[asac] comments for Nov 13 ASAC telecon

[Christine Wilson]

Hi, everyone,

Unfortunately I won't be able to participate in Monday's telecon, as I 
have to teach a class. Here are some comments on some of the items from 
the agenda.

Chris


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.

Although Larry D'Adario's note had a lot of useful comments, it wasn't 
clear to me after reading it what exactly was the impact he would predict 
from changing the lower band limit to 86 GHz from 89 GHz. I think it 
would be helpful if he could focus specifically on the 86 GHz vs 89 GHz 
question (or 84 GHz vs 89 GHz, if the ASAC thinks 84 GHz may be justified 
scientifically) so that we can see more clearly what the effect of 
broadening the bandwidth on the overall system would be.

Of course, part of the motivation for broadening Band 3 and decreasing 
the priority of Band 2 is political, in that we may not end up with all 
10 receiver bands as originally planned. I think it would be helpful to 
focus first on what is needed, scientifically and technically, for Band 
3. The SiO maser line seems to me to be a strong argument.

(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.

In Hasegawa's memo on the scientific merit of the advanced correlator, I 
am a bit confused by his third example, searching for absorption line 
features agains bright nuclei or quasars. This may be because I don't 
work in this field, but I don't understand why we need the enhanced 
correlator to do this science. If the gas to be studied in absorption is
in the Milky Way itself, then the radial velocity would probably be known 
fairly accurately from the rotation curve of the Milky Way. If the gas 
was in another galaxy between us and the quasar, the radial velocity is 
often known to 100-200 km/s from optical spectroscopy. Since super-high 
resolution (< 1km/s) would not be required (at least not in an initial 
search), then a bandwidth of, say 400 km/s with 400 channels would be 
sufficient. Finally, if the gas was in the actual molecular torus around 
an AGN, again the velocity would still be known to 100-200 km/s accuracy, 
and the line-widths would be expected to be relatively narrow (few 100 
km/s, if I'm remembering some recent IRAM work properly). One way 
that I can see that the enhanced correlator would be helpful for 
absorption line studies is if one was doing a blind search; however, such 
a search could probably be done with the existing correlator, at the 
expense of more integration time.