[alma-config] 14 km configuration

[Bryan Butler]

all,

a couple of comments to mark's email:

>There was some concern here in Tucson when Bob Brown announced
>that the largest ALMA configuration was 14 km, whereas the systems
>group had been working with a 10 km assumption.  

dick sramek and i had a brief conversation about this.  as i
understand it, at least in terms of the FO system, they are
carrying a max fiber length of 25 km, which i imagine can be
accomodated with a 14 km diameter design.  there are certainly
other things that are affected by the max baseline length,
but are they really critically dependent on a change from 10 km
to 14 km?  have the systems folks come out and said that this is
a real problem already?

>If we relaxed the shadowing criteria, antennas could fit between
>Chajnantor and Chascon.  I didn't see any mention in Memo 296 what the
>shadowing elevation angle is.  A lot of the shadowing occurs in E-W
>observations, and there is no compelling reason not to relax the E-W
>shadowing (ie, make it 20 or 30 degrees elevation in E-W; no objects on
>the sky are made inaccessibe by that change, just the HA tracks are
>somewhat restricted).  If we relax the shadowing, the inner
>antennas in the donut could go closer to Chascon.  Additionally, the
>increased freedom in antenna placement would likely result in
>a superior configuration and beam.  In evaluating full track
>(u,v) coverage, we would have to consider shadowing by the mountains.

i'm not sure which mask leonia used when he did the work for
memo 296.  the current masks (that i constructed, anyway) include
the following criteria (this is described at
http://www.aoc.nrao.edu/~bbutler/work/alma/configurations/):

 "All masks include constraints that a valid antenna location cannot 
  be shadowed by more than 25 degrees in any direction, and cannot
  be shadowed by more than 15 degrees to the north (defined as within 
  30 degrees of true north), that the location must be interior to
  the science preserve boundary by more than 100 meters, and that the 
  location must be further than 400 meters from both pipelines. 

  They then further require that a valid antenna location cannot have 
  local slope greater than some prescribed value. The values I have
  used below are 5 and 10 degrees for the slope constraint."

>Scientifically, there may be a case for 14 km.  However, as the jump
>to 14 km was made for technical reasons rather than scientific ones,
>I would seek to fit something that was a more respectable jump from
>the 3km configuration (such as 10 km).

i agree that the jump from 3km might be a bit much.  part of the 
problem here is that we never changed the default array diameters
after the change to more antennas.  maybe we should be looking at:
150m, 450m, 1.3km, 4km, 14km?  which keeps the 3:1 scale change
that we had originally, and accomodates the fact that we have a
larger E config (most compact).  in this case, there isn't such
a mismatch between the most spread out 2 configs.  we have bandied 
this about a bit amongst ourselves, but there was never any 
"official" suggestion to do that.

	-bryan