[mmaimcal] Re: Array of 8m antennas

[Al Wootten]

Robert Brown writes:
 > 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.
This is probably true.  I believe there is merit to operation of the small
array at the higher frequencies also but receivers for these frequencies might
take the place of e.g. the 7mm system.  Although 7mm on the small array will
be interesting--for example, see Carlstrom's BIMA results, it will have
low sensitivity and its only unique feature would be the southern hemisphere
location.  One possibility is that the 850 GHz system be deployed first
on the ACA (or THzA) to give us some access to the submm sky while the
technology for Band 10 matures.  However, this array might not be constructed
first.
 > 
 > 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).
Again, if this is the primary purpose, I believe that 6m is the best diameter.
However, sensitivity might be an issue favoring larger diameters, in addition
to the attraction of constant receiver cabin size.
 > 
 > 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.
Stephane presented a number of configuration studies which Viallefond had
worked on at the Tokyo meeting--these are in the notes from that meeting,
which perhaps you have.  One good possibility was to cluster the antennas near
the ALMA Compact Array--say three groups of five.
 > 
 > 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.
I'm not sure that the claim that it is best for imaging is well supported.
 > 
 > 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.
The ACA may be critical in carrying the interferometer calibration to the
single antenna calibration however.  This suggests that the two be located
under the same weather conditions.  For some experiments at higher 
frequencies, it would be very useful to gain sensitivity by cross 
correlating ALMA with ACA antennas, with underilluminated ALMA antennas.
This feature would be compromised by the higher site, as would, I think,
the cross calibration accuracy.  Furthermore, it would be difficult to
maintain the ACA on a much higher site.

See you Thursday!

Clear skies,
Al