[mmaimcal] Correlator comments

[Harvey Liszt]

It has always bothered me that the ALMA and EVLA correlators
will come on line at about the same time and yet be so different.
All synergy has been sacrificed; the EVLA and ALMA will diverge
in their architectures and never be able to help one another.

It would have been more straightforward (not necessarily better)
if the ALMA correlator had been adopted for EVLA in the first
place, but buying into WIDAR for ALMA achieves the same common
endpoint.  In light of the added costs for software development 
for ALMA's 2G device, WIDAR for ALMA makes even more sense.

regards, Harvey


Al Wootten wrote:
> 
> 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

-- 
Harvey Liszt                            work:434.296.0344 fax 0278
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