[mmaimcal] Brown message on small antenna array

[Al Wootten]

From: Robert Brown <rbrown at NRAO.EDU>
To: guillote <guillote at iram.fr>
CC: awootten at NRAO.EDU
Subject: Array of 8m antennas
Date: Mon, 26 Jun 2000 10:20:49 -0400

Stephane:

In preparation for the Japanese visit Wednesday/Thursday I've been
looking at the question of the array of 8m antennas.  I've been calling
this the "ALMA Complement Array" (ACA) just to have a name.  Below is
the logic I've been using; I would appreciate your views on any of this.

1.  For operational simplicity the same receiver (front-end?) package
that is used on the ALMA 12m antennas should be used on the ACA
antennas.  Thus, the door to the receiver cabin on the ACA antennas is
1.1m wide.  I believe this consideration alone drives us to rather
larger antenna diameter, the receiver cabin has to be big.

2..  The primary purpose of the ACA is to measure accurately the
uv-spacing corresponding to the range from 12m to about 1.5*12m.  (There
are many secondary purposes).  Therefore, if the ACA uses 8m antennas
with 1.5D configuration spacings we can accomplish the primary
objective.  (if 1.5D could be made 1.4D this is even better).

3.  Using ring arrays, the simpliest approach to maximize the number of
1.5D spacings is to surround the central element with an antenna ring
with ring radius 1.5D.  Four antennas fit on that ring, so the total
array is 5 antennas.  This is too sparse, not enough 1.5D
interferometers, unusable if even one antenna fails.  We need to add
another ring, this time at radius 3D; 6 antennas fit on that ring.  So
now we have 11 antennas in the ACA.  This is pretty good--it gives many
interferometers at 1.5D.  But it can be improved by "squashing" the
array either E-W or N-S and adding another 2 antennas.  Then it is
really good--you can get 18 interferometers measuring on 12-14 m
spacings, and good coverge out to ~30m which will give plenty of overlap
with ALMA so that ALMA and ACA can be easily put on the same calibration
scale.

4.  I conclude that the ACA should be 11-13 antennas.  An antenna
diameter of 8m seems best both from an imaging and sensitivity
perspective.

5.  ALMA and ACA do not have to be co-located because we can carry the
calibration of one to the other.  There is no need to cross-correlate
ALMA and ACA.  Therefore we have the possibility to locate the ACA at a
higher elevation and take advantage of the 200micron window if that is
seen as desirable.


When the Japanese arrive on Wednesday it is probably easiest if we phone
you.  Can you give me a number to call?  I suspect we will begin about 9
am EDT.

    -Bob



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