[mmaimcal] Munich

[Michael Rupen]

I hate to weigh in on this stuff yet again, but we did think about this stuff
quite a while back.  I've emailed this group before (10sep99) about subarrays;
4 is ridiculously low, though this depends to some extent on how they are 
defined.  Consider
   1 subarray for standard interferometry
   1 subarray for VLBI
   1 subarray for testing
   1 subarray for single dish observations (with however many telescopes)
Sounds great...but what if some of your antennas aren't equipped at all
frequencies?  Surely that will be very common, since even if all the receivers
were built simultaneously it will still take quite a while to put them on
all those telescopes & test them.  So 5 strikes me as an *absolute* minimum,
and I would argue for 6 or more, if you want also to respond to transient
sources without losing the whole array, or observe bright time-variable
sources (e.g. solar flares) simultaneously at multiple frequencies.

On the single dish end: trying to support an instrument as sensitive and
flexible as ALMA with one 12m antenna will be difficult if not impossible.
Mark & I wrote a few words on this back in 1995 (memo 128, p. 7), pointing
out that if you want to make good mosaic'd observations you would have to spend
about 1/4 your time doing total power measurements -- assuming ALL the 
then-40 antennas were useful in those measurements.  Going with only one
single dish is equivalent to the assumption that only a few per cent of
interferometric obs. will require total power data, even under the assumptions
that (1) the single dish is working in total power mode all the time, 
(2) the weather is fine for SD work at the relevant frequencies to match
those interf. obs. (this is not at all clear -- the SD after all will be
MUCH slower than the full array, by an amount which depends on the number
of dishes [is this still 64? or has inflation set in again?], so finding
good weather for high-freq. mosaic support may be difficult), and (3) there
are no problems with cross-calibration, meaning that there are no
benefits to obtaining SD and interf. data simultaneously.  This last seems
quite an assumption, given variable weather; I would think it would help
considerably to have SD and interf. data taken at the same time, given
changes in the atmosphere as well as niggling details like pointing etc.
where calibrations could neatly be applied to both data sets at once. And
of course I'm unhappy with the idea that the array will only seldom do
mosaics; at a minimum I'd like to see that stated explicitly, with a real
scientific justification.

On what timescale does >1 SD need to be defended?  and is there any hope of
actually doing so, or is this another one where we'll make a great noise but
have no impact?  Sorry to be grumpy...some of these decisions just do not
seem sensible.

              Michael