[mmaimcal] Meeting

[Al Wootten]

Folks:

My plane leaves at 430 pm EDT tomorrow so I think we will NOT have a
meeting tomorrow.  However, please look at the ASAC agenda for 17 May
http://www.cv.nrao.edu/~awootten/mmaimcal/asac/asacmay01agenda.html
which is still in the process of buildup for material on the next frequency
bands, the correlator, the new receiver specs and more.

I am working on the charge from the committee to the Proj. Scientists:
The ASAC recommended in the report of its February meeting that "a careful
assessment of the cost/performance tradeoff for SSB vs DSB operation of
the receivers and their consequences for the IF subsystem and correlator 
should be undertaken in the next few months."

I don't know what Hasegawa and Guilloteau have in mind.

I'll try to distribute this asap and I would like comments.  
I could summarize: Of course, a critical
parameter in that examination has to be the mix of line and continuum
observations.  For the sensitivities given by Thompson and D'Addario in
ALMA Memo No. 304 one can show that e.g. at 225 GHz if continuum 
observations occupy
more than 53% of the time for a purely interferometric ALMA, DSB operation
would be preferred, depending on the band, weather and a host
of other variables.  I suspect that ALMA will be operated in line mode
much more often than half of the time.  In total power mode, continuum
enjoys a bigger advantage and DSB operation may be preferred if continuum
observations occupy more than 6% of the time.  Of course, in that case
one must figure out how to calibrate the sidebands, so I think that an
uninteresting case--and we are, after all, building an interferometer!
This all depends to some extent on weather and other assumptions but I
have used those in 304.

We have new numbers for receiver costs, though they have not been blessed
by the Front End Division Heads.  I estimate the cost of a DSB receiver by
using the costing for Band 7 used there.  I estimate the cost of a 2SB
receiver by using the costing for Band 6.  I have estimates of the cost
differential for the LO and backend also, which is nil.  
This gives the cost advantage per band of 2SB vs DSB in about as accurate 
a way as I can imagine. ($2M cheaper for DSB).

Thus, we have dollars for each case and sensitivity estimates for each
case.  We can compare for instance the cost of gaining the sensitivity
we would need to match DSB line observations to SSB line observations
in terms of, for instance, added collecting area.  I conclude we are better
off gaining the sensitivity through deploying SSB receivers.  Building
them is another question of course!

At the moment I am tweaking numbers and words...comments would, of course,
be welcome.  

Clear skies,
Al