[alma-config]misc config comments

[Mark Holdaway]

> I don't think large array configs need to have short spacings.
> Some overlap in spatial frequencies for multiple configuration
> cross calibrations is good, but no need to overdo it. One
> could make joint observations with ACA if there is a case for
> short and long at the same epoch.
> 

Sorry, this message is TOO LONG, but here it is anyway.

Short spacings are required for imaging rather than cross-calibration.

The idea behind extra short spacings in all configurations dates back to
about 1993, and has become a generally accepted constraint in the
configuration work.  In 1993, Robert Braun suggested that the VLA change
it's configurations by adding more short baseline information, reusing
pads in the C, B, and A arrays.  Also at that time, I realized that the
Keto triangle configurations, even for 40 antennas, had a much worse
central hole than the VLA's 27 antennas -- ie, in at least this one
respect, we were going BACKWARDS.

The idea is that the VLA's implementation of multiple configurations
scales the inner hole as well as the longest spacing.  This basically
requires that many "B" array observations also need to have some "C"
array, or even "D" array time.  From SNR equalization arguments, the time
required in "C" array will be about 1/100 the "B" array time (never done
in practice; usually a minimum of a couple of hours is granted in the
smaller configuration), and much less for "D" array, but the astronomer
must wait and organize multiple data sets.

Now: if so little time is required to "fill the hole" in the smaller
configurations, and that hole is really such a small part of the Fourier
plane (just happens to be the MOST IMPORTANT PART!), it seems that it
would take very few antennas being moved closer toegther to fill the hole
using the full time available to the more extended array.  The VLA has now
gone to the shortened C array (a modification of the C array made by
moving two antennas to otherwise vacant D-array pads) to implement this
idea.

Simulations by EVERYONE WHO DOES THEM (except for mosaicing simulations
with total power) basically indicate that if you don't simulate either
multi-configuration data or use an array with extra short spacings, you
make bad images of complex objects.  A minor amount of care in assuring
the presence of short spacings gives you excellent imaging. In fact, the
short spacings often dominate simulation results.

If one wants to act like a black belt, one may have theories about how to
shape the exact Fourier distribution by compining different
configurations' data with different observing times and different
weightings.  Or, if one feels they are really pushing the abilities of the
array on a very complex object the imaging of which would otherwise be
dominated by errors in the extended emission, one can require multiple
configuration data. However, for MANY projects which would otherwise
require some multi-configuration time, a single configuration with some
short spacings will do the job as required.  This is quick for the
astronomer and simple for the support staff, streamlining operations in
a telescope which is otherwise becoming bogged down by it's own
complexity.

Joint ACA-ALMA observations also go in the direction of complication,
while the single-configuration-imaging idea is trying to simplify life for
the astronomer (or scheduling/pipeline coders). Furthermore, if no care is
taken for short baselines in the intermediate configurations, there will
be a gap between the ACA's longest and the ALMA's shortest baselines, so
it won't work.

	-Mark