[mmaimcal] Correlator comments

[Al Wootten]

As some of you know, I made some comments on the EVLA correlator at the
ASAC meeting.  The ASAC is finalizing its report on the correlator.
I wasn't sure from the discussion that my comments were received exactly as
I had intended.  After thinking about using WIDAR on the EVLA in pursuit of
some high z CO line at 35 GHz, and performing the same experiment on ALMA,
I realized ALMA would be backend challenged.  This persuades me of the
usefulness of the 2G correlator.  Carlson estimated to me that rubber stamping
WIDAR for ALMA use would cost about $20M.  The current estimate for the
2G correlator/JP including computing enhancements is about $55M (computing
enhancements must be added to Carlson's design but I'm not competent to do
that).  I summarized some points and sent them to Rafael (who writes 
the report):

'I would comment some portions of the text relevant to my remarks.  I was
trying to make two points which perhaps I didn't fully convey.

'I believe that the characteristics of the WIDAR
correlator, which will be operational before the 2G correlator, strengthen
the case that the ALMA 2G correlator should have similar or better 
specifications;
specifications which the baseline correlator does not fully meet.  The
ASAC might not favor a situation in which e.g. the EVLA/WIDAR completes a 
spectral survey of Orion at 35GHz more effectively than ALMA could, owing to
backend limitations on ALMA.  A similar case would hold for other science
projects given--from a redshift search for a high-z object (particularly
limiting for the ALMA correlator at 35 GHz where its best wide mode frequency
resolution is pretty broad) to absorption against a background object.

'Secondly, it has been my understanding that the correlator designs under 
consideration in Europe have many similarities to the WIDAR correlator.  
WIDAR is a nearly fully
designed correlator while the EU architecture is, to my knowledge, more
sketchy.  I believe that the path to establishing the best possible
architecture, in Chikada-san's unification suggestion, would include
consideration of the successful solution to architectural problems
which WIDAR has achieved. '

Comments?

Clear skies,
Al