[alma-config] the ring array as an array with complete UV coverage

[Leonia Kogan]

Dave,

You wrote:

>The simulations that Kogan did in
>memo 247 are particularly disturbing.  It is hard understand
>why the uniform circular array image in fig. 6 b did not 
>find the three small sources to the right of the field center.  
>It is hard to believe that this is fundamental in the 
>circular array UV coverage.

First of all all three model (not only fig 6b) which I used in my simmulation 
demonstrate disadvantage of the uniform ring array in comparison with my 
configuration. As I know the same result was obtained by J. Conway.

This fact can be explained by the too big field of the model which was
selected in my simmulation. The bigger the field the smaller size of
the array's antenna should be. If the field is to small the ring array
will not have complete UV coverage because the diameter of  the
antennas wiil be less than spacing between the antennas. It can be
illustrated on the language of the sidelobes.  When the antennas have
very small diameter 100% grating lobes appears if UV coverage has an
uniform distribution. Selecting the antenna's diameter to be equal to
the spacing, we are limiting the sky by the primary beam cutting off
the grating lobes. For the smaller antenna primary beam is too large to
suppress the grating lobes and they create a problem in the image
quality.

As I showed in my previous message the used models are close to the
allowed limit but small enough for my third model.

So the disagremment can be explained by the upper explanation only partially.

Thank you

Leonia