From: Dick Crutcher <crutcher@uiuc.edu>
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Subject: [Anasac] polarization, and comments on charge 2
Date: Fri, 18 Feb 2005 17:36:47 -0600
At today's telecon I was asked if the polarization "widgets" on the 345 GHz
receiver could be used to do polarization work even though the dual
polarization receiver option were not implemented. What I said about how
BIMA does polarization work was correct, but for ALMA there are major
implications. First, it would be necessary to switch the "widget" so each
telescope would successively observe R and L circular polarization. It
would be necessary to go through all possible combinations on each baseline
within a UV cell move time, so that one would have the RR, LL, LR, and RL
combinations on each baseline. I don't think this ALMA widget is designed
to switch rapidly; rather, it would be moved in or out of the beam
depending on whether it was to be used or not. So I would guess that a
redesign would be necessary for this very different duty cycle. Moreover,
the switching time would have to be very rapid with the large number of
baselines that ALMA will have (even with only 40 antennas). Again, all four
combinations on each baseline would be needed for every (or better, every
half) UV cell move time. With the very large number of baselines that ALMA
will have, the switch cycle time would have to be very short. This time
could be worked out, but it would probably be comparable to a nutating
subreflector time. Whether this could be made to work, reliably, seems to
me to be problematical. And one would probably spend a very significant
fraction of the time to go through the complete cycle in dead time while
switching rather than in observing. The efficiency would be at best 25%,
since each of RR, LL, LR, and RL would be measured sequentially on each
baseline rather than simultaneously. In practice it would be significanlty
less than 25% due to switching dead time. Bottom line is that I think
without a dual polarization receiver, ALMA would not do polarization
observations.
Comments on charge 2 after each point:
2. ASAC is invited to continue its considerations of this September, 2004
charge, which may be combined with the continued development of ideas for
implementing demonstration science elaborated at the same meeting:
Following thorough assessment of the pros and cons of policies in use at
existing ground- and space-based facilities, including those currently
operated
by the ALMA Executives, ASAC is invited to consider policy recommendations on:
a. how to facilitate joint projects between scientists of different
partners,
COMMENT: One could add a bonus to TAC proposal ratings for joint projects,
and/or have a coordinated, single review for projects across different
projects. Having it reviewed separately by two or three different TACs
would impede such projects, and having it all done by one partner's TAC
would raise issues of whose time gets charged.
b. how to handle large proposals with significant scientific duplication,
COMMENT: Require or strongly encourage collaboration. And/or award part of
the necessary observing time to each team separately, so each could observe
a subset of the total but collaboration at the end would be necessary for a
complete science project.
and
c. whether provision needs to be made at this time for legacy projects
and, if so, what mechanisms should be used for such projects.
These complex, often-contentious issues should be addressed in the spirit of
demonstrating how ASAC believes their recommendations, if adopted, would
maximize ALMA's scientific impact.
COMMENT: I would strongly favor this, along the lines of the SST program.
There are some projects that are so big and so obvious that doing them via
a legacy project with results available immediately to everyone would
maximize science.
Richard M. Crutcher
Professor of Astronomy
University of Illinois
1002 W. Green St.
Urbana, IL 61801
Phone: (217)333-9581
--
AlWootten - 21 Feb 2005