna.

And still the ring array reproduces worse image for all 3 models.

These estimation are very aproximate and I might miss a coefficient 
of order 2 somewhere.

Thanks

Leonia

----- Begin Included Message -----

>From mholdawa at cv3.cv.nrao.edu Sun Feb  6 14:24 MST 2000
X-Authentication-Warning: water.tuc.nrao.edu: mholdawa owned process doing -bs
Date: Sun, 6 Feb 2000 14:24:00 -0700 (MST)
From: Mark Holdaway <mholdawa at cv3.cv.nrao.edu>
X-Sender: mholdawa at water.tuc.nrao.edu
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>
MIME-Version: 1.0
X-Lines: 52
Status: RO


> 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




----- End Included Message -----