[alma-config]Comments on Memos 389 and 390 (fwd)

[Bryan Butler]

dave (and all),

>As I see it, we do need to make some decisions above and beyond
>the basic desire for Gaussian UV coverage that was agreed
>to a year ago.  (I was unable to attend last years meeting and
>am probably repeating a lot of what was said or presented there.)
>1.  How much reuse of pads do we want, range?
>     25-50%
>2.  Do we want to strive for positive PSF (without single
>dish data) out to ? times the synthesized beam?
>     add (how many?) short spacings or not
>3.  What is the criteria for the largest configuration? 
>     Gaussian UV or circle array
>4.  What criteria do you want for the most compact configuration? 
>     Gaussian UV or filled aperture
>5.  Where does the ACA fit in?
>     off to the side or incorporated into the middle of the array
>There are probably other issues that need to be discussed and
>decided.  We should have a list of the these.

sounds an awful lot like starting from scratch to me.  anyway,
most of these were indeed discussed at the PDR.  recommendations
from the committee included:
 - the total number of pads is not to exceed 250.  this then
   determines 'reuse', given number of antennas and number of
   'configurations' (though this is a slippery term in john's
   designs).
 - the largest configuration should maximize resolution - i.e., circle.
 - the compact configuration should maximize brightness temperature
   sensitivity - i.e., pack 'em in (with access constraints - ala 
   leonia's recent designs).
 - the ACA was discussed at great length, with several recommendations.
you can find the report of the PDR on the web at:
http://www.alma.nrao.edu/administration/designreviews/configurationpdr.html 

>This is clearly more than we can accomplish during infrequent meetings
>over the next two months.  

um, yes, this is my point about opening up the discussions
completely again.  if we do so, we will likely not be able to make 
significant progress in the next couple of months.

>We either need weekly meetings with
>a lot of people doing lots of work or a more sensible pace for
>arriving at the final master plan.  To this end what is it that is
>actually needed in December and what is going to be done with it?
>We surely aren't going to be pouring concrete, the multiple competing
>antenna prototypes with different pad requirements are more than a
>year from a decision point.  Basic array layout in terms of where 
>the array center will be and where the buildings will be put could
>be used for initial road construction.  This can be handled by allocating
>a building space in some sensible place and a major access road to
>the building to the mask.  Conway's zoom spiral configurations
>could be used for a first cut to determine where the buildings and access
>road might fit.

on timescales, although we will not be pouring concrete for
probably at least another year (for the pads at least), a campaign
to survey the potential pad locations is not trivial, and will
likely take several iterations, each with many people there for
a significant amount of time - probably occupying at least
1 year and maybe more (depending on manpower).  if everybody
on this list were willing to commit to 3 10-day trips to the 
site in the next year, we could probably do it (although we
probably couldn't afford the travel!).

in terms of initial road construction, i think it's more
complicated than simply knowing where the array center
(which was also specified by the PDR committee) and buildings
are (which is certainly *not* known).

>My feeling is that the most compact configurations and the location of
>the ACA are the most contentious and will take awhile to resolve.

the compact configuration is well in hand, from my perspective.
leonia has several good designs, and it's a matter of choosing
between them.  they follow the recommendation of the PDR
committee, with different road/access designs.

>To this end we should reserve something on the order of 200m diameter
>circle with no interfering buildings or access roads. 

again, this is done, essentially.  you should have a look at
leonia's ALMA memo 355 (although he has newer designs).

the troublesome one is the ACA, which we've (well, not me, but
others on this list) been hitting pretty hard with the simulations.
not where to locate it, necessarily (i think the strawman is to
put it 'on the side' of the compact configuration), but whether
it is *really* needed at all.


	-bryan