A discussion of ALMA configurations on 9 June 1999 was attended by T. Helfer, L. Kogan, M. Yun, B. Butler, Morita, S. Radford, F. Viallefond, J. Richer and A. Wootten. We agreed that setting up a strawperson configuration plan should be accomplished as soon as possible. We further agreed that a standard image library should be formed, with the idea that the first and subsequent stawperson arrays could be confronted with these images to evaluate their performance. A goal of the project is to provide precision total flux images at least to 350 GHz, and to higher frequencies as it appears practical. Existing and proposed configurations should therefore be judged by their performance at 350 GHz, though they should be tested at the extreme frequency ranges of the array also, currently 31 GHz and 900 GHz.
Criteria for acceptable imaging include image fidelity and response to large spatial and intensity dynamic range, including modeling of multiconfiguration observations. A few types of images were specifically mentioned--the VLA image of Cas A, which shows a wealth of structure on many spatial scales--should be employed, scaled to match ALMA observing parameters. Viallefond noted the advantages of fractal images. Although the eye charts which were displayed at the Imaging 99 meeting are excellent for conveying points about image quality to lay audiences, they don't resemble astronomical objects and aren't well-suited to configuration optimization. Currently, the MMA Advisory Committee (US) is canvassing its members for suitable images.
Helfer presented the current MMA/ALMA strawperson configuration, followed by a presentation by Kogan on imaging simulations. A vote was taken on eyeballed image quality, based on Kogan's simulations, which suggested a moderate divergence of opinion and underscored the need for standard criteria. Kogan also discussed a consideration of spiral arrays. One example showed excellent imaging qualities, but with extended low sidelobes compared with 'doughnut' simulations. All agreed that simulations beyond these snapshot types, to include various zenith angles to account for shadowing, pointing errors, and atmospheric conditions, were needed. Helfer agreed to extend the simulated imaging of M51-like objects by current arrays to the strawperson ALMA array. Yun discussed MMA memo No. 265, on the number of configurations needed. The continuously varying array discussed by Webster and by Conway was considered; it was thought that the operations groups should consider the effects on manpower requirements and other factors of this configuration type.
Radford presented the state of our detailed knowledge of Chajnantor terrain. A limited number of sites appear well-suited to the compact configuration. Siting of more extended configurations is even more problematic. Radford and Helfer will mate the strawperson configuration to our existing knowledge of the site to provide a strawperson siting of the array. Richer may be able to do some preliminary reconnaissance when he visits Chajnantor in late June. In situ examination of extended array sitings will commence as possible. This is a challenging undertaking easily foiled by poor weather.
Progress will be monitored at the weekly ALMA ImCal meetings; further face to face meetings may be possible at the August URSI meeting or during the October ALMA science meeting/MAC/SAC meeting. The next US visit to the site will probably occur later in October.