[alma-config] Ultra compact array

[Morita, Koh-Ichiro]

Colleagues,

in Japan, we are now discussing to make smaller dishes (6 ~ 8 m) for 
ALMA.  Many people want to know the merit of "enhanced" ALMA
with such smaller dishes and how many such smaller dishes are desired.

I would like to ask you any comments about following discussion.

Even for the compact configuration of "original" ALMA (64 x 12 m),
there is a central (U,V) hole around (U,V) distance from 0 to 1.3 D (D
= 12 m / \lambda).  Ultra compact array with the smaller dishes can be
recover this (U,V) hole.  

But, for mosaicing imaging with a single dish image, equivalent (U,V)
coverage of mosaicing is smoother than that of single pointing
observations.  So, we still have some response at the central (U,V)
hole.  The sensitivity level depends on illumination pattern of each
antenna but maybe 10 - 30 % of zero spacing sensitivity (Is it correct?).

If we want to recover such a central (U,V) hole by the ultra compact
array with the smaller dishes, the sensitivity at the (U,V) of the
ultra compact array should be larger than the original sensitivity of
homogeneous array.  It means that the number of the smaller dishes
should be 0.1 - 0.3 x (64 x 12^2 / 8^2) ~ 14 - 42.

Is this discussion correct?

Koh-Ichiro
--------------

Koh-Ichiro Morita
Nobeyama Radio Observatory
Nobeyama, Minamimaki, Minamisaku, 
Nagano 384-1305, Japan.
Tel: 81-267-98-4396
Fax: 81-267-98-4339
Email: morita at nro.nao.ac.jp