[mmaimcal] Re: THE NECESSITY OF N > 4 SUBARRAYS FOR ALMA

[Larry D'Addario]

The fact that someone can invent a way to use many subarrays does not
justify their construction.  Although each of the functions in Steve's
list of 8 will sometimes be needed, most will be needed only a small
fraction of the time.  They can therefore all be accomodated with,
say, 3 subarrays by executing each of 6 of Steve's functions
sequentially rather than simultaneously.  The scheduler might have to
think a little harder, but that is the only cost.

I think the VLA experience (reported by Dick Sramek at the recent ALMA
Systems Design PDR) is telling:  the 5-subarray capability has never
been used, and 4 subarrays are operated only on rare occaisions.  Yet
all of Steve's functions are equally applicable to the VLA.

If ALMA were to require 8 subarrays most of the time, it would follow
that most observations require far less collecting area than is
provided by the full array.  If that's true, how can we possibly
justify to the funding agencies the enormous expense and operational
difficulties of the huge 64-element array?

As a matter of design philosophy, applicable not only to this point
but also to any suggestion of a costly enhancement to the telescope,
we should always ask whether the proposed enhancement enables some
science that would be impossible without it.  If so, then the value of
that science (including whether it should be done with this telescope
as opposed to some other) can be weighted against the cost.  If the
enhancement merely makes some observations more convenient or faster,
where the observations are still possible without it, then building
the enhancement is far harder to justify.

--Larry