[mmaimcal] ALMA Correlator

[Al Wootten]

I was just going over the minutes of last Thursday's ASAC meeting 
(http://www.cv.nrao.edu/~awootten/mmaimcal/asac/asacmay01minutes.html)
and some statements there got my dander up.  In particular...

Future correlator science was discussed.  The ASAC minutes state:
"S. Guilloteau
noted the potential imact of a large future correlator. With a large
bandwidth and high sensitivity it will be possible to observe several
lines simultaneously, providing homogeneous data sets. The large bandwidth, 
higher sensitivity and larger number of channels will be
particularly important for high redshift galaxies surveys. Other projects
requiring simultaneous observations of several lines to provide homogeneous data
sets and planetary studies will also greatly benefit. M. Yun commented on the 
high data rates required."

In my opinion, the case for the future correlator is considerably overstated
here.  Several lines may be observed simultaneously with the baseline
correlator providing data sets of equal homogeneity.  The bandwidth of the
baseline correlator is equal to that of the future correlator (we still
have NO DOCUMENT describing the future correlator to the detail available
in ALMA Memo No. 194).  There
is higher sensitivity in the future correlator in the broadest band modes
but the sensitivity is equal for spectral windows such as those which can
be achieved with existing correlators and which will be employed in at least
55% of observations, according to a pessimistic (my opinion) estimate by 
Guilloteau in his DSB/SSB memo.  The resolution of the baseline
correlator in its broadband mode is sufficient to provide good signal on
any known high redshift galaxy, with 25 km/s channels at 1.3mm.  This will
decrease to 50 km/s at 3mm, arguably marginal (opinions?) and really won't
be good at 7mm, where it is 100 km/s or so, but we aren't now planning to
build the latter band in early phases of ALMA.  The datarates
expected of ALMA, 6 MB per second normal, will accommodate the 512 channels
of the baseline correlator while it is not at all clear that this will
be the case for the future correlator.

I would be interested in hearing if others feel similarly.  I will send
the above to the ASAC management unless someone convinces me I shouldn't.

Clear skies,
Al