[alma-config] Ultracompact array

[Ed Fomalont]

Hi Mark and others,

     I have a few questions concerning mosaicing and the inner u-v
hole on u-v coverage.

     Since mosaicing may be used alot for the smaller configurations,
perhaps the u-v coverage should be optimized for more robust mosaicing
at the expense of slightly poorer beam characteristics.

     I believe that true mosaicing depends mostly on fringe spacings
which are between 20% to 50% of the FWHP primary beam size.  It is
with these spacings that the primary beam shape interacts with the
more extended objects, so that short spacing information can be
obtained.  If this is correct, would an array with an excess (probably
slight) of shorter spacings be more mosaic-friendly?

     For the u-v and PSF simiulations, what is the status of the u-v
hole in the middle of the array?  Can it be assumed that somehow the
visibility at these short spacings will be measured (by total-power
phased-array imaging around the source or by mosaicing or ???) - and
thus should be filled in at some level for the simulations?  Or, should
the hole remain for the simulations?  The reason I am asking is that
the distortions caused by this u-v hole may actually dominate certain
types of sidelobe behavior for most reasonable arrays, particularly the
more compact arrays.

     Cheers,  Ed