[mmaimcal] Summary of discussions on M51 imaging by ALMA SD ACA

[Mark Holdaway]

> I suspect that the 25% estimate is low for the ACA but I expect that for
> some of the time when the 12m array is in an extended configuration, the ACA 
> will not be used as often with data collected in that configuration.  During 
> good weather, it may be playing catchup.  There will be some periods when the
> ACA can make good use of high frequency weather but during which the large
> array will be better employed at lower frequencies.  This needs a more
> careful examination, to be sure.  

I think the "25%" number comes from assuming something about the
distribution of projects with configuration;  essentially 100% of
the projects observed in the most compact ALMA configuration will
also use the ACA, and perhaps it is assumed that 25% of all observing
time will be spent in the compact array.  (Folks like Jack Welch or
Frazer Own would have placed this number closer to 30% or 40%, but we
don't know one way or another.)

Now, the scheduling of the campact array will be some block of time
during the year (perhaps multiple times a year), and it will not
have a mopnopoly on good weather conditions.  So, there will be plenty
of time when the ALMA is in more extended configurations and the
weather is great so the ACA can "catch up".

Another consideration is the fact that many configurations will have some
short baselines and might mosaic.  Basically complete snapshot (u,v)
coverage will occur out to configurations of 150m; Mosaicing will be very
common in configurations out to 300m.  (I am assuming that for larger and
larger configurations, the fraction of mosaics will fall off quickly.)  
However, if you are matching sensitivites between the 300m configuration
and the ACA, you need only 1/4 as much sensitivity (ie, 1/16 as much time)
as if you were matching sensitivies with the 150m configuration:  SO,
getting ACA data for larger configurations is a minor consideration.


Note:  in recent times, the filling factor of the compact ALMA array has
gotten smaller than in the original MMA design.  One implication of the
(old) larger filling factor and more compact ALMA configuration is that a
more sensitive ACA would have been required to match it.  (There are other
practical reasons to favor a smaller filling factor other than just to
reduce the demands on the ACA.)


Note:  the ACA adds a LOT of complexity to the dynamic scheduling,
pipeline reduction, and data archive retrieval.  In addition, an
astronomer or pipeline cannot make an image as soon as the source has been
observed, but must wait until it has been observed with both instruments.  
I anticipate astronomers will often choose to forego the inclusion of the
ACA in order to streamline the imaging process and get results in a more
timely manner. However, we are possibly preventing the array from being
able to do this by only designing total power capabilities into 4 ALMA
dishes.


	-M