From: Dick Crutcher <> Sender: To: Cc: 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.


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
Topic revision: r1 - 2005-02-21, AlWootten
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