[alma-config] Offset Array design/Protomemo

[John Conway]

Hi,

1)  I have produced a version of a zoom array centred on one of the
positions identified by Butler and Radford as being smooth and
level. The details of the array can be found at 

http://www.oso.chalmers.se/~jconway/ALMA/SIMULATIONS/SIM6/

This use the same outer circle as earlier designs, probably 
more circularlity can be introduced back in by moving the circle
to the North West a bit. Note that the degree of squinting 
in the beam is relatively  modest for all arrays. Not ethis array
still need to be optimised - I like Mark H suggestion of doing a
multi-reolution beam sidelobe optimation - also the choice
of inner pads to populate is not at all optimised yet, - D-array
requires osme work in this regard.

Note that this version of the zoom array also has added short spacings 
compared to the earlier strawperson. All arrays now have baselines
down to 25m. I agree with Morita-san and Mark H, and disagree
with Min and Leonia. Adding just a few short baselines can dramatcially 
improve the single configuration imaging performace at relatively 
little cost to the sidelobe level (adding one or two inner antennas
will of course decrease the resolution but this can easily be compensated
for by moving out the other 60 antennas a few percent in radius!, and the
degree of 'concentration' only increase a bit.)

To quantify the tradeoff between baseline length dynamic range
and sidelobes  -  using my version of the Kogan sidelobe minimisation
program  in this new array with added short spacings I get 6% peak 
sidelobe within 30 beams comapred to aboyt 4.5% in my earlier 
strawpersons which had much fewer short spacings.


2) I have also started a protomemo on the earlier non-offset 
design (the one being tested by Steven), including comparsions with 
the double ring.  This can be found at 

http://www.oso.chalmers.se/~jconway/ALMA/SIMULATIONS/SIM5/

This is somehwhat long already. You might want to look 
at Sects 4.1  and 4.2

Sect 4.1 which compares the radial density of uv points 
between the ring and spiral arrays. PLEASE NOTE - while it was
true a year ago that the radial distributuio for  25% 'donuts'
ring and zoom spirals was quite different, the remarkable thing
is that the doubling ring and zoom spiral are to first order 
now very similar(!) - despite starting with different philosphies 
- it seems that any array that shares 50% pf its pads between
arrays factors of 2 apart in resolution must be quite condensed. 


Sect 4.2 compares the fraction of filled cells within a uv circle 
set to the largest ring baseline for the ring and spiral arrays, 
For snapshots the spirals have 2% - 5% less filling fraction than 
rings but for long tracks the ratios are reversed. I need to check
this result, but it seems to be due to have more multiply sampled
cells for the case of zoom spiral.