[alma-config] Antenna locations

[John Conway]

Hi,

For the zoom array the pad positions can be found at 

http://www.oso.chalmers.se/~jconway/ALMA/SIMULATIONS/SIM4/

A list of pad positions in SAM56 coordinates us given in
http://www.oso.chalmers.se/~jconway/ALMA/SIMULATIONS/SIM4/PADS
these are based on the FITS terrain map, and should be OK to within
10m or so. The pads are numbered and plots of the pad 
distributions can be found by clicking on the appropriate
array size A- E and plotting the middle image which 
shows the pads for that array superimposed on the 
terrain mask.

Note that both the zoom spiral and the double ring array 
have quite  similar constraints in terms of terrain and 
share essentially the sames outer 3km diameter ring 
and have the same compact array.

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1) As Min said in his message, a main objective would be to
look at the site both strawpersons hav nominally chosen for 
the most compact  part of the array. The site is quite near the edge 
of a quebrada, and both designs have two pads on the 'island'
within this quambra (my pad numbers 140 and 141). I guess
for the compact array we need to check;

i) The general roughness of this site, how much bulldozing 
is required. 

ii) whether having the site so close to an 
quebrada  edge  is feasible given the amount of heavy construction 
machinery there will be.

iii) whether the soil/terrain characteristics are OK 
to allow pads on the 'island' in the quebrada  and whether
there is access via a transporter.

Note that in my mind one objective of the competition of the
two strawperson designs was to compare the imaging properties
of the two design philosphies on quiet literally 
the same 'playing field', thats one reason (the other was 
laziness/desire not to reinvent the wheel)
why I adopted essentially the same centre for the array as
for the Min and Leonia - tha fact that the two designs 
agree on this place does not therefore indicate its a 
magic place. I understand though that 
it may be close optimum if one wants to fit an outer  
circle, then a circle based on this center 
comes out of the the fitting with Leonias program as best. If 
the shape of the outer ring is relaxed somewhat  (see point 
3 below) then a wider range of centres can be used. 

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2) From some simulations I have done I think the uv coverage 
(at least for the zoom array, and probably for the rings) 
is not too badly effected as long as the compact centre 
is not moved by more than 20%-25% of the radius of the 
outer ring. Given this even keeping the same outer 
ring as in the two strawpersons we should consider
other sites for the  compact array.

If we keep the same circular ring as in the present strawpersons 
then  moving NW from the present E-array location there appears 
to be a relatively flat area around

628050, 7453700

and moving East, another at around 

627960, 7453200

Perhaps these could be checked. The first alternative 
position would seem to be prefered  since the terrain seems to
allow us to move the NW  part of the ring futher out toward the 
NW to compensate somewhat for moving the centre.

Basically though any region within the triangle
formed by the present E array position and the 
two alternative positions above might I think be 
accomodated by modifications of the present strawperson 
design(s)  Unfortuanetely if one of the original 3 identified
sites which are near the edges of the Chan site  is selected 
then that requires a substantial change in design.

One could also ask the question another way, from the 
terrain point of view which  place within this 
triangle would those visiting the site consider 
most practical?? If there was a strong preference
for a particular place we could probably design 
around it.

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3) If we have a different outer perimeterm such a Reuleaux
triangle shape the then the whole array could be moved 
somewhat to the N and W. An earlier zoom  design based on this 
perimeter was presented in 

http://www.oso.chalmers.se/~jconway/ALMA/SIMULATIONS/SIM2/

and had the compact array centred on 

628590, 7454000

Overall my impression was the terrain covered was somewhat less
rugged than in the stawperson arrays  and the compact centre was closer
to the gas pipeline. Again this place could be checked.

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4) Finally there is the question of the practically of
some of the pads in the 3km diameter ring common to both 
strawperson designs, Some of the pad positions selected to the 
South and SE  of  the ring are on isolated 'valid' pixels, 
how practical are  these place really?  are they only valid because 
they   are local maxima in elevation. I'm thinking in particular 
the pads labeled 170 - 177 in my design (which are also in the
double ring design at essentially the same places). Driving
to these places and looking at practicallity and access
might be worthwhile. Again if this area is unsuitable 
then this might suggest moving the outer ring somehat
to the NW.

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