The current discussion on the "Exclusive use" area on Chajnantor is clear 
for the 6x6 inner region, but open to more uncertainties for the 14 km  
array because of the two options. If we can manage with Chile to keep the two  
options open for slightly longer, this is fine. However, we may have in the  
course of the negociations to give up some option, or in a more general way 
to relax our request.

Here are some arguments in favor of the Y+ configuration:
1) more continous reconfiguration, which gives more flexibility and 
less time lost in non-optimal hybrid configurations.
2) less roads, so cheaper
3) less roads, so shorter reconfiguration time
4) shorter fibers, so probably better phase stability
5) better expandability towards even longer baselines
6) more flexibility in re-weighting (tapering and/or uniform) data to
adjust angular resolution
7) possibly less pads, because no complex hybrid between the 5 km  
Gaussian
and the 14 km array is needed.

Arguments in favor of the ring configuration (14 km ring) are:
A) Slightly better resolution
B) Better astrometric performance 

Apart for argument 7), which will always result in some tradeoff of cost  
vs imaging quality, for the Y+ configuration, argument 2 is probably the  
weakest. 
The costs of the roads will be smaller, but as pointed out by John  
Conway, we nay need to place antennas at the bottom of some quebrada, 
where pads  could be significantly more expensive.

For the 14-km ring, neithe A) or B) are decisive, since an proper choice  
of the Y+ configuration can produce the same angular resolution (but not  
exactly the same astrometric capabilities because of the different 
distribution  of baselines). Moreover, argument 5) indicates we could 
ultimately (if  needed) get even better result with a Y++ configuration.

The current project thinking is to negociate more land for the Y+(+) 
configuration, rather than to keep the 2 options (Y+ and ring) open.

We would like to ASAC to provide us with a written recommendation  
concerning this approach.


	Stephane Guilloteau