[mmaimcal] Brown message on small antenna array

[Darrel Emerson]

I've been out of email contact for a few days, and only just read Bob's
note.

   Probably someone else has already pointed the following out.

   Bob's statement about the missing spatial frequencies being from
12m to 1.5*12m isn't quite right.  With a 12m antenna the reponsivity
falls to zero at 12m, and with a 1.5D spacing the responsivity of the
interferometer falls to zero at 12m*(1.5-1) = 6m.  So the worst covered
spatial frequency is centred on about 9m.  The 12m as a single dish covers
spatial frequencies below 6m fairly well, and the 1.5D (=18m) spacing
interferometer covers spatial frequencies above 12m fairly well.  So it's
the range between about 6m and 12m we would like the complementary array
to cover best.
  Of course, details of aperture illumination, shadowing and
foreshortening come into it, to name but a few.

		Just a detail,
			cheers,
				Darrel.

On Mon, 26 Jun 2000, Al Wootten wrote:

> 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
> 
> 
> 
> ------- end -------
>