Configurations

Within the Configurations Working Group most of the discussion focuses on two major alternatives for the basic array layout: the spiral zoom array concept described by Conway (ALMA Memos #216, 260, 283, and 291); and the ``doughnut'' array developed by Kogan guided by the goal of achieving minimal sidelobes (ALMA Memos #171, 212, 226, and 247). For both concepts realistic array layouts considering topographic constraints have now been studied (ALMA Memos #292 and 296). Both layouts appear to achieve comparable sidelobe levels, which are of order 6-8% (for snapshots!), with the spiral array producing lower near sidelobes for longer tracks. Consequently, a decision to adopt one or the other design has to be based on a number of factors, including logistics and scientific requirements. For example, guided by experience with the VLA, one might expect that the observers' demand will be highest for the most extended configuration (for maximum resolution) and the most compact one (maximizing surface brightness sensitivity). Such considerations should be included in the choice of array concepts. A need for model images has arisen and a total of five images will be chosen for use with all simulations. More imaging simulations are necessary for arrays involving baselines up to 20 km, where terrain considerations are the major issue. Given ALMA's excellent brightness sensitivity, imaging of thermal emission from gas and dust with such long baselines will open new vistas. Resolutions better than 10 milliarcseconds will be achieved, which are essential for studies of some of ALMA's key science goals, such as the formation of planets. As decisions on antenna pad locations have to be made by late 2000, we recommend that the Configurations Working Group report on progress to the ASAC at our next meeting, after which we can make a final recommendation. Since the large size of the working group might be conducive to excessive discussions, intervention by the project scientists might be necessary to warrant a timely decision process.