[asac] comments for Nov 13 ASAC telecon

[Al Wootten]

Folks

Perhaps I can clarify some issues Christine raised a little.

Christine Wilson writes:
 > New Business
 > 
 > (2) Extension of Band 3 to lower frequencies
 > 
 > As I recall, a large part of the motivation of changing the lower 
 > frequency limit of band 3 to 86 GHz was to be able to access the SiO 
 > maser line at 86.2 GHz, which could be very useful during the initial 
 > testing of the array. This still seems to me to be a strong scientific 
 > and technical argument for pushing the band limit down to 86 GHz.
 > However, I've forgotten what the reason was for pushing it down to 84 
 > GHz.
My notes suggest that the reasons were
1) increased overlap with the VLBA frequency band, which goes down to 80
2) coverage of a methanol maser line at 84 GHz

...snip...

 > (3) Scientific Merit of the Enhanced Correlator
 > 
 > On reading the minutes of the informal correlator meeting, I was reminded 
 > again how much I personally need a "translator" to understand the issues 
 > in the correlator design. Steve Myer's memo from last fall outlining the 
 > main design issues for the baseline correlator was very helpful; I think 
 > we will need another memo in that style again when we are asked to make 
 > recommendations or choose between two or more correlator options.
 > 
 > One small point in those minutes: I thought that, although the baseline 
 > correlator was planned to be built for only 64 antennas, that there was 
 > room in the design to expand it to more antennas if the correlator group 
 > knew how many antennas are planned soon enough. I thought the hard upper 
 > limit was more like 80-90 antennas than 64.
The baseline correlator may be expanded to more antennas in a straightforward
fashion, as you recall.  In my presentation, I passed along the estimates from
John Webber that the cost would scale as (N/64)^1.6 and that the extra time
required would be a few months.  However, when correlator construction
commences, it will be increasingly difficult to accommodate more antennas.


Clear skies,
Al