[alma-config]Re: [mmaimcal]Memo 428

[John Conway]

On Tue, 2 Jul 2002, Stephane Guilloteau wrote:

> 
> Be carefull
> 
>     - we cannot (yet) simply assume the ACA will be build
>     - however, we (astronomers) cannot continue dreaming to build whatever
> we wish for ALMA.
> 
> We have a budget constraint, and within the current uncertainties, saving 1
> M$ or so on 25 pads
> would be more than welcome. I would rather re-affect that 1M$ to a better
> calibration system. In that
> area, we have some unfunded items which would be useful, such as an
> automated FTS and its associated
> software. This would be useful for most projects, while the 25 pads for an
> elongated N/S compact configuration
> are at best only useful for 5 % of the projects.
> 
> Using the inner "spiral" pads and some re-weighting scheme is not optimal
> when considering fixed configurations, but if we add up the overhead time
> and associated losses due to reconfiguring to such special configuration,
> this becomes less obvious.
> 
> Given the imaging results shown by Mel Wright, I would strongly argue
> AGAINST the added complexity
> of the "elongated compact" configuration.
> 

I think we can achieve a fairly round beam out to dec +30 and 
pretty good short spacing coverage down to 15m between antenna
projected antenna centres, with little or no shadowing with 
about 6 - 8 extra pads rather than 20 as suggested 
in Mels memo. Building any more extra pads to get a excellent high 
surface brightness array for declinations above +30 degrees 
is probably a very expensive use of resources (some capability 
for dec >+30 low resolution will of course remain by E-W tapering 
larger arrays).  Targeting excellent capability out to Dec 30 
(which covers the Taurus region) we have 
about 75% of the sky  to play with and quite a lot of 
overlap betwen dec 0 and 30 with N.hemisphere radio
interferometers. 

I am still optimising the compact array concept using 
Mark Holdaways modification of Frederik Boones software 
and beam optimisation, which is why I havn't posted 
anything in the last week or two, however I can put the 
unoptimised  version of  on the web today to show how 
the concept is  looking. Mel can run his software 
calculayting sensitivities etc on that and we can 
decide on this concept for the compact array.


> And remember: the configuration design IS URGENT.
> 

I know and I am working intensively on it. I hope to have 
something by the middle  of next week and then have a free
month after I come back from vacation in August for final touches
and to write  up the documumention. This will the input to 
proposed Site Requirements review in Septemer.
This will be our proposed plan which
the  civil engineers will cost in September and October. 
Depending on that result and the  budget we may have 
to iterate.

 

> The current feeling of most members in the ALMA Management Advisory
> Committee is that we will need some descoping of the project, and these
> people certainly have a lot of expertise on large projects. Having to
> descope a receiver band would be disastrous. So we have to be careful in all
> items which have a small influence of the overall performance of ALMA, but
> result in some quantifiable cost savings..
> 
>         Stephane
> 

Understood. Which is why the documention defending the design is a
as important as the design itself.


 John